Reaction dynamics of oxygen atoms with unsaturated hydrocarbons from crossed molecular beam studies: primary products, branching ratios and role of intersystem crossing

Abstract

We review the progress made in the understanding of the dynamics of the reactions of ground state oxygen atoms, O( 3 P), with unsaturated hydrocarbons (acetylene, ethylene, allene, propyne and propene) which are of great relevance, besides from a fundamental point of view, in combustion chemistry and of interest also in atmosphere-and astro-chemistry. Advances in this area have been made possible by an improved crossed molecular beams (CMBs) instrument with rotating mass spectrometric detection and time-of-flight analysis which features product detection by lowenergy electron soft-ionisation for increased sensitivity and universal detection power. This apparatus offers the capability of identifying virtually all primary reaction channels, characterising the dynamics, and determining the branching ratios (BRs) for these polyatomic multichannel nonadiabatic reactions. The reactive scattering results are rationalised with the assistance of theoretical information from other laboratories on the stationary points and product energetics of the relevant ab initio potential energy surfaces (PESs). For the simpler prototypical systems, such as O( 3 P) + ethylene, detailed comparisons with synergic state-of-the-art quasiclassical trajectory surface-hopping calculations on full dimensional ab initio coupled triplet and singlet PESs have recently been possible. For more complex systems, such as O( 3 P) + propene, comparisons with the results of synergic statistical calculations of BRs on ab initio coupled triplet and singlet PESs, have also been carried out very recently. The combined experimental/theoretical approach has allowed for a better understanding of the mechanism of these reactions. And overall has deepened considerably our understanding of chemical reactivity; in addition, these studies provide an important bridge between CMBs dynamics and thermal kinetics as well as valuable information for improving combustion and astrochemistry models.

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

“… 19 − 23 We emphasize that reliable information on product BFs as a function of temperature and then predictions of channel-specific rate constants as a function of temperature and pressure are crucially needed to improve current combustion models. 19 − 24 …”

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

“…After the characterization of the center-of-mass (CM) angular, T (θ), and translational energy, P ( E ′ T ), distributions for the various contributing channels (see the Supporting Information ), the relative yield of each primary product was estimated following the procedure outlined in previous work. 40 The experimental product BFs are reported in Table 1 , together with selected thermal data measured in conditions where the BF for the C 5 H 6 + CO channel is comparable. It should be noted that phenoxy from channel 1 is partly produced from the direct adiabatic reaction of O( 3 P) on the triplet PES (BF = 0.48 ± 0.14) and partly from the nonadiabatic reaction via ISC to the singlet PES (BF = 0.18 ± 0.06) (see Figure 5 a, Table 1 , and the Supporting Information ), with a total BF of phenoxy + H of 0.66 ± 0.20.…”

Crossed-beam chemical reaction dynamics probed with universal and state resolved ion imaging

“…ISC in O ( 3 P) reactions with unsaturated hydrocarbons has been well documented, and provides a clear contrast to what is seen here. 74 Trajectory surface hopping calculations from Bowman and coworkers clearly revealed the mechanism in that case. For O ( 3 P) + ethylene, there is a diradical addition complex bound by 25 kcal mol -1 that supports a long-lived complex.…”

“…[72][73] ISC is also observed in oxygen atom reactions with unsaturated hydrocarbons, in which case the strongly bound addition complex is long-lived, increasing the probability for the system to the singlet/triplet seam of intersection where ISC occurs. [74][75][76][77][78][79] Here, we show that, even in the absence of heavy atoms or a strongly bound adduct on the initial triplet potential surface, ISC occurs in the course of bimolecular reactions of O( 3 P) with amines. In this case, we propose that ISC is promoted by the near-degeneracy of the singlet and triplet potential surfaces in the exit channel, along with the long-range dipole-dipole interaction and high dimensionality of the system that permits a brief period of recollision, leading finally to the deep hydroxylamine well on the singlet surface.…”