Theoretical Study of the Extent of Intersystem Crossing in the O(3P) + C6H6 Reaction with Experimental Validation.

Carlo Cavallotti,Carlo De Falco,Nadia Balucani,Luna Pratali Maffei,Piergiorgio Casavecchia,Adriana Caracciolo,Gianmarco Vanuzzo

doi:10.1021/acs.jpclett.0c02866

Abstract

The extent of intersystem crossing in the O(3P) + C6H6 reaction, a prototypical system for spin-forbidden reactions in oxygenated aromatic molecules, is theoretically evaluated for the first time. Calculations are performed using nonadiabatic transition-state theory coupled with stochastic master equation simulations and Landau–Zener theory. It is found that the dominant intersystem crossing pathways connect the T2 and S0 potential energy surfaces through at least two distinct minimum-energy crossing points. The calculated channel-specific rate constants and intersystem crossing branching fractions differ from previous literature estimates and provide valuable kinetic data for the investigation of benzene and polycyclic aromatic hydrocarbons oxidation in interstellar, atmospheric, and combustion conditions. The theoretical results are supported by crossed molecular beam experiments with electron ionization mass-spectrometric detection and time-of-flight analysis at 8.2 kcal/mol collision energy. This system is a suitable benchmark for theoretical and experimental studies of intersystem crossing in aromatic species.

Highlights

A detailed understanding of the mechanism of benzene oxidation in atmospheric chemistry, combustion, and astrochemistry is crucial in order to be able to interpret the reactivity of aromatic species, as benzene can be considered as the archetypical member of the family.[1−3] An important mechanism of benzene oxidation involves intersystem crossing (ISC) between a triplet state, accessed following the addition of atomic oxygen O(3P) to benzene (C6H6), and the ground singlet state.[1,4]
The competition between termination and branching has a huge impact on the system reactivity, as it directly impacts the concentration of radicals in the reaction environment
There are contrasting reports concerning the relative importance of reactions 1−3, with seminal crossed molecular beam (CMB) work suggesting that the major pathway leads to the formation of phenoxy and H,17 while a recent kinetic work suggests that in combustion conditions (1000− 1200 K) the dominant pathways are chain termination reactions 2 and 3.1 It should be noted that, because of the complexity of the experiments,[1] the error bars on the measured branching fractions (BFs) are rather high