Coincidence ion yields of CF3–CH3+A TPEPICO spectrum in the scanning-energy mode was recorded for CF3–CH3 from 12–22 eV at a wavelength resolution of 0.3 nm and an ion TOF resolution of 64 ns. The parent ion and the fragments CF3+, CH3+, CF2–CH3+ and CF–CH2+ were detected as the strongest five ions, and their yields are shown in Fig. 5. The parent ion appears weakly at the lowest energy, then with increasing energy the three major fragment ions CF3+, CF2–CH3+ and CH3+ are observed. These four ions are the main fragments within the energy range 12–17 eV. At higher energy, an ion of mass 45 u, almost certainly CF–CH2+, appears gradually and becomes the dominant fragment in the range 18–21 eV. Very weak minor fragment ions are also observed with masses of 33 u (CH2F+) and 64 u (CF2–CH2+), but their yields are not shown in Fig. 5. We comment that as with CHF2-CF3+,[10] but unlike both isomers of C2H2F4+,[11] we did not observe any signal due to CF3–CH2+, corresponding to C–H bond fission. Note that by using a TOF resolution of only 64 ns, a definitive determination of the number of hydrogen atoms in a fragment ion can be problematic, but we are confident of these assignments.
Within the energy range of the ground ionic state, the cation CF3–CH3+ is observed with an appearance energy of 12.98 ± 0.04 eV. This signal is relatively weak and appears over a narrow energy range, suggesting that the X̃ state of CF3–CH3+ is bound only for a small range of low vibrational levels in the Franck–Condon envelope. The slow rise of the ion yield in the threshold region is due to the small Franck–Condon factor at threshold. Electron impact studies have observed an ionisation threshold of 13.26 eV.[23] Due to the electron energy resolution being significantly inferior, the results from our photon-impact experiment should be more accurate.
The CF3+ fragment ion has an AE298 of 13.25 ± 0.05 eV. This fragment is the most intense and dominates until 15.5 eV. Using the procedure of Traeger and McLoughlin,[30] this value of AE298 converts into an upper limit of 13.41 ± 0.05 eV for ΔrH°298 for the reaction CF3–CH3 → CF3+ + CH3 + e− (Table 1). C–C bond cleavage can also produce CH3+ as the fragment ion, where we measure AE298 to be 14.25 ± 0.05 eV. As above, this value can be used to determine an upper limit of ΔrH°298 for the reaction CF3–CH3 → CH3+ + CF3 + e− to be 14.41 ± 0.05 eV (Table 1). G2 calculations predict ΔrH°298 for these two reactions to be very similar, 13.52 and 14.28 eV, respectively. Our experimental values can be used to determine an average ΔfH°298 for the parent neutral molecule. Using literature values of ΔfH°298 for CH3,[28] CH3+,[35] CF3[36] and CF3+,[37] a lower limit of −751 ± 10 kJ mol−1 is determined for ΔfH°298(CF3–CH3). This value is in excellent agreement with −771 kJ mol−1 from our G2 calculation, and independent theoretical values of −746 ± 2 and −755 from Chen et al[21]. and Zachariah et al.[18] By assuming that there are no exit channel barriers or kinetic shifts in either reaction we equate our lower limit value with the absolute value. Henceforth, therefore, we use ΔfH°298(CF3–CH3) = −751 ± 10 kJ mol−1.
The CF2–CH3+ fragment ion signal appears with a threshold of 14.10 eV ± 0.05 eV, corresponding to an upper limit of ΔrH°298 for the reaction CF3–CH3 → CF2–CH3+ + F + e− of 14.26 eV. The signal increases slowly, then rises rapidly from approximately 15.5 eV. From 14.1–17.8 eV, the peaks in the ion yield of CF2–CH3+ match the bands in the TPES. An interesting point is that the emergence of the second threshold at 15.5 eV corresponds to the onset of the B̃-state band in the TPES. The ab initio calculation shows that this band is associated with the electronic state caused largely by electron loss from the (HOMO − 2) F 2pπ nonbonding orbital. The (HOMO − 3) orbital also has F 2pπ nonbonding character. The similarity of the ion signal with the shape of the B̃ and C̃ bands in the TPES suggest that CF2–CH3+ is produced directly via C–F bond cleavage by an impulsive mechanism from these electronics states of the parent cation without prior internal energy conversion to the ground state. From the upper limit of ΔrH°298, we determine ΔfH°298(CF2–CH3+) ≤ 546 ± 11 kJ mol−1. We were not able to measure the kinetic energy release in the dissociation CF3–CH3+ → CF2–CH3+ + F (Section 6), but by analogy with the 1,1,2 isomer we can asssume that it may be considerable. It is likely, therefore, that the true enthalpy of formation of this ion is significantly lower than this value. A G2 calculation, for instance, predicts ΔfH°298(CF2–CH3+) to be 443 kJ mol−1, and Lias et al[29]. give an indirect value of 458 kJ mol−1. These data are therefore self-consistent, and suggest that the AE298(CF2–CH3+) lies well above the thermochemical threshold energy of CF2–CH3+ + F + e−. The daughter ion is therefore likely to be formed with the release of significant kinetic energy. We comment that, in principle, it should be possible to determine experimentally the absolute value of ΔfH°298(CF2–CH3+) by measuring the kinetic energy release into CF2–CH3+ + F as a function of photon energy, and extrapolating the linear graph to determine the photon energy at which the kinetic energy release would be zero.[38] This procedure would then yield the dissociative ionisation energy, i.e. ΔrH°0 for the reaction CF3–CH3 → CF2–CH3+ + F + e−. Unfortunately, whilst this experiment has been successfully employed for the ground electronic state of polyatomic cations which are repulsive in the Franck–Condon region (e.g. CF4+ and SF6+),[38] we have not been able to yield equivalent data for repulsive, excited electronic states.[10,11] Possible reasons for the failure of such experiments are described elsewhere.[10,11] We can, therefore, only confirm an experimental upper limit for ΔfH°298(CF2–CH3+) of 546 ± 11 kJ mol−1.
Above 17 eV, minor ions are observed. The strongest is CF–CH2+ which appears with a threshold of 17.1 ± 0.1 eV. Energetically, this minor ion can only form with HF + F as neutrals (Table 1). As the direct three-body dissociation CF3–CH3 → CF–CH2+ + HF + F + e− seems unlikely, we propose a two-step mechanism to form CF–CH2+. The first step involves the loss of a F atom to produce CF2–CH3+, the second step (CF2–CH3+ → CF–CH2+ + HF) proceeds via a tight transition state and HF elimination. The ion yield curves of CF–CH2+ and CF2–CH3+ support this suggestion, since the increase of the CF–CH2+ signal corresponds exactly to the decrease of the CF2–CH3+ signal. The second step will almost certainly involve a barrier in the exit channel, and could explain why the AE298 of CF–CH2+ bears no relation to the energy of the dissociation channel CF–CH2+ + HF + F + e−; the former lies ca. 1.5 eV higher in energy. Such a three-body dissociation through a sequential two-step mechanism has already been suggested to explain the products of dissociative photoionisation of CFCl2–CH3[39] and CHF2–CH3.[40] Above 18 eV, CF–CH2+ becomes the dominant ion fragment. Very weak signals due to the minor ions CF2–CH2+ (mass 64 u) and CH2F+ (mass 33 u) are also observed above 16.5 eV. In the latter case, the most likely accompanying neutral fragment is CHF2, so these products can only form via both H- and F-migration across the C–C bond.
Coincidence ion yields of CHF2–CH2F+The TPEPICO spectrum in the energy range 11.8–24.0 eV was measured with an optical resolution of 0.3 nm. The ion yields are shown in Fig. 6. The parent ion appears at lowest energy from the ground ionic state. As the photon energy increases, a C–C bond fragmentation reaction takes place, followed at higher energy by cleavage of a C–F bond. This can be seen in the ion yields for CHF2+, CH2F+, and CHF–CH2F+ or CHF2–CH2+. As with the 1,1,1 isomer, C–H bond cleavage is not observed. These four major ions are the dominant fragments until 16 eV, when a new reaction channel involving two or more bond cleavages opens, possibly with intra-molecular proton transfer. The fragment CF–CH2+ gradually becomes the dominant ion in the higher photon energy region, and we note that an ion of mass 45 u was also dominant with hν > 18 eV for 1,1,1 trifluoroethane (Section 5.2.1). As for CF3–CH3, we have used the procedure of Traeger and McLoughlin[30] to convert the AE298 of the major fragment ions, determined from an extrapolation of the linear portion of the ion yield to the baseline, into an upper limit for the enthalpy of the unimolecular reaction at 298 K, ΔrH°298, in order to determine unknown values of enthalpies of formation at this temperature. For the minor ions, we only compare AE298(CF–CH2+) with ΔrH°298 for the possible dissociation reactions to infer what the accompanying neutrals may be.
From the onset of ionisation, 11.88 eV, up to ca. 12.5 eV, the parent ion forms exclusively, implying that low vibrational levels of the ground state of the parent cation are bound and lie below the first dissociation threshold. The parent ion intensity decreases sharply when the fragmentation channel to produce CHF2+ becomes energetically allowed. CHF2+ has an AE298 of 12.50 ± 0.04 eV, and is the predominant ion from ca. 12.8 to 16.0 eV. The AE298 can be converted into an upper limit of the enthalpy change for the reaction CHF2–CH2F → CHF2+ + CH2F + e− at 298 K of 12.65 ± 0.04 eV. This value is in excellent agreement with the value of ΔrH°298, 12.76 eV, derived by us from G2 calculations for the enthalpy of formation of reactants and products of this reaction. The other possible ionic product from cleavage of the C–C bond, CH2F+, has an AE298 of 13.19 ± 0.04 eV, corresponding to ΔrH°298 ≤ 13.34 ± 0.04 eV. This latter value is only in reasonable agreement with our G2 calculation for the enthalpy change for the reaction CHF2–CH2F → CHF2 + CH2F + + e− of 13.08 eV. Both CHF2+ and CH2F+ form by cleavage of the C–C bond, and are the expected products for dissociation by removing a σ electron from the HOMO of CHF2–CH2F. As with the 1,1,1 isomer, the good agreement of both energies with theory implies no exit channel barriers or kinetic shifts in either fragmentation channel. Combining these experimental values of ΔrH°298 with literature values for the enthalpies of formation of CHF2 (−237 kJ mol−1), CH2F (−33 kJ mol−1), CH2F+ (833 kJ mol−1)[29] and CHF2+ (604 kJ mol−1),[10] a refined, average enthalpy of formation at 298 K for CHF2–CH2F of −671 ± 12 kJ mol−1 is deduced. This value is in excellent agreement with our G2 calculation, −680 kJ mol−1, and other literature values in the range −656 to −665 kJ mol−1.[16,41]
At a photon energy of 14.51 ± 0.05 eV, corresponding to ΔrH°298 ≤ 14.65 ± 0.05 eV,[30] the signal from an ion of mass 65 u increases rapidly. This signal, corresponding to F-atom loss from the parent ion, approximately matches the drop in the ion signal of CHF2+. It is not possible to differentiate the two isomers CHF–CH2F+ or CHF2–CH2+ in the TOF-MS. G2 calculations predict ΔrH°298 to be 13.72 eV for the reaction CHF2–CH2F → CHF–CH2F+ + F + e− and 12.42 eV for the reaction CHF2–CH2F → CHF2–CH2+ + F + e−. Both channels are therefore open at the AE298 threshold of 14.51 eV. Formation of the other isomer with mass 65 u, CF2–CH3+, involves both fission of a C–F bond and H-atom migration, and seems unlikely. The energy of 14.51 eV is close to the onset of the (HOMO − 3) C̃ excited state of CHF2–CH2F+ centred at 15.97 eV, and the yield of this fragment ion follows closely the threshold photoelectron signal of the C̃ state. Molecular orbital calculations predict that the C̃ state of CHF2–CH2F+ is produced by electron removal from a F 2pπ non-bonding orbital localised predominantly on the CHF2 group (Section 4.2). It seems likely, therefore, at least near threshold, that CHF–CH2F+ is the dominant component and arises from the dissociation CHF2–CH2F → CHF–CH2F+ + F + e−. Careful analysis of the ion yield shows a two-step increase, with a second threshold at ca. 15.2 eV. It is possible that this second threshold is due either to rapid dissociative ionisation from a different electronic state of the parent ion or to formation of a different isomer of C2H3F2+; we note that ionisation from the (HOMO − 4) orbital followed by impulsive F-atom loss may lead to significant production of the isomer CHF2–CH2+. From ΔrH°298 ≤ 14.65 ± 0.05 eV, we determine ΔfH°298 (CHF–CH2F+) ≤ 663 ± 13 kJ mol−1. However, since the dissociation CHF2–CH2F+ → CHF–CH2F++F has a considerable kinetic energy release (Fig. 7 and Section 5.3), it is likely that the absolute enthalpy of formation of this ion is significantly lower than this value. A G2 calculation, for example, predicts ΔfH°298(CHF–CH2F+) to be 565 kJ mol−1, and Lias et al[29]. quote 543 kJ mol−1 determined from the proton affinity of CHFCHF.
As observed in the dissociative photoionisation of other fluorine-substituted ethanes,[10,11,34] a rapid impulsive mechanism involving cleavage of the C–F bond often occurs when the molecular orbital from which the electron has been removed has mainly F 2pπ lone pair character. If this mechanism is occurring, the fragment ion + F atom will have considerable translational kinetic energy, and can lead to a large difference between the observed dissociative ionisation threshold and the calculated energy of reaction. From the calculated electron densities of the molecular orbitals, and the fact that a large fraction of the available energy is deposited into translation kinetic energy of fragments in the CHF–CH2F+ or CHF2–CH2+ + F decay channel (Table 4), it seems likely that CHF–CH2F+ or CHF2–CH2+ is produced directly and impulsively from the C̃ and/or D̃ excited electronic states of CHF2–CH2F+ without prior internal conversion to the ground state. These states of the parent ion are then showing the characteristics of isolated-state behaviour, a phenomenon which is expected in small cations but is unexpected in polyatomic cations with as many as eight atoms.
At higher photon energies, the ion CF–CH2+ (possibly with a very small component of ions with mass 44 and 46 u) gradually increases, and for photon energies above ca. 17 eV this ion becomes dominant. This ion was also the dominant, minor ion for dissociative photoionisation of CF3–CH3 with hν > 18 eV. The observed AE298 of the ion, 16.21 ± 0.05 eV, is significantly higher than the only possible thermochemical reaction energy, 14.76 eV, for the three-body fragmentation CHF2–CH2F → CF–CH2+ + F + HF + e−; fragmentation to CF–CH2+ + F2 + H + e− at 19.07 eV, or even to CF–CH2+ + 2F + H + e− at 20.71 eV, are both forbidden energetically As with the 1,1,1 isomer, the increase of the CF–CH2+ signal roughly matches the decrease of the C2H3F2+ signal. This may imply that CF–CH2+ is formed via a two-step mechanism. First, a fluorine atom is lost from the C̃ or D̃ excited states of the parent ion through an impulsive mechanism as described above to form an isomer of C2H3F2+, then formation of CF–CH2+ occurs from C2H3F2+via a tight transition state, an exit channel barrier, and HF elimination. We note that the difference between the AE298(CF–CH2+) and the energy of the dissociation channel CF–CH2+ + HF + F + e− is ca. 1.5 eV, the same value as with HF + F elimination from the 1,1,1 isomer (Section 5.2.1).