Spin symmetry and interaction mechanisms in free radical reactions

Abstract

The principles underlying the spin-allowed and spin-forbidden free radical reactions are sought. Based on symmetry, the interactionmechanism and the corresponding transmission matrix elements for the mixing of reactant/product hyperpotential surfaces ar elucidated. Magnetic multipole, electrostatic, spin-orbit and spin-spin interactions are considered. Four examples are used to illustrate various cases: (1) Xe ( 3P u) + N 2 ( 1Σ g +) → Xe( 1S g) + N 2 ( 3II g), which satisfies Wigner's selection rule for the conservation of total spin angular momentum and for which spin and orbital interactions must be compatible, all of the above interactions are found. (2) The perpendicular mode of photodissociation in O 3 * ( 1B 1) → O( 3P g) + O 2( 3E g −), which is orbitally forbidden and for which only spin-spin interaction is found. (3) The hypothetical reaction CH( 4Σ −) + CH( 2Σ +) → C 2H 2 (Ã 1A u), which is spin forbidden but is made allowed by the C 2h geometry of the reaction coordinate. Only spin interactions are found. (4) The triplet cyclopropylidene to orthogonal singlet allene transformation, which is a Hückel to Möbius transformation, and the reaction coordinate produces orbital angular momentum changes to compensate for the spin change. The detailed interaction matrix elements shown for each channel of reaction give a qualitative indication of the branching ratios for different reaction paths and may be compared with future experimental measurements.