Dissociation Dynamics of Isotopologs of CH₅ Studied by Charge Exchange of CH₅⁺ with Cs and Quasiclassical Trajectory Calculations

Abstract

The product branching fractions and dissociation dynamics for the dissociation of CH₅, CH₂D₃, CHD₄, and CD₅ have been experimentally measured following charge exchange of the parent cations with Cs. These results are compared with quasiclassical trajectory calculations run on an ab initio potential energy surface for CH₅. In all cases it is found that dissociation channels involving the ejection of a single atom (H or D) are dominant. The statistically weighted branching fractions show that in the mixed isotopologs, ejection of a hydrogen atom is favored over a deuterium atom. The results are consistent with the fluxional nature of CH₅(+) isotopologs--no evidence was found for quantum localization upon deuteration in the comparison of the experimental and theoretical branching fractions, possibly as a result of the vibrational temperature of the cations (estimated to be as high as 1660 K for high frequency modes). This comparison of experiment and theory provides a test of the CH₅ potential energy surface at high levels of excitation.

High-dimensional ab initio potential energy surfaces for reaction dynamics calculations

“…We have used it in studies of the high energy charge-exchange reaction H 3 O + + Cs -[H 3 O] -OH + H 2 , H 2 O + H, 160 using a similar ''Franck-Condon'' model briefly described above and in more detail elsewhere. 69,70 Finally, we note that Han and co-workers have very recently reported diabatic potentials and couplings and quantum wavepacket calculations for coplanar OH* + H 2 and D 2 . 161,162 These are impressive calculations and although a final state analysis of the H 2 O product was not done, the OH and H 2 ro-vibrational distributions were calculated.…”

“…We close this subsection on the H + CH 4 reaction by noting that the ZBB2 PES has also been used in a joint experimental QCT study of the charge exchange reaction CH 5 + + Cs -[CH 5 ] -CH 4 + H, CH 3 + H 2 69 and mixed isotoplogs. 70 In these calculations the initial zero-point phase space of the highly fluxional CH 5 + , which was obtained using an accurate PI PES, 71 was projected ''vertically'' onto the high energy portion of the CH 5 ZBB2 PES and then trajectories were initiated there. Agreement with experiment was quite good for the branching ratios to the various products channels and also the translational energy distributions.…”

Theoretical spectroscopy using molecular dynamics: theory and application to CH5+ and its isotopologues

“…Thus, in spite of its pronounced fluxionality, the motion of all five protons in CH 5 + must be strongly correlated such that the structure of CH 5 + can be pictured, but only on average, as consisting of a H 2 moiety attached to a CH 3 tripod along the lines of what ab initio PI simulations predicted ten years earlier; 92 a classical animation of the scrambling process can be found on the web. 129 The fluxional character of CH 172 For the mixed isotopologues investigated ejection of a H atom is favored over that of deuterium and the results are found to be consistent with a fluxional nature of the CH 5 + isotopologues. 172 Yet, the issue of ''quantum-induced symmetry breaking'', 88 i.e.…”

“…129 The fluxional character of CH 172 For the mixed isotopologues investigated ejection of a H atom is favored over that of deuterium and the results are found to be consistent with a fluxional nature of the CH 5 + isotopologues. 172 Yet, the issue of ''quantum-induced symmetry breaking'', 88 i.e. the preferred localization of H and D in the moiety and the tripod of mixed isotopologues, as seen in the IR spectra, 88 could not be investigated, 172 possibly as a result of a high vibrational temperature of the parent cations (of E1660 K for high frequency modes); see also ref.…”

“…125 This puzzling question has been settled based on a series of measurements of the vibrational spectrum not only of CH 5 + itself but of all its isotopologues. 88,96,[132][133][134] Much in parallel with experimental efforts the structures and energies, 4,92,110,130,131, chemical bonding and reactions, 84,93,110,111,130,135,137,149,154,160,[165][166][167][168][169][170][171][172][173][174][175] (ro-)vibrational states and transitions, 131,147,151,155,157,162,163,[176][177][178][179][180][181][182][183] as well as dynamics 129…”