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

Abstract

The extent of intersystem crossing in the O( 3 P) + C 6 H 6 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.

Crossed-Beam and Theoretical Studies of the O(3P, 1D) + Benzene Reactions: Primary Products, Branching Fractions, and Role of Intersystem Crossing

“…The reactivity that follows addition of O( 3 P) to benzene is well-described by the C 6 H 6 O potential energy surface shown in Figure 1 (see section 2.2 and ref ( 34 )), which represents a very significant improvement with respect to previous PESs. 24 , 48 In fact, as discussed in ref ( 34 ) and below, besides the multireference character of some important aspects of the triplet and singlet PESs, which required appropriate high-level quantum treatment to obtain more accurate energies, it is the detailed treatment of ISC that was missing for this system until the present study (see section 2.2 ). 34 This turned out to be crucial for reliable treatment of the reaction kinetics and dynamics.…”

“…The barrier so calculated is −0.9 kcal/mol, in good agreement with the value of −1.3 kcal/mol determined at the CCSD(T) level and 1.3 kcal/mol lower than determined in our previous work. 34 …”

“… 20 The T2/S0 H SO coupling in this system, square-averaged over the three triplet–singlet coupling elements, is about 35 cm –1 for both ISC1 and ISC2, thus exhibiting a small dependence on the MECP geometry. 34 …”

“…On the basis of the theoretical part of the present work, 34 which supersedes previous theoretical studies, 24 , 48 the energetically available reactive channels for the O( 3 P, 1 D) + benzene reactions are the following: The reported enthalpies of reaction at 0 K are those calculated in this work (the values in parentheses are experimental values from recommended Δ H 0 0 ); 52 C 5 H 6 stands for 1,3-cyclopentadiene and C 5 H 5 for the cyclopentadienyl radical. It should be noted that the H abstraction channel leading to OH + C 6 H 5 (phenyl) is endoergic by about 10 kcal/mol for O( 3 P) and exhibits a high energy barrier (of about 12 kcal/mol); 48 therefore, it is expected to contribute negligibly to the O( 3 P) reaction and to be minor also for the O( 1 D) reaction 51 (see the SI ).…”

“… Triplet (T1, T2) (red lines) and singlet (S0) (blue lines) potential energy surfaces for the O( 3 P) + C 6 H 6 reaction (kcal/mol) (also see ref ( 34 )). Intersystem crossing (ISC) structures were determined at the ωB97X-D/6-311+G(d,p) level using unrestricted (ISC1) and restricted (ISC2) wave functions.…”