Reactive scattering dynamics in atom+polyatomic systems: F+C2H6→HF(v,J)+C2H5

Abstract

State-to-state scattering dynamics of F+C2H6-->HF(v,J)+C2H5 have been investigated at Ecom=3.2(6) kcalmol under single-collision conditions, via detection of nascent rovibrationally resolved HF(v,J) product states with high-resolution infrared laser absorption methods. State-resolved Doppler absorption profiles are recorded for multiple HF(v,J) transitions originating in the v=0,1,2,3 manifold, analyzed to yield absolute column-integrated densities via known HF transition moments, and converted into nascent probabilities via density-to-flux analysis. The spectral resolution of the probe laser also permits Doppler study of translational energy release into quantum-state-resolved HF fragments, which reveals a remarkable linear correlation between (i) HF(v,J) translational recoil and (ii) the remaining energy available, Eavail=Etot-E(HF(v,J)). The dynamics are interpreted in the context of a simple impulsive model based on conservation of linearangular momentum that yields predictions in good agreement with experiment. Deviations from the model indicate only minor excitation of ethyl vibrations, in contrast with a picture of extensive intramolecular vibrational energy flow but consistent with Franck-Condon excitation of the methylene CH2 bending mode. The results suggest a relatively simple dynamical picture for exothermic atom+polyatomic scattering, i.e., that of early barrier dynamics in atom+diatom systems but modified by impulsive recoil coupling at the transition state between translationalrotational degrees of freedom.

Rotationally resolved reactive scattering: Imaging detailed Cl+C2H6 reaction dynamics

“…4 and 8, and Table I͒. The latter was calculated to be 5.2 kcal/ mol for butyl radical, 35 while for the relaxation of the ethyl radical from the F-H-C 2 H 5 transition state geometry, 4.2 kcal/ mol was obtained in calculations by Whitney et al 36 In a sense, even though the forward scattered ethyl radical has substantial internal excitation, it may yet be a spectator in the reaction. 4 and 8.…”

Detailed benchmark ab initio mapping of the potential energy surfaces of the X + C2H6 [X = F, Cl, Br, I] reactions

“…Besides the main H-abstraction reaction path and the higher-energy H-substitution studied also for halogen + methane reactions, in the X + C 2 H 6 [F, Cl, Br, I] reactive system methyl-substitution appears as a new mechanism and is expected to occur at lower energies then H-substitution. For these reactions so far only the H-abstraction channel (X = F, Cl) [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] or its reverse reaction (X = Br, I) [34][35][36][37][38][39][40][41] have been investigated.…”

“…Later state-to-state scattering experiments also proposed an early-barrier TS from the mostly vibrationally excited HF products, however, a significant amount of vibrationally non-excited HF molecules implied energy transfer from HF to the ethyl unit during the H-abstraction reaction. 15 Direct dynamics simulations were performed by Troya and coworkers using a reparametrized semiempirical Hamiltonian, based on MP2/aug-cc-pVDZ geometry optimization and CCSD(T)/ aug-cc-pVDZ energy computation on the stationary points, for the reactions of fluorine with small alkanes including ethane, where the effect of the length of the alkane chain on the dynamics was studied. 16 Espinosa-Garcia et al have very recently reported the first analytical valence-bond -force-field-based PES of the F + C 2 H 6 -HF + C 2 H 5 reaction based on CCSD(T)/triple-zeta ab initio data and depending on 63 adjustable parameters.…”

Dynamics of Bimolecular Reactions in Solution

“…Reactions of F atoms with hydrocarbons at the gas-liquid interface, shown in the bottom row of Table 1, also result in lower product vibrational excitation than for the corresponding gas-phase reactions (97,109). However, the dynamical origins are different from those for reactions in bulk liquids.…”