Quantum aspects of roaming dynamics
Authors:   , Foley Casey D. 
Published: April 2021

Decades of experimental and theoretical achievements built on the foundations of thermochemistry and quantum mechanics have birthed the field of molecular dynamics where intimate details of individual reactive events can be mapped. On the ground state of formaldehyde, two dissociation pathways are commonly known, one proceeding over a high barrier through a three-center, tight transition state to give molecular products, and another via homolytic simple bond fission to radical products. A third, distinct ground state dissociation pathway first seen in formaldehyde 15 years ago, the 'roaming' mechanism, involves incipient radicals, but the H atom samples a large, flat region of the PES, roaming in the van der Waals region, leading to an intramolecular H + HCO reaction. Roaming reactions have been visualized from a classical perspective with only a few exceptions in the literature despite the likely quantum nature of the process. A major objective of the presented work is to reveal quantum aspects of roaming reactions. Here, a few different molecules (C3H3Cl, HDCO, and H2CO) were investigated with the goal of characterizing roaming radicals on the ground state, where roaming observations have primarily occurred. Knowledge of the excited states of propargyl chloride was necessary to understand the experimental observations from investigation of its ground state. Ultraviolet (UV) photodissociation and state-specific detection with velocity map imaging of Cl, Cl*, and C3H3 were interpreted with the aid of multireference calculations to characterize the nature of the electronic excitations. A series of triplet states were identified to preferentially dissociate to Cl or Cl*. Infrared multiphoton excitation and infrared multiphoton dissociation (IRMPD) of propargyl chloride enhanced UV processes and allowed for radical dissociation at threshold. IRMPD on the ground state produced HCl following isomerization to 1-chloroallene, where a roaming-like transition state with Cl in an abstraction geometry is adopted. Accurate H and D atom ground state radical thresholds of singly deuterated formaldehyde, HDCO, were obtained from velocity map imaging to aid future studies HDCO dynamics studies. The different radical thresholds, arising from the difference in zero-point energies, is a purely quantum phenomenon. PHOFEX spectra over a wide range were collected, creating a library of known frequencies of rotational lines that can be used to study the dynamics of different vibrational bands and the energy dependence of different processes. Heats of formation of HDCO and DCO were also determined. Finally, a detailed examination of the photochemical dynamics in both ortho and para nuclear spin isomers of formaldehyde is provided, exploring the full range of parent rotational levels from J = 0 to 4, initially prepared via excitation of specific rotational lines of a range of vibrational bands on the first singlet excited state (2141, 2161, 2143, 2241, 2243). Measurements of the entire CO (v = 0) product rotational distributions are combined with velocity map imaging of selected CO product states to obtain a complete picture of the photochemical dynamics for each specific parent rotational level. Distributions are found to vary dramatically with small changes in total energy, effects not captured at all by classical treatments. Full quantum state correlations reveal interactions among three formaldehyde dissociation continua, yielding rich insight into the dynamics of this highly excited molecule as it decays to products. An orbiting resonance dominates the dynamics, though other possible dynamical phenomena are noted.