Real-time tracking of the entangled pathways in the multichannel photodissociation of acetaldehyde.

Chung-Hsin Yang,Surjendu Bhattacharyya,Kopin Liu,Lihong Liu,Wei-Hai Fang

doi:10.1039/d0sc00063a

Chung-Hsin Yang, Surjendu Bhattacharyya + Show 3 more

Open Access

https://doi.org/10.1039/d0sc00063a

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Abstract

The roaming mechanism, an unconventional reaction path, was discovered more than a decade ago in the studies of formaldehyde photodissociation, H2CO → H2 + CO. Since then, observations of roaming have been claimed in numerous photochemical processes. A closer examination of the presented data, however, revealed that evidence for roaming is not always unequivocal, and some of the conclusions could be misleading. We report here an in-depth, joint experimental and theoretical study of the title reaction. By tracking the time-evolution of the pair-correlated product state distributions, we decipher the competing, interwoven reaction pathways that lead to the radical (CH3 + HCO) and molecular (CH4 + CO) products. Possible roaming pathways are then elucidated and a more precise descriptor of the phenomenon is delineated.

Highlights

The concept of the transition state (TS) is central to the understanding of chemical reactivity.[1]
A notable example, which sparked a surge of interest in non-TS dynamics, is the UV photodissociation of formaldehyde (H2CO).[5,6]
Theoretical analysis of the trajectory revealed that roaming arises from the incipient H/HCO radical channel where the two fragments do not have sufficient kinetic energy along the reaction coordinate to dissociate, but instead orbit each other and eventually undergo a radical recombination reaction by direct abstraction of the H atom from HCO at a long range to form H2 + CO, thereby emerging with distinct pair-correlated product distributions

Summary

Introduction

Coproduct accompanied by a low total kinetic energy release (TKER). Theoretical analysis of the trajectory revealed that roaming arises from the incipient H/HCO radical channel where the two fragments do not have sufficient kinetic energy along the reaction coordinate to dissociate, but instead orbit each other (i.e., roaming with signi cant kinetic energy tied to the centrifugal motion) and eventually undergo a radical recombination reaction by direct abstraction of the H atom from HCO at a long range to form H2 + CO, thereby emerging with distinct pair-correlated product distributions. Since roaming has been claimed in numerous unimolecular dissociation processes,[7] including the closely related acetaldehyde (CH3CHO) photodissociation.[8,9,10,11,12,13,14] In those studies, experimental evidence for roaming is almost universally, with a few exceptions,[8,14] based on the observation of a bimodal rotational state distribution (or sometimes a distribution with two Boltzmann rotational temperatures) for one of the molecular fragments. With the aid of concurrent theoretical calculations [Methods], this set of two-dimensional – time and pair-correlation – results enable us to disentangle the multiple, interwoven dissociation pathways

Two-dimensional perspective: time and pair-correlation

Uncovering the dissociation pathways

Implications and outlook

Methods

Findings

Conflicts of interest