Theoretical studies of intersystem crossing effects in the O+H2 reaction

Abstract

We present a general procedure for studying intersystem crossing effects in bimolecular chemical reactions, along with an application of this to the O+H2 reaction. In this procedure, we use previously derived singlet and triplet potential energy surfaces that were based on high quality multireference configuration interaction (MRCI) nonrelativistic electronic structure calculations, and the coupling surface is obtained from lower level complete active space self-consistent field (CASSCF) calculations using the effective nuclear charge one-electron Breit–Pauli expression for the spin-orbit interaction. We find that the resulting spin-orbit splittings match the known values for O(3P), O(1D), and OH(2Π) sufficiently accurately to be useful for dynamics calculations. Also, the electronic basis can be truncated to seven states (1 3A′, 1 3A″, and 1 1A′) without seriously distorting these asymptotic splittings. We show that the seven states may be exactly decoupled into a set of four, which contains the singlet, and a set of three states from the triplets. We find that the spin-orbit matrix elements vary smoothly with geometry, so that a relatively simple function can be used to interpolate matrix elements for all geometries. The cross sections for reaction are calculated using a trajectory surface-hopping (TSH) approach in conjunction with a “diabatic” representation based on the nonrelativistic potentials and the CASSCF spin-orbit coupling matrix. An application of this approach is presented to the O+H2 reaction, using the 1 1A′ state of Dobbyn and Knowles, and 1 3A′ and 1 3A″ states of Walch and Kuppermann [slightly modified so that they are asymptotically degenerate in the product (H+OH) region]. The states show a singlet–triplet (S–T) crossing that is generally on the product side of the barrier on the triplet surfaces. The TSH results indicate that only a few percent of the trajectories undergo intersystem crossing (either from singlet to triplet, or vise versa) at the S–T crossing, so the effect of these transitions on measurable properties of the reaction dynamics is small. However, those trajectories that undergo triplet to singlet transition have much higher product rotational excitation than those that react on the triplet alone. We find that a much larger fraction of trajectories (20%–40%) undergo hopping between the two triplet states, and this leads to an averaging of the dynamical results for the two states.

A Diabatic Representation Including Both Valence Nonadiabatic Interactions and Spin−Orbit Effects for Reaction Dynamics

“…[24][25][26] Besides the fine-structure splitting, the second important effect of SOC is that it causes spin-forbidden processes to become partially allowed through interaction and mixing of states of different spin multiplicity. 8,[27][28][29][30][31][32][33][34] The most common occurrence is the interaction between singlet and triplet states, as in the bimolecular O( 3 P, 1 D) + H 2 → OH( 2 Π) + H reaction, 37,38 and in photodissociation of systems such as HCl, 39,40 HBr, [41][42][43][44][45] CH 3 I, [46][47][48][49][50] ICN, [51][52][53][54] BrCH 2 Cl, [55][56][57][58] or BrCH 2 COCl. [59][60][61][62][63][64] In organic photochemical reactions, the spin-orbit interaction between a triplet state and states of singlet multiplicity promotes decay of the triplet state by phosphorescence and/or intersystem crossing.…”

Coriolis coupling and nonadiabaticity in chemical reaction dynamics

“…Further, Goldfield and coworkers 23 74 The issue was previously studied by Schatz and coworkers using trajectory surface hopping (TSH) method within a ''mixed'' representation. 75 78 ) and the spin-orbit coupling matrix of Maiti and Schatz,76 are used in the nonadiabatic timedependent wave packet scattering calculation carried out by Chu et al 74 In Table 1, the branch ratio of the spin-orbit-induced nonadiabatic transitions, determined from the calculated quan- P 0 , 1 D 2 )þH 2 is presented. 74 At the fine structure level, the largest spin-orbit-induced nonadiabatic transitions occur between the 3 A 0 and the 3 A@ states, in particularly for the case of O( 3 P 0 )þH 2 such nonadiabatic transitions account for more than 50% of the overall reactivity.…”

Quasi-classical trajectory calculation of the isotopic effect on integral cross sections and alignments of O(3P)+HD→OH+D and OD+H at Erel=0.2–1.0eV

“…OH + H on these related energy surfaces 1 3 A 0 , 1 3 A 00 , and 1 1 A 0 are available, e.g. computed by Schatz and coworkers [5][6][7], Dobbyn and Knowles [8], Braunstein et al [9], Balakrishnan [10], Chu et al [11,12], and Wu [13]. However, the effects of the hydrogen isotope deuteron (D) upon this reaction have not been investigated thoroughly until now.…”