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Back | Show only these items: | Ionization-induced switch in aromatic molecule–nonpolar ligand recognition: Acidity of 1-naphthol+ (1-Np+) rotamers probed by IR spectra of 1-Np+–Ln complexes (L = Ar/N2, n ≤ 5)

1
Ionization-induced switch in aromatic molecule–nonpolar ligand recognition: Acidity of 1-naphthol⁺ (1-Np⁺) rotamers probed by IR spectra of 1-Np⁺–L_n complexes (L = Ar/N₂, n ≤ 5)

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

The interaction of the trans (t) and cis (c) rotamers of the 1-naphthol cation (1-C₁₀H₈O⁺ = 1-Np⁺ = 1-hydroxynaphthalene⁺) with nonpolar ligands in the ground electronic state is characterized by IR photodissociation spectra of isolated 1-Np⁺–L_n complexes (L = Ar/N₂) and density functional calculations at the UB3LYP/6-311G(2df,2pd) level.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj2

Size-dependent frequency shifts of the O–H stretch vibration (Δν₁) and photofragmentation branching ratios provide information about the stepwise microsolvation of both 1-Np⁺ rotamers in a nonpolar hydrophobic environment, including the formation of structural isomers, the competition between H-bonding and π-bonding, the estimation of ligand binding energies, and the acidity of t/c-1-Np⁺. t-1-Np⁺ is predicted to be more stable than c-1-Np⁺ by 9 kJ mol⁻¹, with an isomerization barrier of 38 kJ mol⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

The OH group in t-1-Np⁺ is slightly more acidic than in c-1-Np⁺ leading to stronger intermolecular H-bonds.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

Both 1-Np⁺ rotamers are considerably less acidic than the phenol cation because of enhanced charge delocalization.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

The 1-Np⁺−Ar spectrum displays ν₁ bands of the more stable H-bound and the less stable π-bound t-1-Np⁺–Ar isomers.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

Only the more stable H-bound dimers are identified for t/c-1-Np⁺–L₂.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

Analysis of the Δν₁ shifts of the H-bound dimers yields a first experimental estimate for the proton affinity of the t-1-naphthoxy radical (∼908 ± 30 kJ mol⁻¹).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

The Δν₁ shifts of 1-Np⁺–L_n (n ≤ 2 for Ar, n ≤ 5 for N₂) suggest that the preferred microsolvation path begins with the formation of H-bound 1-Np⁺–L, which is further solvated by (n−1) π-bound ligands.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con3

Ionization of 1-Np−L_n drastically changes the topology of the intermolecular interaction potential and thus the preferred aromatic substrate–nonpolar ligand recognition pattern.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con4

Introduction

Hydroxyarenes are fundamental molecules in chemistry, biology, and life sciences, and display a variety of interesting properties arising from both the hydroxy group and the aromatic π-electron system.^1,2

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot1

In particular, they are strong photoacids, that is the acidity of the OH group increases drastically upon electronic excitation and ionization, which eventually may promote proton transfer to a suitable solvent.^2–4

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

The properties of such proton transfer processes, which are among the most fundamental reactions occurring in chemical and biological systems, depend sensitively on the environment.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

Consequently, (micro)solvated hydroxyarenes have become popular model systems to investigate energetics and dynamics of photo-induced proton transfer in both isolated complexes and in solution under controlled solvation conditions.^2–20

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot2

The present work reports quantum chemical calculations and infrared (IR) spectra of 1-naphthol cations (1-Np⁺ = 1-C₁₀H₈O⁺ = 1-hydroxynaphthalene⁺) solvated by a well-defined number of nonpolar ligands (L = Ar/N₂), providing direct access to the acidity of the OH group in the cation ground state and its dependence on stepwise solvation in a hydrophobic environment.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj3

Significantly, aromatic molecules (A) with acidic functional YH_k groups (e.g., Y = N, O) offer two major recognition sites for inert ligands L. L can bind to the aromatic π-electron system (π-bond) or to one of the k acidic protons of YH_k (H-bond).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

Alternative binding sites, such as H-bonds to aliphatic or aromatic CH protons, are usually less stable.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

The preferred binding site in A–L dimers depends on several factors, including the degree of electronic excitation and charge state of A, the acidity of the YH_k group, and the ligand type (polar or nonpolar).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

For example, complexes of neutral A with rare gas atoms have π-bound equilibrium structures in the singlet electronic ground state (S₀).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac6

Examples include Ar complexes of benzene (Bz–Ar),²¹ phenol (Ph–Ar, YH_k = OH),^22,23 aniline (An–Ar, YH_k = NH₂),²⁴ and indole (In–Ar, YH_k = NH).²⁵

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

The major contributions to the A–Ar attraction are dispersion forces between Ar and the π-electron system of A, which favor π-bonding.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac8

In fact, H-bound isomers have not been detected for any aromatic A–Ar complex in S₀.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

The situation changes for A–N₂ dimers, because the quadrupole moment of N₂ leads to additional electrostatic interactions with the polar YH_k group of A, so that the H-bond may energetically compete with the π-bond.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

For example, N₂ favors H-bonding to Ph^22,26 but π-bonding to less acidic An²⁷ as well as Bz.^28,29

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac11

In general, the topology of the interaction potential of A⁺–L radical cation dimers differs qualitatively from that of neutral A–L, because of the significant additional electrostatic and inductive attraction arising from the positive charge.^{8,13,15,30,31}

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac12

As a consequence, neutral and ionic A⁽⁺⁾–L dimers often possess rather different equilibrium structures and interaction energies.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac13

For example, Ph⁺–Ar,^32–35 An⁺–Ar,³⁶ and In⁺–Ar³⁷ have H-bound global minima in the ground electronic state of the cation (D₀), whereas the π-bound isomers are only local minima.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac14

Similarly, Ph⁺–N₂,^22,32,33,38 An⁺–N₂,³⁹ and In⁺–N₂³⁷ prefer H-bonds over π-bonds in D₀.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac15

Because of the additional charge–quadrupole interaction, the H/π-bonds in A⁺–N₂ are stronger than in A⁺–Ar.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac16

Similar to A⁺–Ar/N₂, the Ar and N₂ dimers of protonated aromatic molecules (AH⁺) with acidic YH_k groups (e.g., PhH⁺–Ar/N₂)^40–42 also favor H-bonds over π-bonds.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac17

In contrast, the most stable A(H)⁺–Ar/N₂ dimers of A(H)⁺ ions without YH_k groups, such as Bz(H)⁺–Ar/N₂,^21,43–45 feature π-bonds, because CH protons are only weakly acidic even in the cation ground state.⁴⁴

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac18

The ionization-induced switch in the preferred ligand binding site in A⁽⁺⁾–L imposes important consequences for spectroscopic studies of A⁺–L prepared by photoionization of A–L.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot3

Often, A⁺–L is generated by resonance enhanced multiphoton ionization (REMPI) of A–L formed in a supersonic expansion.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac19

Spectroscopic information of A⁺–L may then be extracted, for example, from photoionization efficiency spectra (PIE),⁴⁶ mass analyzed threshold ionization (MATI),^21,47 zero kinetic energy photoelectron (ZEKE) spectra,¹³ or IR photodissociation.^8,10

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac20

However, all these ionization techniques suffer from the Franck–Condon principle, which prevents significant population of the most stable A⁺–L isomer for dimers with significantly different neutral and ionic global minimum structures.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac21

This situation is, however, quite common for A⁽⁺⁾–L dimers with both polar and nonpolar L.^31,32,48,49

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac22

To avoid the limitations of REMPI cluster ion generation, in the present work cold A⁺–L_n complexes are produced in a supersonic plasma expansion created by electron ionization (EI) of a molecular beam.⁵⁰

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met1

This EI ion source generates A⁺–L_n complexes via EI of A followed by cluster aggregation.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met1

Consequently, the EI source produces predominantly the most stable isomer of a given A⁺–L_n complex, independent of the most stable structure of neutral A–L_n.

Type: Method | Advantage: Yes | Novelty: Old | ConceptID: Met1

This is in contrast to the REMPI ion source, which often generates only local minima of A⁺–L_n.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac23

For example, EI-IR spectra of Ph⁺–Ar_n,^32–35 An⁺–Ar_n,³⁶ and In⁺–Ar_n³⁷ were interpreted with global minimum structures, which completely escaped detection in the corresponding REMPI-IR and other photoionization spectra (PIE, ZEKE, MATI).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac24

1-Np is one out of the two possible bicyclic monohydroxyarenes (1/2-Np).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac25

It has two planar isomers, which differ by the orientation of the OH group with respect to the aromatic ring (Figs. 1 and 2), namely the trans (anti) and cis (syn) rotamers denoted t-1-Np and c-1-Np, respectively.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac26

Spectroscopic information about the structures and relative energies of both rotamers is available from a variety of techniques, including (dispersed) laser-induced fluorescence,^20,51,52 REMPI,^19,53 and IR spectra.^12,19

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac27

Information on the corresponding t/c-1-Np⁺ cation rotamers is provided by ZEKE,⁵³ MATI,¹⁶ and REMPI-IR spectra.¹⁹

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac28

The results for t/c-1-Np⁽⁺⁾ relevant for the present work can be summarized as follows.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac29

In S₀, t-1-Np is slightly more stable than c-1-Np (ΔE = 220 ± 50 cm⁻¹)⁵¹ and its OH group is somewhat more acidic as demonstrated by the smaller O–H stretch frequency (ν₁/cm⁻¹ = 3655 vs. 3663).¹²

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac30

Interestingly, the energy order of both rotamers is reversed in S₁ (ΔE = −59 cm⁻¹).⁵³

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac31

In D₀, t-1-Np⁺ is again more stable than c-1-Np⁺ (ΔE = 495 cm⁻¹), indicating that the potential for OH rotation is quite sensitive to both electronic excitation and ionization.⁵³

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac32

The acidity of t-1-Np⁽⁺⁾ is substantially increased upon ionization, as revealed by the large decrease in ν₁ from 3655 to 3579 cm⁻¹.¹⁹

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac33

No spectroscopic information is available for the acidity of c-1-Np⁺.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac34

Spectroscopic and theoretical information about neutral and ionic 1-Np⁽⁺⁾–L_n complexes with L = Ar/N₂ is rather limited.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot4

REMPI,^19,54 hole-burning,⁵⁴ and IR spectra¹⁹ of t-1-Np–Ar are consistent with a π-bound structure in S₀, in agreement with quantum chemical calculations.⁵⁴

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac35

Similarly, the REMPI-IR spectrum of t-1-Np⁺–Ar in the O–H stretch range was interpreted with a π-bound isomer in D₀.¹⁹

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac36

No evidence for an H-bound 1-Np⁽⁺⁾–Ar isomer has been reported for both S₀ and D₀.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac37

Calculations predict π-bound t-1-Np–N₂ to be more stable than the H-bound isomer in S₀, and preliminary REMPI and hole-burning spectra appear to be consistent with this view.⁵⁴

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac38

To the best of our knowledge, no spectroscopic information is available for any c-1-Np⁽⁺⁾–L_n complex in both S₀ and D₀.^9,16,19,20

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac39

Such information is difficult to obtain because of the usually low population of the less stable c-1-Np rotamer in molecular beams and steric hindrance involved in the formation of H-bound c-1-Np⁽⁺⁾–L complexes.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac40

Several aspects motivated the present EI-IR spectroscopic and theoretical study on size-selected 1-Np⁺–L_n complexes with L = Ar and N₂.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj4

(1) These clusters serve as a suitable model for solvation of the highly acidic 1-Np⁺ cations in a nonpolar solvent.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot5

Particularly interesting is the competition between H-bonding and π-bonding and its consequences on the cluster growth.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot6

As Ar and N₂ are weak proton acceptors, proton transfer from 1-Np⁺ to the solvent may not be expected in D₀ even for complete solvation.⁵⁵

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot6

(2) As the ability of the OH group to form H-bonds to a ligand is correlated with its acidity, the EI-IR spectra of H-bound 1-Np⁺–L dimers directly probe the acidity of both 1-Np⁺ rotamers and eventually enable a first estimate of the unknown proton affinity of the 1-naphthoxy radical.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot7

The detection of c-1-Np⁺–(N₂)_n in the present work corresponds to the first observation of c-1-Np⁺ complexes and enables thus direct comparison of both 1-Np⁺ rotamers.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot8

H-bound 1-Np⁺–L isomers are also characterized by density functional calculations, because theoretical studies of the interaction potential in these dimers are not available.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj5

(3) Comparison of 1-Np⁺–L_n with Ph⁺–L_n^22,32–35 and H₂O⁺–L_n^56–59 reveals the influence of substitution of a mono- and bicyclic aromatic ring on the acidity of ROH⁺ cations.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot9

The acidity is expected to decrease in the order H₂O⁺ > Ph⁺ > 1-Np⁺ because of increasing charge delocalization in the aromatic ring¹⁹.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp1

Experimental

EI-IR photodissociation spectra of mass-selected 1-Np⁺–L_n complexes (L = Ar/N₂, n ≤ 5) were recorded in a tandem quadrupole mass spectrometer (QMS1/2) coupled to an EI ion source and an octopole ion trap.⁵⁰

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

Briefly, 1-Np⁺–L_n were generated in a plasma expansion created by EI of a pulsed supersonic molecular beam.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

The expansion gas was produced by seeding 1-Np vapor (T ≈ 350 K) in Ar or N₂ at stagnation pressures of 3–10 bar.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

As described previously,³³ the dominant production mechanism of 1-Np⁺–L_n starts with EI of 1-Np near the nozzle orifice, which is followed by three-body association in the high-pressure region of the expansion:1-Np + e → 1-Np⁺ + 2e1-Np⁺–L_n−1 + L + M → 1-Np⁺–L_n + M (M = L, 1-Np)This reaction sequence produces mainly the most stable 1-Np⁺–L_n complexes and to smaller extent less stable isomers.^{33–37,60,61}

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac41

Alternative mechanisms involving the formation of neutral 1-Np–L_n complexes and subsequent EI play only a minor role.³³

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac42

Fig. 3 shows a mass spectrum of the ion source recorded after optimization for 1-Np⁺–(N₂)_n generation.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

The spectrum is dominated by N_n⁺ and [X–(N₂)_n]⁺ cluster ions, including X = 1-Np and the impurity H₂O. The intensity ratios of 1-Np⁺–(N₂)_n are of the order of 80∶8∶2∶1 for n = 0–3, indicating the formation of weakly-bound noncovalent complexes.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

The central part of the plasma expansion was extracted through a skimmer into QMS1, which was tuned to the mass of 1-Np⁺–L_n.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

The mass-selected 1-Np⁺–L_n beam was then injected into an octopole ion guide, where it interacted with a tunable IR laser pulse.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

Resonant vibrational excitation of 1-Np⁺–L_n induced the evaporation of one or more ligands: 1-Np⁺–L_n + hν → [1-Np⁺–L_n]* → 1-Np⁺–L_m + (n − m)L Other photodissociation channels were not observed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

1-Np⁺–L_m fragments generated in the octopole were mass-selected by QMS2 and monitored as a function of the laser frequency (ν) to obtain the EI-IR spectra of 1-Np⁺–L_n.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

Several fragment channels (m) were observed for 1-Np⁺–(N₂)_n parent complexes with n > 3.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

Consequently, IR spectra were recorded simultaneously in the two dominant fragment channels.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp7

As an example, Fig. 4 shows the mass spectrum obtained for resonant ν₁ excitation of t-1-Np⁺–(N₂)₄.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

Major fragment channels are m = 0 and m = 1 arising from laser-induced dissociation and m = 3 produced by metastable decay in the octopole.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

To separate laser-induced fragmentation from interfering metastable decay, the ion source was triggered at twice the laser repetition rate and signals from alternating triggers (laser on, laser off) were subtracted.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp8

As the IR spectra recorded in different fragment channels are similar, only the spectra obtained in the dominant channels are shown.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

IR radiation was generated by a single-mode optical parametric oscillator laser system and calibrated to an accuracy of better than 1 cm⁻¹ by analyzing atmospheric water absorptions along the IR laser path⁶².

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp9

Density functional calculations

Density functional theory (DFT) calculations were carried out for t/c-1-Np⁺ and their H-bound dimers at the UB3LYP/6-311G(2df,2pd) and UB3LYP/6-31G* levels of theory⁶³ to characterize the intermolecular H-bonds and their effects on the t/c-1-Np⁺ properties.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

As the results are qualitatively similar for both basis sets, only those obtained with the larger basis are reported.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

The t/c-1-Np⁺(–L) properties relevant for the present work are summarized in Table 1 and Figs. 1 and 2.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

For comparison, additional calculations were carried out for t/c-1-Np, Ph⁽⁺⁾, and Ph⁺–L at the same theoretical level.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod3

Table 1 includes structural and energetic attributes of the intermolecular H–L bond, such as length (R_H–L), angle (θ_O–H–L), dissociation energy (D_e), and intermolecular stretch frequency (ν_s), as well as relevant properties of the intramolecular O–H bond, namely length (R_O–H), stretch frequency (ν₁), and IR intensity (I₁).

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod4

In general, all coordinates were relaxed during the search for stationary points.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod5

Intermolecular interaction energies were counter-poise corrected for basis set superposition error.⁶⁴

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod6

Harmonic vibrational frequencies were scaled by a factor of 0.9516 to bring the calculated ν₁ frequencies of both t-1-Np⁺ and Ph⁺ into agreement with the experimental values, ν₁ = 3579 and 3534 cm⁻¹.^19,65

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod7

First, the bare t/c-1-Np⁽⁺⁾ rotamers are considered.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj6

Geometry optimization results in planar equilibrium structures of 1-Np in both S₀ and D₀, consistent with available spectroscopic data.^52,53

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

Fig. 2 compares the potentials for internal OH rotation in 1-Np and Ph calculated for S₀ and D₀ including zero-point energy contributions.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj7

In S₀, t-1-Np is predicted to be more stable than c-1-Np by ΔE = 360 cm⁻¹, in reasonable agreement with the value of ΔE = 220 ± 50 cm⁻¹ estimated from the analysis of the UV spectrum recorded in an absorption cell.⁵¹

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

The barrier for internal rotation from t-1-Np toward c-1-Np, V_b = 1087 cm⁻¹, is somewhat smaller than in Ph, V_b = 1151 cm⁻¹.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

The latter value is close to the experimental barrier determined from rotationally resolved UV spectroscopy in a molecular beam, V_b = 1215 ± 10 cm⁻¹,⁶⁶ suggesting that the employed theoretical level is of sufficient accuracy to yield reliable internal rotation potentials.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

100

In D₀, t-1-Np⁺ is also calculated to be more stable than c-1-Np⁺ and the energy difference, ΔE = 746 cm⁻¹, is approximately twice that in S₀.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

101

The difference in stability of both rotamers in D₀ and S₀ corresponds to the difference in their adiabatic ionization potentials, and the predicted value is compatible with the experimental measurement, 386 vs. 281 cm⁻¹.⁵³

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

102

The barrier for OH rotation from t-1-Np⁺ toward c-1-Np⁺, V_b = 3198 cm⁻¹, is significantly larger than in S₀.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

103

The corresponding barrier for Ph⁺ is even higher, V_b = 4514 cm⁻¹.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs9

104

The barrier height of hydroxyarenes is usually correlated with their C–O bond strength.^52,66

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac43

105

In general, the stronger and shorter the C–O bond, the more hindered the internal rotation.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac43

106

Specifically, R_C–O/Å∼1.363 > 1.318 > 1.307 and V_b/cm⁻¹ = 1087 < 3198 < 4514 for 1-Np, 1-Np⁺, and Ph⁺.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs10

107

Apparently, electron donation from the OH group into the aromatic π-electron system is more efficient in D₀ than in S₀ and larger for Ph⁺ as compared to 1-Np⁺.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

108

In contrast to the C–O bond, the O–H bond strength decreases along the series c-1-Np⁺ > t-1-Np⁺ > Ph⁺, as is signaled by the increasing bond length (R_O–H/Å = 0.9666 < 0.9675 < 0.9714) and the decreasing stretch frequency (ν₁/cm⁻¹ = 3587 > 3579 > 3534).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

109

The rise in the acidity of the OH group along this series is also mirrored by the increasing positive partial charge on the OH proton.⁴⁴

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

110

In general, the differences between the t/c-1-Np⁺ rotamers are significantly smaller than between t/c-1-Np⁺ and Ph⁺.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

111

Interestingly, the IR oscillator strengths for ν₁ of c-1-Np⁺ and t-1-Np⁺ differ by a factor of 2.4 (I₁/km mol⁻¹ = 121 vs. 289).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs11

112

The H-bound 1-Np⁺–L dimers feature trans-linear proton bonds of L to the acidic OH group of 1-Np⁺, leading to planar equilibrium geometries with C_s symmetry (Fig. 1).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs12

113

Because of steric hindrance arising from the interaction with the proton on the second aromatic ring (C₍₈₎H), the deviation from linearity is much larger for c-1-Np⁺–L (∼150°) as compared to t-1-Np⁺–L (∼170°).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

114

The C₍₈₎H–N and C₍₈₎H–Ar separations of 2.71 and 3.07 Å in c-1-Np⁺–L are close to the values expected from van der Waals radii (R_vdW/Å = 1.1, 1.5, 1.9 for H, N, Ar).⁶⁷

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

115

The anisotropy of the long-range charge–quadrupole and charge–induced dipole interactions aligns the N₂ ligand in such a way that the molecular axis points toward the positive charge,^{30,32,37,39,41,44,60,68–71} resulting in nearly linear H⋯N–N configurations in t/c-1-Np⁺–N₂ (θ_H–N–N = 175/171°).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res17

116

The increase in the acidity along the series c-1-Np⁺< t-1-Np⁺< Ph⁺ is directly correlated with the intermolecular interaction strength, leading to shorter H–L bonds (R_H–L/Å = 2.59 > 2.46 > 2.35 for L = Ar and 2.11 > 2.02 > 1.95 for L = N₂) with larger stretch frequencies (ν_s/cm⁻¹ = 46 < 49 < 73 for L = Ar and 88 < 94 < 117 for L = N₂) and higher dissociation energies (D_e/cm⁻¹ = 287 < 372 < 685 for L = Ar and 1205 < 1336 < 1731 for L = N₂).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res18

117

The effects of H-bonding on the intramolecular O–H bonds are an elongation (ΔR_O–H), a reduction in the stretch frequency (Δν₁), and an enhancement in the IR oscillator strength (ΔI₁).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res19

118

Again, the magnitude of these effects is correlated with the H-bond strength and increases along the series c-1-Np⁺< t-1-Np⁺< Ph⁺: ΔR_O–H/Å = 0.0016 < 0.0027 < 0.0044 for L = Ar and 0.0048 < 0.0068 < 0.0099 for L = N₂; −Δν₁/cm⁻¹ = 34 < 57 < 92 for L = Ar and 91 < 134 < 195 for L = N₂; ΔI₁/km mol⁻¹ = 182 < 445 < 537 for L = Ar and 453 < 977 < 1060 for L = N₂.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs13

119

These theoretical results demonstrate that IR spectroscopy in the O–H stretch range is a suitable tool to probe both the acidity of the t/c-1-Np⁺ rotamers and their ability to form H-bonds.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con5

120

Similar to previous studies on (para-halogenated) Ph(H)⁺–Ar_n and Ph(H)⁺–(N₂)_n,^{32–35,40–42} the EI-IR spectra of the corresponding 1-Np⁺–Ar_n and 1-Np⁺–(N₂)_n complexes clearly demonstrate that the H-bonds are more stable than the π-bonds.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res20

121

Hence, the energetically most favorable 1-Np⁺–L_n cluster growth begins with the formation of a H-bound 1-Np⁺–L dimer, to which further π-bound ligands are attached.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res21

122

The exact position of the π-bound ligands is however not obvious (e.g., over the first or the second aromatic ring) and difficult to determine from both the present spectroscopic and theoretical approaches.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res22

123

As DFT calculations do not properly describe dispersion forces, π-bound 1-Np⁺–L isomers were not investigated theoretically.³²

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac44

124

On the other hand, previous studies revealed that π-bound ligands have only very minor effects on the O–H bond properties^19,32,35.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac45

IR spectra

125

The EI-IR spectra of 1-Np⁺–Ar_n (n ≤ 2) and 1-Np⁺–(N₂)_n (n ≤ 5) recorded in the O–H stretch range are shown in Figs. 5 and 6.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp10

126

Table 2 summarizes the band maxima and widths of the ν₁ transitions observed (A–C), along with their isomer assignments.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs14

1-Np⁺–L Dimers

127

Fig. 5 compares EI-IR spectra of 1-Np⁺–N₂ (a, b) and 1-Np⁺–Ar (c) with the REMPI-IR spectrum of t-1-Np⁺–Ar (d).¹⁹

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs15

128

The EI-IR spectrum of 1-Np⁺–Ar displays two transitions, which are attributed to the ν₁ fundamentals of the H-bound (A) and π-bound (B) isomers of t-1-Np⁺–Ar, respectively.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res23

129

These assignments are based on the band positions, relative IR intensities, and band profiles, as well as the comparison with the DFT calculations and the REMPI-IR spectrum.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res23

130

The more intense band at 3538 cm⁻¹ (A) is assigned to ν₁ of H-bound t-1-Np⁺–Ar (Fig. 1b).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res24

131

The derived complexation-induced red shift of −41 cm⁻¹ from the corresponding t-1-Np⁺ transition (3579 cm⁻¹)¹⁹ is consistent with the calculated value (−57 cm⁻¹).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res25

132

The blue-shaded band contour with a sharp P-branch head is characteristic for excitation of a proton donor stretch vibration, because the intermolecular H-bond becomes stronger and shorter in the excited state, resulting in larger rotational constants.^30,32,61,70

Type: Result | Advantage: None | Novelty: None | ConceptID: Res26

133

Band B at 3580 cm⁻¹ is assigned to ν₁ of π-bound t-1-Np⁺–Ar, in good agreement with the REMPI-IR spectrum (Fig. 5d).¹⁹

Type: Result | Advantage: None | Novelty: None | ConceptID: Res27

134

The modest complexation-induced blue shift of +1 cm⁻¹ and the rather symmetric band profile support this assignment, because intermolecular π-bonding has almost no influence on the O–H bond.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res28

135

There are actually other binding sites, which also have little impact on the O–H bond, such as H-bonding to aromatic C–H bonds.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs16

136

Although at present these alternative binding sites cannot be completely ruled out from both the experimental and the theoretical point of view, the presently favored assignment for B is to a π-bound isomer and this interpretation is used as working hypothesis for further discussion in this paper.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res29

137

This scenario is supported by theoretical and spectroscopic data obtained for BzH⁺–Ar,^44,45 Bz⁺–Ar,^43,72–74 and PhH⁺–Ar,⁴² which indicate that intermolecular CH–Ar bonds in A(H)⁺–Ar are less stable than π-bonds.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac46

138

The EI-IR spectrum of t-1-Np⁺–Ar clearly demonstrates that the H-bound isomer is the global minimum on the intermolecular potential, whereas the π-bound structure is a less stable local minimum.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res30

139

Nonetheless, the H-bound isomer could not be detected in the REMPI-IR spectrum (Fig. 5d),¹⁹ because it was obtained through resonant ionization of π-bound t-1-Np–Ar.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs17

140

In contrast to both t-1-Np⁺–Ar isomers, spectroscopic indication of the corresponding c-1-Np⁺–Ar dimers in the EI-IR spectra is only tentative.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs18

141

The ν₁ frequency of bare c-1-Np⁺ has not been measured,¹⁹ and the calculated value is slightly larger than for t-1-Np⁺ (+8 cm⁻¹).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs19

142

Some of the EI-IR spectra recorded for 1-Np⁺–Ar actually reveal a very weak band at 3595 cm⁻¹ (indicated by an asterisk in Fig. 5c) appearing about 15 cm⁻¹ to the blue of π-bound t-1-Np⁺–Ar.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs20

143

At first glance, this signal may be attributed to ν₁ of π-bound c-1-Np⁺–Ar, which could occur in detectable abundance only under certain experimental conditions.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res31

144

If this interpretation were correct, a minor part of the blue wing of band A may arise from ν₁ of H-bound c-1-Np⁺–Ar (Fig. 1d).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res31

145

However, according to the analysis of the 1-Np⁺–N₂ spectra (vide infra), the abundances of c-1-Np⁺ and its Ar complex are probably too small for observing ν₁ of π-bound c-1-Np⁺–Ar even under favorable (e.g., warm) expansion conditions.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res31

146

Thus, the interpretation of the weak 3595 cm⁻¹ band remains open.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res31

147

Fig. 5 compares EI-IR spectra of 1-Np⁺–N₂ recorded under warm (a) and cold (b) expansion conditions, as revealed from the widths of the observed rotational band contours.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj8

148

The cold spectrum (b) displays a single blue-shaded band at 3467 cm⁻¹ (A), which is assigned to ν₁ of H-bound t-1-Np⁺–N₂ (Fig. 1a).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res32

149

The measured red shift of −112 cm⁻¹ upon H-bonding is slightly smaller than the calculated value of −134 cm⁻¹.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs21

150

Moreover, it is roughly three times larger than the one for t-1-Np⁺–Ar (−41 cm⁻¹) because of the stronger H-bond.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res33

151

In addition to band A, the warm 1-Np⁺–N₂ spectrum (a) shows a transition at 3499 cm⁻¹ (C), which is absent in the cold spectrum (b).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs22

152

This band is attributed to ν₁ of H-bound c-1-Np⁺–N₂ (Fig. 1c).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res34

153

The −88 cm⁻¹ shift from calculated ν₁ of c-1-Np⁺ (3587 cm⁻¹) agrees with the prediction (−91 cm⁻¹).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs23

154

Both EI-IR spectra of 1-Np⁺–N₂ do not show any absorption near 3580 cm⁻¹, implying that the abundances of the π-bound t/c-1-Np⁺–N₂ isomers were below the detection limit under both warm and cold conditions.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res35

155

The ratios of the integrated ν₁ band intensities observed in the EI-IR spectra of 1-Np⁺–L can be used to estimate the relative abundances of the H-bound and π-bound isomers in the plasma expansion, using the calculated ν₁ IR oscillator strengths of t/c-1-Np⁺ and H-bound t/c-1-Np⁺–L (Table 1) and assuming that π-bound t/c-1-Np⁺–L have the same ν₁ IR intensities as bare t/c-1-Np⁺.^32,36,39

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod8

156

For t-1-Np⁺–Ar, this procedure results in an abundance ratio of N_H∶N_π≈1.6, on the basis of the experimental ν₁ intensity ratio of ≈4 and the theoretical ratio I_H∶I_π ≈ 2.5.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs24

157

For t-1-Np⁺–N₂, a lower limit for N_H∶N_π > 9 can be estimated from the achieved signal-to-noise ratio (≈40) and I_H∶I_π ≈ 4.4.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs25

158

The larger abundance of the H-bound t-1-Np⁺–L isomers suggests that they are more stable than the π-bound dimers, because the EI source produces predominantly the most stable isomer of a given complex.^33,50

Type: Result | Advantage: None | Novelty: None | ConceptID: Res36

159

Moreover, the energy difference between both isomers appears to be significantly larger for t-1-Np⁺–N₂ than for t-1-Np⁺–Ar, resulting in a less efficient production of the π-bound isomer for the N₂ complex.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res37

160

Similar to previous studies on Ph⁺–Ar and In⁺–Ar,^34,37 the relative intensity ratio of the ν₁ bands of H-bound and π-bound t-1-Np⁺–Ar depended on the expansion conditions, confirming that both transitions arise indeed from two different isomers.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res38

161

The spectrum in Fig. 5c was obtained when the conditions were optimized for the production of π-bound t-1-Np⁺–Ar.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs26

162

As the relative population of the H-bound isomer increased for decreasing effective source temperature, this isomer corresponds to the global minimum of the t-1-Np⁺–Ar potential, whereas the π-bound structure is a less stable local minimum.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res39

163

The population ratio of the H-bound isomers of t-1-Np⁺–N₂ and c-1-Np⁺–N₂ in the warm spectrum of 1-Np⁺–N₂ (Fig. 5a) is deduced as N_t∶N_c ≈ 8 from the experimental (≈18) and predicted (≈2.2) ν₁ intensity ratios.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs27

164

As the N₂ complexation energies are similar for both 1-Np⁺ rotamers (1336 and 1205 cm⁻¹), the c-1-Np⁺ concentration in the plasma expansion is estimated to be roughly one order smaller than that for t-1-Np⁺.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res40

165

This result is consistent with the production mechanism described in section 2 and the internal rotation potentials in Fig. 2.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs28

166

Assuming a nozzle temperature of T ≈ 350 K, the energy difference of c-1-Np and t-1-Np of ΔE ≈ 220 cm⁻¹ corresponds to a thermal population ratio of N_t∶N_c ≈ 2..5⁵¹

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs29

167

Adiabatic cooling in the supersonic expansion increases this ratio only slightly because both isomers are separated by a significant barrier (V_b = 1087 cm⁻¹ corresponds to T ≈ 1560 K).⁷⁵

Type: Result | Advantage: None | Novelty: None | ConceptID: Res41

168

For example, N_t∶N_c ≈ 8 was observed in a previous molecular beam experiment.²⁰

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac47

169

Assuming similar cooling of the t/c-1-Np rotamers in our expansion and mainly vertical ionization of the EI process, eqn. (1a), a similar ratio is obtained for the ionic t/c-1-Np⁺ rotamers because isomerization is strongly hindered by a large barrier in D₀ (V_b = 3198 cm⁻¹).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res42

170

Owing to comparable complexation energies for H-bonding and also π-bonding for both 1-Np⁺ rotamers, the t/c-1-Np⁺–L dimer population ratios should be similar to the t/c-1-Np⁺ monomer ratios, eqn. (1b), i.e.N_t∶N_c ≈ 3–10 depending on the efficiency of adiabatic cooling of neutral t/c-1-Np.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp2

171

Thus, the deduced ratio of N_t∶N_c ≈ 8 for the H-bound t/c-1-Np⁺–N₂ isomers is within the expected range.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res43

172

If the weak 3595 cm⁻¹ band in the 1-Np⁺–Ar spectrum were indeed due to ν₁ of π-bound c-1-Np⁺–Ar, the relative abundance of π-bound t/c-1-Np⁺–Ar (and also t/c-1–Np⁺) would be derived as N_t∶N_c ≈ 2.1 from the experimental (≈5) and predicted (≈2.4) ν₁ intensity ratios.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp3

173

This rather small ratio would imply an unexpectedly high population of both c-1-Np⁺ and π-bound c-1-Np⁺–Ar and makes this tentative assignment questionable.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res44

Larger 1-Np⁺–L_n complexes

174

Fig. 6 reproduces the EI-IR spectra of 1-Np⁺–Ar_n (n ≤ 2) and 1-Np⁺–(N₂)_n (n ≤ 5) recorded in the dominant fragment channels (Table 3).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs30

175

The 1-Np⁺–Ar spectrum (a) is dominated by two transitions at 3538 (A) and 3580 (B) cm⁻¹ assigned to ν₁ of H-bound and π-bound t-1-Np⁺–Ar (section 4.1).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res45

176

The 1-Np⁺–Ar₂ spectrum (b) reveals only a single band at 3539 cm⁻¹ (A), which is shifted by merely +1 cm⁻¹ from band A of H-bound t-1-Np⁺–Ar.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs31

177

Consequently, this band is assigned to ν₁ of a t-1-Np⁺–Ar₂ isomer, which is obtained by adding a π-bound ligand to H-bound t-1-Np⁺–Ar, denoted t-1-Np⁺–Ar₂(H/π).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res46

178

A blue shift of +1 cm⁻¹ is also observed for attachment of a π-bound Ar ligand to bare t-1-Np⁺.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs32

179

The ν₁ transitions of t-1-Np⁺–Ar₂ isomers with two π-bound ligands, t-1-Np⁺–Ar₂(2π), are expected near 3580 cm⁻¹ (B).

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp4

180

As the 1-Np⁺–Ar₂ spectrum lacks any signal in that frequency range, the abundance of t-1-Np⁺–Ar₂(2π) isomers is below the detection limit.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res47

181

The abundance ratio of t-1-Np⁺–Ar₂(2π) and t-1-Np⁺–Ar₂(H/π) is estimated to be less than 5%.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs33

182

This result confirms that the H-bond between Ar and t-1-Np⁺ is stronger than the π-bond.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res48

183

Hence, the preferred solvation sequence in small t-1-Np⁺–Ar_n complexes begins with the formation of an H-bound t-1-Np⁺–Ar dimer core, which is further solvated by (n − 1) π-bound ligands.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res49

184

The 1-Np⁺–N₂ spectrum (Fig. 6c) displays two transitions at 3467 (A) and 3499 (C) cm⁻¹ assigned to ν₁ of the H-bound isomers of t-1-Np⁺–N₂ and c-1-Np⁺–N₂ (section 4.1).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res50

185

Attachment of four further N₂ ligands causes only small incremental blue shifts, suggesting that the observed t/c-1-Np⁺–(N₂)_n complexes have one H-bound and (n−1) π-bound ligands.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res51

186

Fig. 7 visualizes the ν₁ frequencies of the most stable isomers of t-1-Np⁺–Ar_n, t-1-Np⁺–(N₂)_n, and c-1-Np⁺–(N₂)_n as a function of the cluster size n.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs34

187

Table 3 lists the photofragmentation branching ratios measured for resonant ν₁ excitation of the H/(n − 1)π isomers of t-1-Np⁺–Ar_n and t-1-Np⁺–(N₂)_n.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs35

188

In agreement with previous studies on related systems,^{33,37,39,42–44,50,57,60,61} the range of photo-induced fragment channels (m) for a given parent cluster size (n) is rather narrow (Fig. 4).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res52

189

This information is used to roughly estimate ligand binding energies within the framework of a simple model, which assumes that the absorbed photon energy (ν₁) is available for subsequent ligand evaporation.³³

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj9

190

The ligands are classified in H-bound and π-bound ones with dissociation energies D₀(H) > D₀(π), and all π-bound ligands are assumed to have the same binding energy.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod9

191

Larger t-1-Np⁺–(N₂)_n complexes evaporate on average about five π-bound ligands upon ν₁ excitation (∼3500 cm⁻¹), yielding an estimated dissociation energy of the order of D₀(π) ∼ 650 ± 150 cm⁻¹.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs36

192

Using this information, the branching ratios observed for n = 4 and 5 lead to 500 < D₀(H)/cm⁻¹ < 2000, i.e.D₀(H) ∼ 1250 ± 750 cm⁻¹, which agrees with the predicted value, D_e(H) = 1336 cm⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res53

193

Within the cluster size range investigated, the ν₁ bands assigned to c-1-Np⁺–(N₂)_n display photofragmentation branchings similar to the ν₁ bands of t-1-Np⁺–(N₂)_n, confirming that D₀(π) and D₀(H) are roughly comparable for both 1-Np⁺ rotamers.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res54

194

The t-1-Np⁺–Ar₂(H/π) trimer evaporates both ligands upon ν₁ excitation, implying that D₀(H) + D₀(π) < 3540 cm⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res55

Further discussion

195

The EI-IR spectra of 1-Np⁺–Ar_n are dominated by ν₁ absorptions assigned to Ar complexes of the more abundant t-1-Np⁺ rotamer.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs37

196

Moreover, the relative ν₁ intensities of the H-bound and π-bound dimers suggest that the H-bond of Ar to t-1-Np⁺ is more stable than the π-bond, D₀(H) > D₀(π).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res56

197

This conclusion is in line with the lack of detection of the 1-Np⁺–Ar₂(2π) isomer.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res57

198

The binding energies of the π-bound dimers of Bz⁺–Ar, para-difluorobenzene⁺–Ar, In⁺–Ar, and Ph⁺–Ar were accurately measured as 512 ± 3, 572 ± 6, 537 ± 10, and 535 ± 3 cm⁻¹, respectively.^15,23,76,77

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac48

199

As the strength of the π-bond of Ar to an aromatic cation A⁺ is apparently rather insensitive to the detailed structure of A⁺,⁴⁴D₀(π) of t/c-1-Np⁺–Ar may be estimated as ∼550 ± 50 cm⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res58

200

(This is contrast to neutral A–Ar dimers, where D₀(π) increases with the number of aromatic rings¹⁵.) The t-1-Np⁺–Ar₂ photofragmentation data show that D₀(H) + D₀(π) < 3540 cm⁻¹, yielding 500 < D₀(H)/cm⁻¹ < 3040.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res59

201

The upper limit can be further reduced because D₀(H) of t/c-1-Np⁺–Ar should be somewhat lower than D₀(H) of Ph⁺–Ar (670 ± 140 cm⁻¹).³³

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs38

202

First, Δν₁ of H-bound t-1-Np⁺–Ar is much smaller than Δν₁ of H-bound Ph⁺–Ar (1.15 vs. 2.0%),³³ because t-1-Np⁺ is less acidic than Ph⁺.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res60

203

Second, the N_H∶N_π ratio of t-1-Np⁺–Ar in the EI source is lower than for Ph⁺–Ar under comparable conditions.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs39

204

When the conditions are optimized for the generation of the π-bound isomers, N_H∶N_π ≈ 1.6 for t-1-Np⁺–Ar and ≈4 for Ph⁺–Ar.³⁵

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs40

205

This comparison suggests that D₀(H) − D₀(π) is larger for Ph⁺–Ar than for t-1-Np⁺–Ar.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res61

206

Because D₀(π) of Ph⁺−Ar and t-1-Np⁺–Ar are comparable, D₀(H) of Ph⁺–Ar provides an upper limit for D₀(H) of t-1-Np⁺–Ar, resulting in 500 < D₀(H)/cm⁻¹ < 800 or D₀(H) ≈ 650 ± 150 cm⁻¹ for t-1-Np⁺–Ar.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res62

207

Thus, the calculated dissociation energy of D_e = 372 cm⁻¹ substantially underestimates the true interaction energy, in line with conclusions previously drawn for UB3LYP/6-311G(2df,2pd) calculations of related Ar complexes.^32,35,36,41

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con6

208

Surprisingly, despite the underestimated interaction, this theoretical level slightly overestimates the Δν₁ shifts.

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209

In the limit of adiabatic separation of inter- and intramolecular degrees of freedom, the Δν₁ shifts correspond to the changes in the intermolecular binding energies upon ν₁ excitation.

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210

The −41 cm⁻¹ shift for H-bound t-1-Np⁺–Ar implies that the interaction increases by 5–8%, resulting in D₀(H) ≈ 690 ± 150 cm⁻¹ in the v₁ = 1 state.

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211

On the other hand, the +1 cm⁻¹ shift for π-bound t-1-Np⁺–Ar means that ν₁ excitation only slightly destabilizes the intermolecular π-bond (∼550 ± 50 cm⁻¹) by less than 0.3%.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res66

212

Available REMPI,^19,54 hole-burning,⁵⁴ and IR spectra¹⁹ of t-1-Np–Ar have been interpreted with a π-bound global minimum structure.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac49

213

This view is supported by recent quantum chemical calculations at the ri-MP2/ccpVTZ level, which predict the π-bond to be more stable than the H-bond in S₀.⁵⁴

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac50

214

No experimental evidence for the less stable H-bound isomer has been reported so far.^19,54

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215

The π-bound isomer optimizes the dispersion forces between Ar and the π-electron system of the aromatic ring, which dominate the attractive part of the intermolecular potential in S₀.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res67

216

On the other hand, the EI-IR spectrum of t-1-Np⁺–Ar clearly shows that the H-bond is the preferred intermolecular recognition site in D₀, because of the additional induction forces between the charge distribution in t-1-Np⁺ and the polarisability of Ar.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res68

217

In particular, the large positive partial charge on the acidic O–H proton causes the substantial additional stabilization of the H-bond on ionization.⁴⁴

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac52

218

The ionization-induced switch in the preferred recognition motif from π-bonding to H-bonding has now been demonstrated for several A–Ar and A–CH₄ dimers involving aromatic molecules A with acidic functional YH_k groups (Y = O, N) and appears to be a general phenomenon.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac53

219

Investigated molecules include A = (para-halogenated) Ph,^32–35 An,³⁶ In,³⁷ and 1-Np.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac54

220

H-bonds to Ar are also the most stable binding pattern for protonated aromatic molecules (AH⁺) featuring acidic functional groups, such as PhH⁺ or 1-NpH⁺.^40–42,78

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac55

221

Significantly, Ar and CH₄ prefer intermolecular π-bonds over H-bonds to A(H)⁺ without acidic substituents, such as Bz(H)⁺.^43–45

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac56

222

Aromatic and aliphatic C–H bonds in A(H)⁺ are only little acidic, so that dispersion forces (favoring π-bonds) override the induction forces (favoring H-bonds).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac57

223

The drastic change in the intermolecular potential and particularly in the geometry of the most stable structure of 1-Np–Ar induced by ionization explains why the H-bound t-1-Np⁺–Ar isomer has completely escaped detection in the REMPI-IR spectrum of this cation dimer (Fig. 5d).¹⁹

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac58

224

In this experiment, t-1-Np⁺–Ar dimers were prepared by REMPI of π-bound t-1-Np–Ar via the intermediate S₁ state.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp11

225

Consequently, the Franck–Condon factors for populating the H-bound global minimum of t-1-Np⁺–Ar are nearly vanishing.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs41

226

This example demonstrates the severe limitations of photoionization techniques for the characterization of the most stable isomers of cluster cations in cases where the cluster structure changes drastically upon ionization.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con7

227

Such significant structural changes are, however, typical for ionization of a large variety of complexes and the rule rather than the exception.^31,48,49

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac59

228

Similar to Ph(H)⁺–N₂,^32,33,41,42 An⁺–N₂,³⁹ and In⁺–N₂,³⁷ the H-bound t/c-Np⁺–N₂ isomers correspond to the global minima on their intermolecular potentials.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac60

229

The lack of absorptions of π-bound t/c-Np⁺–N₂ in the EI-IR spectra demonstrates that these are substantially less stable local minima.^32,39

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac61

230

Similarly, π-bound isomers were not observed in the EI-IR spectra of An⁺–N₂ and Ph(H)⁺–N₂.^32,39,41

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac62

231

In contrast, π-bound In⁺–N₂ could be weakly observed suggesting that π-bonding becomes competitive with H-bonding for A(H)⁺–N₂ dimers with less acidic A(H)⁺.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res69

232

In line with this trend, N₂ complexes of A(H)⁺ without acidic functional groups, such as Bz(H)⁺, prefer π-bonds with N₂ over H-bonds.^43–45

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac63

233

The photofragmentation data of 1-Np⁺–(N₂)_n yield a binding energy of the order of D₀(π) ∼ 650 cm⁻¹ for both 1-Np⁺ rotamers.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs42

234

This value is similar to the dissociation energies of other π-bound A(H)⁺–N₂ complexes, such as Ph⁺–N₂ (D₀ = 750 ± 150 cm⁻¹),³³ An⁺–N₂ (D₀ = 700 ± 200 cm⁻¹),³⁹ BzH⁺–N₂ (D₀ ∼ 800 cm⁻¹),⁴⁴ and the carbenium isomers of PhH⁺–N₂ (D₀ = 750 ± 150 cm⁻¹).⁴²

Type: Result | Advantage: None | Novelty: None | ConceptID: Res70

235

Similar to the corresponding Ar complexes, the dissociation energies of π-bound A(H)⁺–N₂ are relatively insensitive to the detailed structure of A(H)⁺.⁴⁴

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs43

236

This is in contrast to the strength of the H-bonds in A(H)⁺–N₂, which depends sensitively on the acidity of the proton donor group of A(H)⁺.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res71

237

Photofragmentation data for 1-Np⁺–(N₂)_n yield D₀(H) = 1250 ± 750 cm⁻¹ for both 1-Np⁺ rotamers.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs44

238

According to the ν₁ band shifts and the calculations, t-1-Np⁺ is somewhat more acidic than c-1-Np⁺, resulting in slightly stronger H-bonds.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs45

239

However, D₀(H) of both t/c-1-Np⁺–N₂ dimers should be significantly smaller than D₀(H) of Ph⁺–N₂, which was measured as 1640 ± 10 cm⁻¹.³⁸

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac64

240

The Δν₁ shifts of −112 and −88 cm⁻¹ correspond to the increases of the H-bond strengths in t-1-Np⁺–N₂ and c-1-Np⁺–N₂ upon ν₁ excitation.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res72

241

Presently, it is not established which type of intermolecular bond is favored by 1-Np-N₂ in S₀. ri-MP2/ccpVTZ calculations for t-1-Np–N₂ predict the π-bond to be more stable than the H-bond.⁵⁴

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac65

242

REMPI and hole-burning spectra suggest that only one isomer is produced in the supersonic expansion and that this isomer is probably π-bound.⁵⁴

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac66

243

Interestingly, Ph–N₂ prefers H-bonding,^22,26,38 whereas N₂ dimers of less acidic An and Bz have π-bound global minima in S₀.^27–29

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac67

244

Apparently, the most favorable recognition motif in neutral A–N₂ dimers is determined by a subtle balance between dispersion, induction, and electrostatic forces, which is governed by the polarisability and permanent multipole moments of A as well as the acidity of the YH_k group (if present).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac68

245

In contrast, the EI-IR spectra of t/c-1-Np⁺–N₂ as well as Ph(H)⁺–N₂,^32,33,41,42 An⁺–N₂,³⁹ and In⁺–N₂,³⁷ clearly show that H-bonding is the stronger interaction type for the cation.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac69

246

This result suggests that this trend is quite common for A(H)⁺–N₂ dimers involving acidic A(H)⁺ ions.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac70

247

Similar to A(H)⁺–Ar, A(H)⁺–N₂ without acidic groups, such as Bz(H)⁺–N₂, feature intermolecular π-bonds in the cation ground state.^43–45

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac71

248

On the other hand, when the number of aromatic π-electrons and thus the dispersion forces are reduced to a minimum, such as in cyclic C₃H₃⁺ with only two π-electrons, N₂ actually prefers H-bonding to the aromatic CH protons over π-bonding.^60,69,71

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac72

249

The optimal 1-Np⁺–L interaction is for any angular orientation significantly stronger than the L–L attraction, D_e ∼ 100 cm⁻¹ for both Ar₂ and (N₂)₂^79,80 Thus, the 1-Np⁺–L_n cluster growth is mainly driven by the 1-Np⁺–L dimer potential, because three-body forces are small for ion complexes with nonpolar ligands.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac73

250

Consequently, the solvation sequence for the most stable 1-Np⁺–L_n complexes starts with the formation of H-bound 1-Np⁺–L, which is further solvated by (n − 1) π-bound ligands.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res73

251

A similar cluster growth was deduced for Ph(H)⁺–L_n and An⁺–L_n with L = Ar and N₂, which also starts with the solvation of the acidic protons of the OH₍₂₎ and NH₂ groups by H-bound ligands, before further π-bound ligands are attached to the aromatic ring.^{33,36,39,40,42}

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac74

252

Fig. 7 plots the ν₁ frequencies of the most stable isomers for a variety of A(H)⁺−Ar_n (a) and A(H)⁺–(N₂)_n (b) complexes as a function of the cluster size n.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs46

253

The A(H)⁺ ions include monohydroxyarene radical cations (A⁺ = Ph⁺, t-1-Np⁺, c-1-Np⁺) as well as carbenium ions of PhH⁺, which all possess a single OH group.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs47

254

The plots mirror the preferred evolution of the solvation (sub)shells in these clusters, denoted H/(n − 1)π.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res74

255

The first H-bound ligand induces a large ν₁ red shift because of the destabilization of the O–H bond upon intermolecular H-bonding.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res75

256

The size of Δν₁ is correlated with the strength of the intermolecular interaction.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs48

257

Thus, |Δν₁| is larger for N₂ dimers than for Ar dimers and increases for rising OH acidity in the order c-1-Np⁺< t-1-Np⁺ < PhH⁺ < Ph⁺, which is also reflected by decreasing ν₁ frequencies of the bare monomers (n = 0).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs49

258

The H-bound dimers (n = 1) are further solvated by π-bound ligands, which induce small incremental blue shifts of ν₁.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs50

259

Thus, π-bound ligands slightly destabilize the intermolecular H-bond to the first ligand via noncooperative three-body forces, which in turn stabilize the intramolecular O–H bond.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res76

260

As expected, the ν₁ frequencies are not converged at the largest cluster size investigated (n ≤ 7), because the first solvent shell around A(H)⁺ is not complete yet.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res77

261

The ν₁ shift for t-Np⁺–(N₂)₅ of −77 cm⁻¹ amounts to 2.15%.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs51

262

Unfortunately, neither Ar nor N₂ matrix isolation studies are available for any of these aromatic A(H)⁺ ions,⁸¹ preventing comparison of the A(H)⁺–L_n cluster band shifts with the bulk limit (n → ∞).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac75

263

The complexation-induced red shift in the proton donor stretch vibration, Δν_X–H, of H-bound X–H⁺–L dimers is correlated with the difference in the proton affinities (PA) of the two bases X and L.^{18,30,33,70,82}

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac76

264

The smaller PA(X)–PA(L), the stronger the intermolecular H–L bond and the larger Δν_X–H.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac76

265

This relation may be used to estimate unknown PA values of bases X from IR spectra of their X–H⁺–L dimers.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod10

266

Recently, this procedure was applied to XH⁺ = In⁺ to obtain the first experimental determination of the PA of the indolyl radical.³⁷

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac77

267

Fig. 8 plots the relative red shifts |Δν_X–H|/ν_X–H as a function of PA(X) for a series of H-bound XH⁺–N₂ and XH⁺–Ar dimers, including XH⁺ = SiOH⁺, Ph⁺, In⁺, An⁺, and t-1-Np⁺ (Table 4).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs52

268

The shifts are larger for L = N₂ compared to L = Ar, because PA(N₂) > PA(Ar) (494 > 369 kJ mol⁻¹).⁸³

Type: Result | Advantage: None | Novelty: None | ConceptID: Res78

269

For a given L, the shifts decrease in the order XH⁺ = SiOH⁺ > Ph⁺ > t-1-Np⁺ > In⁺ > An⁺ because of increasing PA(X).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res79

270

The PA of the t-1-naphthoxy radical has not been measured so far.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac78

271

Linear interpolation from the known PA values of the anilino and phenoxy radicals yields PA ≈ 908 ± 5 kJ mol⁻¹ for t-1-naphthoxy (Fig. 8).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs53

272

As the uncertainty of this method for determining PA values is not well documented, the error is enlarged to ±30 kJ mol⁻¹, yielding 908 ± 30 kJ mol⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res80

273

The PA of c-1-naphthoxy is predicted to be larger by around 10 kJ mol⁻¹ according to the smaller Δν₁ values.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs54

274

This example demonstrates that IR spectroscopy of cluster ions can be used to selectively probe thermochemical properties, such as the PA, of specific conformers of transient radicals.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res81

275

The attraction in weakly H-bound XH⁺–L dimers arises mainly from electrostatic and inductive forces.^30,56

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac79

276

In general, the H-bond strength correlates with the positive partial charge located on the intermediate proton, q_H.⁴⁴

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac80

277

The larger q_H, the more acidic the OH group and the larger the donation of electron density into the aromatic ring.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac81

278

Because delocalization of the positive charge is more pronounced for 1-Np⁺ than for Ph⁺, as also indicated by the lower ionization potential, the donation of electron density from the OH group to the ring is smaller for 1-Np⁺.¹⁹

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac82

279

Consequently, the dissociation energies of H-bound 1-Np⁺–L are systematically lower than those of Ph⁺–L.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac83

280

This trend is not only observed for nonpolar ligands, such as Ar and N₂ (Table 1), but also for polar ligands, such as H₂O: D₀(H)/cm⁻¹ = 6520 ± 50 and 5547 ± 75 for Ph⁺–H₂O and t-1-Np⁺–H₂O, respectively.^15–17

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs55

281

On the other hand, t-1-Np appears to be slightly more acidic than Ph in S₀, as is indicated by the slightly stronger H-bond to H₂O: D₀(H)/cm⁻¹ = 2035 ± 69 and 1916 ± 50 for t-1-Np–H₂O and Ph–H₂O, respectively.^9,15

Type: Result | Advantage: None | Novelty: None | ConceptID: Res82

282

Spectroscopic and theoretical data demonstrate that neutral 1-Np is a much stronger H-bond donor than H₂O. For example, the dissociation energy of t-1-Np–H₂O (D₀ = 2035 ± 69 cm⁻¹)⁹ is approximately twice that of H₂O–H₂O (D₀ ∼ 1200 cm⁻¹),^84,85 leading to a larger red shift of the proton donor stretch frequency, −148 vs. −56 cm⁻¹.^12,86

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac84

283

The reversed situation is observed for the cationic species, for which H₂O⁺ is a much better H-bond donor than 1-Np⁺.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac85

284

For example, the dissociation energy of H-bound H₂O⁺–Ar is much larger than that of 1-Np⁺–Ar, D₀ ∼ 2100 ± 700 vs. 650 ± 150 cm⁻¹, accompanied by larger frequency shifts, Δν₁ = −541 vs. −41 cm⁻¹.^56,57

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac86

285

The delocalization of the positive charge in the aromatic π-electron system implies that the OH group in 1-Np⁺ is much less acidic than those of H₂O⁺.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac87

286

In conclusion, the acidity of neutral ROH increases along the series H₂O < Ph < 1-Np, whereas for the ROH⁺ cations the trend is reversed, H₂O⁺ > Ph⁺ > 1-Np⁺.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con8

287

The Δν₁ shifts of A(H)⁺–Ar_n and A(H)⁺–(N₂)_n as a function of n (Fig. 7) demonstrate that the acidity of the OH group strongly depends on both the solvent type and the degree of solvation.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res83

288

Asymmetric solvation of A(H)⁺ by the weak proton acceptors Ar and N₂ in the first solvation shell indicates that the acidity of the OH group is most enhanced for the H-bound dimers, because ν₁ is smallest for n = 1.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res84

289

Further solvation with π-bound ligands reduces the acidity again.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs56

290

This effect is typical for interior ion solvation by surrounding polar and nonpolar ligands, because three-body polarization forces are noncooperative.^{50,56,87–90}

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac88

291

Thus, even in the limit n → ∞, proton transfer from A(H)⁺ to the solvent is not expected to occur for these A(H)⁺–Ar_n and A(H)⁺–(N₂)_n complexes.⁵⁵

Type: Result | Advantage: None | Novelty: None | ConceptID: Res85

292

In contrast, for strong proton acceptors, such as L = H₂O and NH₃, which are able to form H-bonded solvent networks, the PA of the L_n cluster increases drastically with n.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res86

293

Thus, for such A(H)⁺–L_n complexes, the H-bond between A(H)⁺ and the L_n network becomes stronger as n increases, because three-body polarization forces are large and cooperative.^88,89

Type: Result | Advantage: None | Novelty: None | ConceptID: Res87

294

Consequently, proton transfer from A(H)⁺ to L_n can be observed for complexes larger than a critical size.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res88

295

For example, proton transfer occurs in Ph⁺–(H₂O)_n for n > 2 ⁸ and in Bz⁺–(H₂O)_n for n > .3^88,89

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac89

296

At present, it is not established how many H₂O ligands are required to induce proton transfer in t/c-Np⁺–(H₂O)_n.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac90

297

ZEKE and MATI spectra of t-1-Np⁺–H₂O show that the proton is not transferred for n = .1^16,17

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac91

298

As both 1-Np⁺ rotamers are less acidic than Ph⁺, it is expected that at least n > 2 is required for t/c-Np⁺–(H₂O)_n.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac92

Concluding remarks

299

The acidity of the t/c-1-Np⁺ rotamers and their microsolvation in nonpolar hydrophobic solvents have been investigated by DFT calculations and IR spectra of size-selected 1-Np⁺–Ar_n (n ≤ 2) and 1-Np⁺–(N₂)_n (n ≤ 5) complexes.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj10

300

Analysis of the n- and L-dependent complexation-induced frequency shifts of the O–H stretch vibration (Δν₁) and photofragmentation branching ratios of 1-Np⁺–L_n provides information about the microsolvation of both 1-Np⁺ rotamers in Ar and N₂.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot10

301

The IR spectra demonstrate that the preferred ion–ligand binding motif between 1-Np⁺ and Ar/N₂ is H-bonding to the acidic OH group, whereas π-bonding to the aromatic ring is less favorable.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con9

302

Consequently, the preferred 1-Np⁺–L_n cluster growth begins with the formation of H-bound 1-Np⁺–L dimers, which are further solvated by (n − 1) π-bound ligands.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con10

303

In general, the H/π-bonds in 1-Np⁺–(N₂)_n are stronger than those in 1-Np⁺–Ar_n, mainly because of the additional charge–quadrupole interaction in the N₂ complexes. t-1-Np⁺ is found to be slightly more acidic than c-1-Np⁺ but both 1-Np⁺ rotamers are considerably less acidic than Ph⁺.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con11

304

Increasing charge delocalization causes the acidity of ROH⁺ cations to decrease along the order H₂O⁺ > Ph⁺ > 1-Np⁺, a trend opposite to the one observed for the corresponding neutral ROH molecules.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con12

305

The Δν₁ shifts of H-bound t-1-Np⁺–L yield a first experimental estimate for the proton affinity of the t-1-naphoxy radical as ∼908 ± 30 kJ mol⁻¹, demonstrating that IR spectroscopy of cluster ions can be used to probe thermochemical properties of transient radicals.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con13

306

The most stable 1-Np⁺–Ar structure (H-bound) differs qualitatively from that of the neutral dimer (π-bound), emphasizing the large impact of ionization on the interaction potential and the preferred recognition motif between aromatic molecules and nonpolar ligands.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con14

307

The ionization-induced switch in the preferred binding type in A⁽⁺⁾–Ar from π-bonding to H-bonding has now been established for a large variety of A⁽⁺⁾ molecules with acidic functional YH_k groups (Y = O, N) and seems to be a general phenomenon.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con15

308

Significantly, the t/c-1-Np⁺–L_n complexes were generated in an EI cluster ion source, which predominantly produces the most stable structure of a given cluster ion.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con16

309

The global minimum structure found for t-1-Np⁺–Ar (H-bound) differs from the geometry observed in photoionization spectra (π-bound), demonstrating that the EI source is more generally applicable than photoionization for the spectroscopic characterization of global minima of cation complexes.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con17

Introduction

Experimental

Density functional calculations

IR spectra

1-Np+–L Dimers

Larger 1-Np+–Ln complexes

Further discussion

Concluding remarks

1-Np⁺–L Dimers

Larger 1-Np⁺–L_n complexes