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Raman spectroscopy of cryosolutions: the van der Waals complex of dimethyl ether with fluoroform

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

Raman spectra in the 3200–350 cm^–1 range were obtained for solutions in liquid krypton, at 131 K, containing dimethyl ether, fluoroform and mixtures of them.

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

In the spectra of the mixed solutions bands were detected that must be assigned to the 1∶1 complex between the ether and fluoroform.

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

The fluoroform C–H stretching in the complex is detected at 18.7 cm^–1 above its value in monomer fluoroform, confirming the blue shifting hydrogen bonding in the complex.

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

Analysis shows that the Raman intensity of this mode is not significantly affected by complexation.

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

The advantage of the complementarity of the IR and Raman spectra of cryosolutions is demonstrated by the detailed analysis of the methyl deformation modes of the ether.

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

Introduction

Molecular cryospectroscopy using infrared (IR) spectroscopy to detect solutes in low temperature rare gas solutions has been proven to be a viable technique in the study of weak molecular complexes.^1,2

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

In contrast, Raman spectra of cryosolutions have been studied far less, and the Raman literature on the formation of weak complexes in cryosolution is virtually non-existent.

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

The reasons for this are the weakness of the Raman effect and the low solubility of most compounds in typical cryosolvents.

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

While the second drawback can be overcome in IR by using cells with longer pathlengths, the low concentrations call for the use of high sensitivity detection in Raman.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met2

State-of-the-art detectivity is offered by the use of a CCD camera, and we have used a detector of this type in this study.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met3

A complex which has recently drawn attention is the one formed between fluoroform and dimethyl ether (DME), F₃CH·O(CH₃)₂.

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

The complexation enthalpy of this complex was determined to be 12.6(3) kJ mol^–1 in liquid argon,³ which puts it in the realm of weak complexes.

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

The complex is held together by a hydrogen bond formed between the fluoroform hydrogen and the DME oxygen atoms.

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

This interaction has been characterized by both calculation and experiment³ to belong to the class of blue shifting hydrogen bonds.

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

The blue shift of the fluoroform C–H stretch, by 17.7 cm^–1 for F₃CH·O(CH₃)₂ in liquid argon, is accompanied by a decrease of the IR intensity of the fundamental stretching transition of the hydrogen bonded C–H bond.

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

This decrease, by an order of magnitude, makes the transition hard to detect in the IR spectra.³

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

In this study we have investigated the possibility of (i) recording Raman spectra of relatively dilute solutions, in a typical cryosolvent, of fluoroform and dimethyl ether, and (ii) observing the formation of a complex between them.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa1

In view of its unusual behaviour in IR, we have studied in some detail the Raman intensity of the fluoroform C–H stretch upon formation of the complex.

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

Ab initio predictions of spectral intensities for complexes held together by blue shifting hydrogen bonds hitherto have exclusively concentrated on the IR spectra.

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

In view of our interest in the Raman characteristics of blue shifting hydrogen bonds we filled this gap by carrying out MP2 calculations of the Raman intensities of monomers and complex.

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

Experimental

Dimethyl ether and fluoroform, with stated purities of 99+% and 98+%, respectively, were obtained from Sigma Aldrich.

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

The solvent gas, krypton, with stated purity of 99.998% was provided by L’Air Liquide.

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

All compounds were used without further purification.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met4

Raman spectra were obtained by using a Dilor XY spectrometer, consisting of a subtractive double f = 80 cm monochromator coupled to a single f = 80 cm spectrograph.

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

The scattered Raman signal was imaged onto a liquid nitrogen cooled charge-coupled device (P88231 EEV CCD) with a resolution of 770 × 1152.

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

The 514.5 nm line of a Spectra-Physics argon ion laser was used for Raman excitation.

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

The power of the incident laser beam was set at 1 W. The polarization of the laser beam was controlled with a polarization rotator and measurements were made with the incident beam either perpendicular (“polarized spectrum” or parallel (“depolarized spectrum”) to the direction of measurement.

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

The light scattered at 90° was focused on the entrance slit, where a polarization scrambler was inserted to eliminate the intensity distortion by apparatus polarization.

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

The spectra were measured with resolutions of 1.5 and 0.8 cm^–1.

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

The Raman cell consisted of a 70 cm long Pyrex tube with an outer diameter of 10 mm.

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

The tube is sealed at the lower end to contain the solution.

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

At the other end the tube is fused to a 1 L glass flask, which can be accessed via a Young PR10RA stopcock.

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

The tube is fitted on the outside with a PT 100 thermoresistor for temperature measurements.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp12

The cell is dimensioned to be inserted into a Leybold exchange gas cryostat, using standard vacuum fittings.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp13

In a typical run the evacuated cell is filled at room temperature with the desired gas mixture to a pressure of 2 bar.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp14

Cooling of the cell to a temperature below the normal boiling point of krypton then causes condensation of an amount of liquid that fills the cell to approximately 2 cm above the bottom of the tube, allowing convenient laser excitation of the sample.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp15

Raman measurements in this study were made in the 3200–350 cm^–1 range, with the solutions kept in a 1 K temperature interval around 131 K. Full thermodynamic characterization of the complex equilibium requires spectra of the solutions to be recorded in a sufficiently wide temperature interval.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp16

Due to the very limited liquid range of krypton at 1 bar, such a temperature study imposes the use of pressures inside the cell of up to 15 bars.

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

The present cell has not been designed to safely hold such pressures.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp17

Consequently, no variable temperature studies can be performed with it, so that the Raman spectra could not be used to determine the complexation enthalpy.

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

The previous cryosolution study of the complex³ in liquid argon focused on the fluoroform C–H stretch.

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

For comparison, we have repeated that study using liquid krypton as solvent, and using concentrations similar to those for the Raman spectra.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met5

The latter are significantly higher than those used previously,³ so that an IR cell with a pathlength of 1 mm had to be used.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met5

The IR experiments were performed as described before.³

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

Theoretical information on the geometry of the complex and on its vibrational spectra was obtained by carrying out ab initio calculations, at the MP2 = FULL/6-31+G(d,p) level.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met7

During the geometry optimization and the force field calculation, corrections for basis set superposition error were taken into account explicitly using the CP-corrected gradient techniques developed by Simon et al.⁴

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met8

All calculations were performed using Gaussian.98⁵

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met9

The Raman intensities were calculated by numerical differentiation of the dipole derivatives with respect to the electric field, using the FREQ = RAMAN and DENSITY = CURRENT keywords.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met10

Results and discussion

Using ab initio calculations at the MP2/6-311++G(d,p) level, the structures of the monomers and of the 1∶1 complex have been optimized.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met11

The resulting geometries are very similar to those obtained at the MP2/6-31++G(d,p) level³ and require no further comment.

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

The more important vibrational frequencies and corresponding IR and Raman intensities for these molecules have been collected in Table 1.

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

The intermolecular van der Waals modes of the complex are not listed, because they are not directly of interest to the present study.

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

In the discussion of the spectra below, the fundamentals of the monomers are described by their number in the Herzberg system, as in Table 1, with the monomer indicated as as superscript.

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

The symmetry species of the normal coordinate is indicated, when deemed necessary, in parentheses.

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

The symmetry species for monomer DME have been derived with the heavy atom skeleton in the (y,z)-plane.

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

The point groups for DME and fluoroform are C_2v and C_3v, respectively, but the complex has the reduced symmetry C_s.

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

To facilitate comparison, however, fundamentals of the complex are described by the symbol, including the symmetry species, of the corresponding monomer transition.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met12

The actual species for the complex can easily be derived using the correlations a₁, b₁ → a′ and a₂, b₂ → a″ for the DME moiety and a₁ → a′ and e → a′ + a″ for the fluoroform moiety.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met13

The occurence of bands in the spectra of mixed solutions that are not present in either of the single monomer solutions allows the conclusion that a complex is formed between DME and fluoroform.

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

The Raman frequencies of these observed complex bands are compared with the corresponding monomer frequencies in Table 2, together with the Raman, IR and ab initio complexation shifts.

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

Figs. 1 and 2 illustrate the new bands observed in the regions of (a₁) and (a₁), respectively.

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

In both figures the top panel gives the Raman spectra while the lower panel gives the IR spectra.

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

In order to allow direct comparisons of complex/monomer relative intensities, the spectra shown were recorded from solutions in which the concentration of the solute(s) was the same in both types of spectroscopy.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met14

The Raman spectra in Fig. 1 show the presence of a complex band in the spectrum of the mixed monomer solution at 3051.8 cm^–1, on the high frequency side of the monomer (a₁) band.

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

With the highest DME mode observed at 2990.5 cm^–1, the assignment of that complex band to a DME CH₃ stretching mode requires a complexation shift of more than 60 cm^–1.

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

In view of the weakness of the complex and of the weak perturbation of the DME methyl groups, such a shift is unrealistically high.

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

Therefore, we assign the 3051.8 cm^–1 band to (a₁) in the complex.

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

Its frequency is 18.7 cm^–1 higher than the corresponding monomer band at 3033.1 cm^–1, in very good agreement with the value of 17.7 cm^–1 determined at 92 K using IR spectroscopy in liquid argon.³

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

Thus, the 3051.8 cm^–1 Raman band clearly confirms the blue shifting hydrogen bond between DME and fluoroform.

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

The experimental conditions used in the present experiments are such that for the mixed solution the complex band is hardly visible in the IR spectrum, which exemplifies the above mentioned problem caused by the decrease of the IR intensity of the C–H stretch upon complexation.

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

From the relative intensities in the top panel then follows that the Raman intensity of this mode is affected much less.

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

A more quantitative appreciation of this will be discussed in a following paragraph.

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

Other complex bands observed in the CH stretching region are compiled in Table 2.

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

They are due to DME, but have not been assigned in more detail.

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

This is because of the problems with the assignments of the DME fundamentals in this region.⁶

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

Table 2 indicates that the highest and the lowest of the bands in this region appear at 2990.5 and 2811.7 cm^–1.in the monomer, and at 2992.6 and 2811.7 cm^–1 in the complex.

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

In the depolarized Raman spectrum these bands are considerably weaker than in the polarized spectrum, which shows that they must be assigned as νDME1(a₁) and νDME2(a₁), respectively.

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

The six complex bands found between νDME1(a₁) and νDME2(a₁) are also polarized in Raman, and, therefore, must be due to overtones or combination bands that derive their intensity from Fermi resonance with the a₁ fundamentals.

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

These resonances do not permit meaningful comparison between the complexation shifts of the outer two complex bands and the (harmonic) ab initio results.

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

For the complex band in the IR spectrum, shown in the lower panel of Fig. 2, is much sharper than the corresponding monomer mode: the measured full width at half height equals 1.0 cm^–1 for the complex band, against 5.0 cm^–1 for the monomer band.

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

In the Raman spectrum, top panel of Fig. 2, both monomer and complex bands are very narrow.

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

Because polarized Raman band widths of totally symmetric bands are predominantly determined by vibrational relaxation, this shows that the monomer IR contour is significantly broadened by rotational relaxation.

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

The ab initio results predict that for the Raman intensity in the monomer differs little from that in the complex, the ratio being 1.051.

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

This ratio is in line with expectations for a mode of a weak complex that is localized in atoms not directly involved in the hydrogen bond.

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

If the ab initio ratio is accepted to be correct, it can be used to transform the experimental band area ratio for this mode into the monomer/complex concentration ratio.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met15

The monomer/complex Raman band area ratio for the 698/694.7 cm^–1 doublet assigned to was determined from least squares band fitting to be 1.03(5).

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

Combining this value with the ab initio Raman intensity ratio leads to a monomer/complex concentration ratio for the studied solution equal to 1.08(5).

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

Also using least squares band fitting, the monomer/complex Raman band area ratio for in the same solution is calculated to be 0.99(7).

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

With the above concentration ratio this can be transformed into the Raman intensity ratio for this doublet, resulting in a value of 0.92(11), which is in reasonable agreement with the ratio derived from the ab initio Raman intensities in Table 1, equalling 0.70.

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

Hence, always in the assumption that the ab initio intensity ratio for the doublet is valid, the above result confirms what was anticipated in the previous paragraph: in contrast with the IR intensity, the Raman intensity of the C–H bond stretching of fluoroform engaged in a blue shifting hydrogen bond with DME, does not significantly differ from that of the monomer.

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

Fig. 1 then makes clear that blue shifting hydrogen bonding may escape detection in IR as a consequence of the intensity behavior, but that this is much less likely to happen when Raman spectroscopy is used.

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

The data in Table 2 show that complex bands have been observed for a number of other fundamentals.

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

It can be seen that in all cases the observed directions match the ab initio ones, and that also the quantitative agreement in general is good.

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

Over the years, dimethyl ether has been used as a model compound in numerous studies, many of which focus on the Lewis base properties of the compound.