Theoretical studies of spin-forbidden radiationless decay in polyatomic systems: insights from recently developed computational methods

We consider spin-forbidden radiationless decay of a bound electronic state which is metastable owing to a crossing with a dissociative potential energy surface of different spin-multiplicity. The minimum energy point on the spin-allowed surface of intersection of the bound and dissociative potential energy surfaces (the minimum energy crossing point) represents a key bottleneck along the minimum energy dissociation path, and frequently represents the transition state for this process. Thus the characterization of the minimum energy crossing point yields valuable mechanistic insights into this class of reactions, providing information concerning the feasibility of radiationless decay and the likely decomposition products. Traditional approaches for determining this point using multireference CI wave functions are computationally costly. However, a recently introduced algorithm, which uses analytic energy gradients and determines this point directly, that is without prior characterization of the surface of intersection, has reduced considerably the requisite computational effort. This algorithm is used to consider postulated spin-forbidden radiationless decay channels in tetrahedral N4, N4(1A1) --> N4(3A'') --> N2(X1-SIGMA(g)+) + N2(A3-SIGMA(u)-) and in the methoxy cation, CH3O+(3A2) --> CH3O+(1A') --> HCO+(X1-SIGMA+) + H-2(X1-SIGMA(g)+). For the methoxy cation it is shown, using CI expansions as large as 2.5 million configuration state functions, that DELTA-E = 15.4 kcal/mol, where DELTA-E is defined as the difference between the energy at minimum energy crossing structure and the energy at the minimum on the bound state potential energy surface. Thus the indicated reaction provides a low energy decomposition pathway, For tetrahedral N4, DELTA-E for the indicated process is 28.2 kcal/mol which is half the barrier for the spin-allowed decay channel.