Intersystem crossing and dynamics in O( ³ P) + C ₂ H ₄ multichannel reaction: Experiment validates theory

Abstract

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...

Combined crossed beam and theoretical studies of the C(1D) + CH4 reaction

“…50) and C 2 H 4 . [51][52][53] Some interesting indications also come from the comparison of our experimental findings with those obtained for the VUV photodissociation of ethylene, which has been widely investigated over many years (see Refs. 19 and 20, and references therein).…”

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

“…To model combustion systems, however, this kind of information is as relevant as the global rate coefficients, since the products of one elementary reaction are the reactants of a subsequent one in the complex scheme of elementary reactions that account for the global transformation which occurs in combustion environments. In the case of the title reactions, an additional complication is associated to the possible occurrence of intersystem crossing (ISC) from the triplet to the underlying singlet PES, which might open other reaction channels not accessible on the triplet PES [16][17][18][19]. Furthermore, not only they are partial, but they have been obtained mostly at room temperature, that is, far from the actual combustion conditions.…”

“…To solve this problem, soft photoionisation (PI) by tunable VUV synchrotron in CMB machines was first developed at the Advanced Light Source (ALS) of Berkeley (USA) [50][51][52][53] and, later, at the National Synchrotron Radiation Research Centre in Hsinchu (Taiwan) [54][55][56][57][58][59]. Table-top alternatives include VUV laser ionisation [41,[60][61][62][63] or, as pioneered in our laboratory, soft electron-ionisation (EI) by means of an ioniser with tuneable electron energy [16,17,[64][65][66][67][68][69][70][71][72][73]. Soft EI cannot compete with the high selectivity and resolution of the synchrotron or laser radiation.…”

A general method to describe intersystem crossing dynamics in trajectory surface hopping

“…In this paper we term these states "MCH" states (referring to the "molecular Coulomb Hamiltonian," see below for a detailed discussion), while in the literature they are also referred to as "spin-diabatic" states. [160] Surface-hopping using these states has been adopted in several cases, either assuming a constant value for the SOC matrix elements [100,101] or with geometry-dependent SOC. [107,109,161,162] Finally, the most advanced schemes to describe ISC employ a diagonalization of the electronic Hamiltonian including SOC, and basically conduct Tully's surface hopping using the obtained eigenstates.…”

“…[99] Full-dimensional dynamical studies can be performed using extensions of the trajectory surface hopping methodology. Different schemes have been employed to investigate ISC in collision reactions like O 1 C 2 H 4 , [100][101][102][103] O 1 N 2 , [104] Na 1 HCl, [105] or S 1 H 2 [106] as well as in the dynamics of O 2 on Al surfaces. [107] Trajectory surface hopping was also used to treat ISC in molecules like sulphur dioxide, [108] acrolein, [109] acetone, [110] pentanal, [111] 2-butene, [112] 6-thioguanine, [113] cytosine, [114,115] uracil, [116] as well as few transition-metal complexes.…”