Probing the Exit Channel of the OH + CH₃OH → H₂O + CH₃O Reaction by Photodetachment of CH₃O^–(H₂O)

Abstract

Transition state dynamics of bimolecular reactions can be probed by photodetachment of a precursor anion when the Franck–Condon region of the corresponding neutral potential energy surface is near a saddle point. In this study, photodetachment of anions at m/z = 49 enabled investigation of the exit channel of the OH + CH3OH → H2O + CH3O reaction using photoelectron-photofragment coincidence spectroscopy. High-level coupled-cluster calculations of the stationary points on the anion surface show that the methoxide–water cluster CH3O–(H2O) is the stable minimum on the anion surface. Photodetachment at a 3.20 eV photon energy leads to long-lived H2O­(CH3O) complexes and H2O + CH3O products consistent with both direct dissociative photodetachment and resonance mediated processes on the neutral surface. The partitioning of total kinetic energy in the system indicates that water stretch and bend excitation is induced in dissociative photodetachment and evidence for long-lived complexes consistent with vibrational Feshbach resonances is reported.

Probing Anion–Molecule Complexes of Atmospheric Relevance Using Anion Photoelectron Detachment Spectroscopy

“…Showing the versatility of this general strategy for studying reaction dynamics, Continetti and co-workers probed the methoxide−water [CH 3 O − •(H 2 O)] ion−molecule complex as a precursor to the exit channel of the CH 3 OH + OH → CH 3 O + H 2 O reaction. 137 The methoxide anion and neutral have been studied extensively using anion photodetachment methods; 138,139 The PE spectrum of CH 3 O − has short, well-resolved vibrational progressions, though affected by vibronic coupling, with an origin at 1.57 eV. 139 The e − KE distribution resulting from detachment of CH 3 O − •H 2 O, shown in Figure 9 taken from ref 137, shows a ∼1 eV solvent shift.…”

“…The methoxide anion and neutral have been studied extensively using anion photodetachment methods; , The PE spectrum of CH 3 O – has short, well-resolved vibrational progressions, though affected by vibronic coupling, with an origin at 1.57 eV . The e – KE distribution resulting from detachment of CH 3 O – ·H 2 O, shown in Figure taken from ref , shows a ∼1 eV solvent shift. However, the broadness of the electron detachment signal coupled with the narrow kinetic energy release of the neutral CH 3 O and H 2 O daughters along with supporting calculations suggested a relatively long-lived neutral complex with both bend and stretch excitation of the H 2 O “product” molecule, again providing insight into the dynamics of this reaction.…”

“…See text for additional details. Reprinted with permission from ref . Copyright 2022 American Chemical Society.…”

Methanediol CH2(OH)2 and hydroxymethyl CH2OH+: key organic intermediates on the path to complex organic molecules

“…These results can help experiments using PPCS techniques to detect the elusive CH 2 OH + and CH 2 (OH) 2 precursors in a reaction with OH • radicals or OH + cations (Benitez et al 2022).…”

“…Additionally, Inostroza et al, in 2019 predicted that CH 3 O -+ H 2 O was the most frequent outcome after impacts made by OH -. This reaction was recently studied (Benitez et al 2022) by photoelectron-photofragment coincidence spectroscopy (PPCS). The authors described the transition state structure and showed that CH 3 O -H 2 O is a stable minimum.…”