Selected Ion Flow Tube Study of the Reactions between Gas Phase Cations and CHCl₂F, CHClF₂, and CH₂ClF

Abstract

rate, but the reagent ions SF 3 + , NO + , NO 2 + and SF 2 + do not react. Surprisingly, although all of the observed product channels are calculated to be endothermic, H 3 O + does react with CHCl 2 F. On thermochemical grounds, Xe + appears to react with these molecules only when it is in its higher-energy 2 P 1/2 spin-orbit state. In general, most of the reactions form products by dissociative charge transfer, but some of the reactions of CH 2 ClF with the lower-energy cations produce the parent cation in significant abundance. The branching ratios produced in this study and by threshold photoelectron-photoion coincidence spectroscopy (preceding paper) agree reasonably well over the energy range 11 − 22 eV. In about one fifth of the large number of reactions studied the branching ratios are in excellent agreement and appreciable energy resonance between an excited state and the ground state of the ionized neutral exists, suggesting that these reactions proceed exclusively by a long-range charge transfer mechanism.Upper limits for the enthalpy of formation at 298 K of SF 4 Cl (−637 kJ mol -1 ), SClF (−28 kJ mol -1 ) and SHF (−7 kJ mol -1 ) are determined. 3 IntroductionThe study of ion-molecule reactions is of importance in many areas of science, such as plasmas found in industrial applications and in the interstellar medium. 1,2 These fundamental processes underpin the complex reactivity that is evident in these systems. One such mechanism is that of charge transfer, which can occur over either a long-range or a short-range. The former model states that as an ion (A + ) makes an approach toward a neutral reagent (BC), the ionic charge induces a dipole interaction in the neutral. At a critical separation between the two species, the potential curves of A + −BC and A−BC + cross, thus allowing an electron to jump from the neutral to the ion. Factors that exhibit a marked preference for the occurrence of this process include energy resonance between the ground electronic state and an ionic state of the neutral and the extent of shielding in the molecular orbital from which the electron is removed. A guide to the possible energy resonances can be found in the photoelectron spectrum (PES) of the neutral species. For the molecular reagent cations, the Franck-Condon factor for neutralising A + can also be important. If the long-range process is unfavourable, then the two species move closer together. The resulting intimate interaction can perturb the relevant potential surfaces to such an extent that a crossing is stimulated, thereby leading to short-range charge transfer. Note that, in this case, although the FranckCondon factors involved are perturbed, they still need to be appreciable in the isolated molecule for this process to transpire. Short-range charge transfer can compete with chemical reactions, where bonds are broken and formed. As neither a curve crossing nor a Franck-Condon factor is required for a chemical reaction to occur, this channel can proceed efficiently. A thorough review of the aforementioned ...

Threshold photoelectron–photoion coincidence spectroscopy study of CHCl2F+, CHClF+2 and CH2ClF+: Steric influence of the chlorine, fluorine and hydrogen atoms

“…This ordering of ion fragments is in accordance with those reported by the other photon-initiated data and by our own selected ion flow tube (SIFT) experiment. 5,12 Our AE 298 value for CF + is in excellent agreement with those of the PIMS and MBEIMS research, with the EIMS data proving to be anomalous again ( Table 2). It should be noted that CCl + /CHCl + ion signal is not observed in the energy range studied.…”

“…These ions were also not observed by previous electron impact data 3,4 or by our SIFT study. 12 This observation is unsurprising, as the C-Cl bond is the weakest in CHClF 2 . Therefore, fission of this bond is the primary dissociation event, as it is energetically the most favourable process.…”

“…Breakdown diagrams, which display the probability of formation for each fragment ion as a function of photon energy, are subsequently produced for comparison with results of the reactions between each HCFC and a variety of small gas-phase cations. 12 The analysis of TPEPICO experiments performed at high time resolution yields the mean translational kinetic energy release for unimolecular fragmentation processes. The results can be compared to model predictions in order to reveal the allocation of excess energy in the ionic products and the mechanism of dissociation.…”

Threshold photoelectron photoion coincidence spectroscopy and selected ion flow tube reactions of CHF3: comparison of product branching ratios§

“…Briefly, each reagent ion of interest is produced in a high pressure electron impact ion source containing an appropriate source gas [21]. The cation is injected through a quadrupole mass filter into a flow tube holding ca.…”

“…So long as there is some overlap of vibrational wavefunction between BC and BC þ at the RE of A þ , the evidence from similar-sized molecules (e.g. CF 4 [20], CHClF 2 and CHCl 2 F [21]) is that the magnitude of the photoionization Franck-Condon factor for BC is not as important as originally thought in determining the efficiency of such a reaction. We note that if this long-range charge transfer mechanism operates, then the branching ratios for fragmentation of (BC þ ) (*) , where (*) donates the possibility of BC þ being in an excited electronic state, are expected to be independent of how this state is produced.…”