The reaction of O((3)P) with C(2)H(4), of importance in combustion and atmospheric chemistry, stands out as paradigm reaction involving not only the indicated triplet state potential energy surface (PES) but also an interleaved singlet PES that is coupled to the triplet surface. This reaction poses great challenges for theory and experiment, owing to the ruggedness and high dimensionality of these potentials, as well as the long lifetimes of the collision complexes. Crossed molecular beam (CMB) scattering experiments with soft electron ionization detection are used to disentangle the dynamics of this polyatomic multichannel reaction at a collision energy E(c) of 8.4 kcal∕mol. Five different primary products have been identified and characterized, which correspond to the five exothermic competing channels leading to H + CH(2)CHO, H + CH(3)CO, CH(3) + HCO, CH(2) + H(2)CO, and H(2) + CH(2)CO. These experiments extend our previous CMB work at higher collision energy (E(c) ∼ 13 kcal∕mol) and when the results are combined with the literature branching ratios from kinetics experiments at room temperature (E(c) ∼ 1 kcal∕mol), permit to explore the variation of the branching ratios over a wide range of collision energies. In a synergistic fashion, full-dimensional, QCT surface hopping calculations of the O((3)P) + C(2)H(4) reaction using ab initio PESs for the singlet and triplet states and their coupling, are reported at collision energies corresponding to the CMB and the kinetics ones. Both theory and experiment find almost an equal contribution from the triplet and singlet surfaces to the reaction, as seen from the collision energy dependence of branching ratios of product channels and extent of intersystem crossing (ISC). Further detailed comparisons at the level of angular distributions and translational energy distributions are made between theory and experiment for the three primary radical channel products, H + CH(2)CHO, CH(3) + HCO, and CH(2) + H(2)CO. The very good agreement between theory and experiment indicates that QCT surface-hopping calculations, using reliable coupled multidimensional PESs, can yield accurate dynamical information for polyatomic multichannel reactions in which ISC plays an important role.
The Oð 3 PÞ þ C 2 H 4 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 þ CH 2 CHO, H þ CH 3 CO, H 2 þ CH 2 CO, CH 3 þ HCO, CH 2 þ CH 2 O) 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 crosssections. 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, CH 3 þ HCO and H þ CH 2 CHO. The agreement between experimental and theoretical functions is excellent, implying that theory has reached the capability of describing complex multichannel nonadiabatic reactions.oxygen atom reactions | polyatomic reaction dynamics | quasiclassical trajectory surface-hopping calculations | crossed beam reactive scattering C omparisons between experimental scattering cross-sections and theoretical predictions, by both quantum-mechanical (QM) and quasiclassical trajectory (QCT) methods on accurate ab initio potential energy surfaces (PESs) for benchmark threeatom (1-4), four-atom (5), and recently noncomplex forming fiveatom (e.g., Cl þ CH 4 ; ref. 6) reactions have greatly advanced our understanding of chemical reactivity over the last decade.Nonetheless, experimental and theoretical investigations of the dynamics of more complex polyatomic reactions, with numerous competing product channels (e.g., the title reaction) still represent a major challenge for both experiment and theory. Experimentally, a major challenge is to study all the open channels with the same degree of accuracy and under the same experimental conditions. This is a prerequisite to identify the primary reaction products and determine their relative importance (branching ratios, BRs). A "universal" detection method to interrogate all product channels on the same footing, such as mass-spectrometry (MS), is required, and this can be best pursued in crossed molecular beam (CMB) experiment...
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