Intersystem crossing and dynamics in O(3P) + C2H4 multichannel reaction: Experiment validates theory

• Authors: Bina Fu, Emory University; Yong-Chang Han, Emory University; Joel M Bowman, Emory University; Luca Angelucci, Dipartimento di Chimica—Università degli Studi di Perugia Via Elce di Sotto; Nadia Balucani, Dipartimento di Chimica—Università degli Studi di Perugia Via Elce di Sotto; Francesca Leonori, Dipartimento di Chimica—Università degli Studi di Perugia Via Elce di Sotto; Piergiorgio Casavecchia, Dipartimento di Chimica—Università degli Studi di Perugia Via Elce di Sotto
• Journal Title: Proceedings of the National Academy of Sciences
• Volume: Volume 109, Number 25
• Publisher: National Academy of Sciences | 2012-06-19, Pages 9733-9738
• Type of Work: Article | Post-print: After Peer Review
• Publisher DOI: http://doi.org/10.1073/pnas.1202672109
• Subjects: Chemistry, General; Physics, Molecular
• Keywords: oxygen atom reactions; polyatomic reaction dynamics; quasiclassical trajectory surface-hopping calculations; crossed beam reactive scattering

Abstract

The O(3P) + C2H4 reaction, of importance in combustion and atmospheric chemistry, stands out as a paradigm reaction involving triplet- and singlet-state potential energy surfaces (PESs) interconnected by intersystem crossing (ISC). This reaction poses challenges for theory and experiments owing to the ruggedness and high dimensionality of these potentials, as well as the long lifetimes of the collision complexes. Primary products from five competing channels (H + CH2CHO, H + CH3CO, H2 + CH2CO, CH3 + HCO, CH2 + CH2O) and branching ratios (BRs) are determined in crossed molecular beam experiments with soft electron-ionization mass-spectrometric detection at a collision energy of 8.4 kcal/mol. As some of the observed products can only be formed via ISC from triplet to singlet PESs, from the product BRs the extent of ISC is inferred. A new full-dimensional PES for the triplet state as well as spin-orbit coupling to the singlet PES are reported, and roughly half a million surface hopping trajectories are run on the coupled singlet-triplet PESs to compare with the experimental BRs and differential cross-sections. Both theory and experiment find almost equal contributions from the two PESs to the reaction, posing the question of how important is it to consider the ISC as one of the nonadiabatic effects for this and similar systems involved in combustion chemistry. Detailed comparisons at the level of angular and translational energy distributions between theory and experiment are presented for the two primary channel products, CH3 + HCO and H + CH2CHO. The agreement between experimental and theoretical functions is excellent, implying that theory has reached the capability of describing complex multichannel nonadiabatic reactions.