Photoelectron–Photofragment Coincidence Studies on the Dissociation Dynamics of the OH–CH₄ Complex

Abstract

Photoelectron−photofragment coincidence (PPC) spectroscopy was used to characterize the energetics and dynamics of the OH + CH 4 → H 2 O + CH 3 reaction initiated by photodetachment of the OH − (CH 4 ) anion complex. PPC measurements at a photon energy of 3.20 eV yielded stable (OH−CH 4 + e − ) and dissociative (OH + CH 4 (ν 1 or ν 3 , v = 0, 1) + e − ) channels. The main channel is dissociation to OH + CH 4 + e − with a low kinetic energy release (KER), peaking at 0.04 eV. Interpretation of the experimental results was supported by quantum chemistry and quasiclassical trajectory calculations. The anion potential energy surface was constructed at the correlated coupled cluster singles, doubles, and perturbative triples level with augmented correlation consistent polarized valence triple-ζ basis set, and previously calculated neutral potential energy surfaces were used. Quasiclassical simulation of the dynamics of the OH−CH 4 complex was carried out by selecting the momenta and coordinates from the Wigner distribution for the anion, providing the starting point for 4000 trajectories on the neutral potential energy surface. In agreement with the experimental results, most of the trajectories yield slowly recoiling OH + CH 4 reactants while some are trapped in the entrance channel van der Waals well.

Dissociative Photodetachment Dynamics of the OH–(C2H4) Anion Complex

“…32,40,41 The photoelectron spectrum of OH − (C 2 H 4 ) is consistent with a hydroxyl anion core inducing structural changes in the hydrocarbon, similar to the OH − (CH 4 ) system previously reported. 36 The dominant peak in the photoelectron spectra is broadened by 0.06 eV compared to OH − , also shown in Figure 2, due to excitation of low-frequency vdW modes of the neutral complex. The shoulder ∼0.37 eV lower in eKE could be attributed to the symmetric (3149 or 3167 cm −1 ) or asymmetric (3232 or 3258 cm −1 ) stretches of the C 2 H 4 moiety in the vdW complex but cannot be resolved with the current experimental resolution.…”

“…This model assumes OH acts as a spectator and is rotationally cold based on a previous DPD study of OH−CH 4 where OH fragments were found to be produced with minimal rotational excitation. 36 Thus, the rotational energy is imparted only to the C 2 H 4 photofragment, and the following expression for rotational energy, E R , is used:…”

“…The evidence for production of a long-lived weakly bound van der Waals complex in the entrance channel for the neutral reaction is similar to the results found in a previous study of OH − (CH 4 ). 36 The DPD of OH − (C 2 H 4 ) is characterized by a low kinetic energy release (KER) of the dissociating OH + C 2 H 4 fragments. Given this low KER, the stable contribution cannot be unambiguously separated from the dissociative channel apart from comparing their respective photoelectron spectra.…”

“…The dual pulsed valve setup allows for a rational approach in forming cluster anions 35 and was used in a previous study to form OH − (CH 4 ) anions. 36 A low gas density flow of 5:45:50 N 2 O:Ar:CH 4 , respectively, passed through a coaxial pulsed discharge generating a plasma that intersected a more intense pulsed supersonic beam of ∼10% C 2 H 4 in Ar, entraining hydroxide anions and allowing subsequent interactions and clustering with C 2 H 4 . A 1 keV electron beam counterpropagating the main expansion was used to stabilize the discharge.…”

Benchmark ab initio characterization of the abstraction and substitution pathways of the OH + CH4/C2H6 reactions

“…17,18,28,30,31 The stationary points along the HA pathway were characterized by various ab initio and density functional methods reporting pre-and postreaction complexes separated by a transition state for each reaction. 19,22,23,27,28,33,35,38 In the case of the OH + CH 4 system, the CH 4 Á Á ÁOH and H 2 OÁ Á ÁCH 3 complexes were also studied by different experimental techniques using stimulated Raman, infrared and electronic excitation, 24,36,37 photoelectron-photofragment coincidence, 34 and infrared spectroscopy in helium nanodroplets. 26,32 Furthermore, detailed reactive scattering experiments were performed for the different isotope-variants of the OH + CH 4 -H 2 O + CH 3 reaction by Liu and co-workers.…”