Feshbach resonances in the exit channel of the F + CH3OH → HF + CH3O reaction observed using transition-state spectroscopy

Abstract

The transition state governs how chemical bonds form and cleave during a chemical reaction and its direct characterization is a long-standing challenge in physical chemistry. Transition state spectroscopy experiments based on negative-ion photodetachment provide a direct probe of the vibrational structure and metastable resonances that are characteristic of the reactive surface. Dynamical resonances are extremely sensitive to the topography of the reactive surface and provide an exceptional point of comparison with theory. Here we study the seven-atom F + CHOH → HF + CHO reaction using slow photoelectron velocity-map imaging spectroscopy of cryocooled CHOHF anions. These measurements reveal spectral features associated with a manifold of vibrational Feshbach resonances and bound states supported by the post-transition state potential well. Quantum dynamical calculations yield excellent agreement with the experimental results, allow the assignment of spectral structure and demonstrate that the key dynamics of complex bimolecular reactions can be captured with a relatively simple theoretical framework.

Stereodynamical control of product branching in multi-channel barrierless hydrogen abstraction of CH3OH by F

“…More recently, we reported a global PES of the title reaction with all 15 internal degrees of freedom (DOFs) by tting 121 000 points calculated at the explicitly correlated (F12a) version of CCSD(T) with the AVDZ basis set and core electrons frozen. 43 This level of electronic structure theory was found to yield results comparable to the benchmark ab initio results obtained by Schaefer and co-workers. 19 The chemically accurate tting was performed with the permutation invariant polynomialneural network (PIP-NN) method, 44,45 which has been successfully applied to several reactive systems for high delity tting of their PESs.…”

“…The QCT method used to investigate the dynamics is well established and the details of the calculations can be found in the ESI. † Briey, trajectories are calculated at collision energies of 1.0, 2.0, 3.0, 5.0, 10.0, 15.0, 20.0, 25.0, and 30.0 kcal mol À1 on the PIP-NN PES 43 for the title reaction interfaced to the VENUS chemical dynamics program. 49…”

“…37,38 The reaction channel (R1) has also been probed by the photodetachment of the F À (HOCH 3 ) anion, whose geometry resembles TS1 for this neutral reaction. 40,42,43 The photoelectron spectrum provides a direct probe of the vibrational structure and metastable resonances that are characteristic of the PES of the neutral reaction. In particular, the experiment revealed spectral features associated with manifold vibrational Feshbach resonances and bound states in the exit channel well.…”

“…In particular, the experiment revealed spectral features associated with manifold vibrational Feshbach resonances and bound states in the exit channel well. 43 In sharp contrast to the numerous experimental investigations on the title reaction, relevant theoretical research is scarce, especially on the reaction dynamics. Based on the information of the stationary points computed with Mfller-Plesset perturbation theory 17,18 and at the G2 level of theory, 18 the kinetics of the two reaction channels were investigated in an attempt to explain the observed anomalously large k R1 /(k R1 + k R2 ) branching ratio.…”

“…In this work, we report an extensive theoretical investigation on the reaction dynamics of this multi-channel system, focusing on the reaction mechanism, as well as stereodynamics and its impact on the microcanonical branching ratio. These calculations were carried out on the globally accurate PES, which has been used successfully to simulate the photoelectron spectrum 43 and photoelectron-photoion coincidence spectrum of the F À (HOCH 3 ) anion 48 and to reproduce the experimental canonical rate coefficients and branching ratios well. 47 Here, integral and differential cross sections are computed for both product channels using QCT, which shed light on the mechanism of this multi-channel reaction.…”