Spin uncoupling in molecular hydrogen activation by platinum clusters

Complete active space multiconfiguration self-consistent field (CAS-MCSCF) and multireference configuration interaction (MRCI):calculations for H-2 re actions with platinum atom and small clusters (Pt-2, Pt-3) have been perfor med with account of spin-orbit coupling (SOC). Relativistic effective core potential basis sets an used. It is shown that the D-3(2) State of the Pt a tom, which is only 2.2 kcal/mol higher in energy than the ground state comp onent, D-3(3), is reactive in hydrogen insertion reactions. The activation barrier (about 2 kcal/mol) is formed by a very efficient singlet-tripler (S -1-D-3(2)) avoided crossing determined by SOC in the 5d shell of the metal, and a very stable singlet (1)A(1) ground state product PtH2 is produced by Pt(D-3(2)) atom insertion into the hydrogen. Despite of the fact that asym ptotic states in the entrance channel differ by 2 in angular momentum quant um number, a small mixing between the S-1 and D-1 states obtained at the MR CI level at the beginning of the reaction leads to a drastic change in the j-j coupling scheme and finally produces the S-T intersystem crossing. Othe r states are nonreactive, since they have a barrier higher than 15 kcal/mol . A simple concerted insertion of platinum dimer into H2 molecule is studie d in order to simulate spin uncoupling produced by CI between the ground si nglet state of the reactants and the double-tripler state (1)[Pt-2((3)Sigma (g)(-)) + H-2(Sigma(u)(+))]. This type of spin uncoupling is typical for al l studied reactions even when they do not follow the concerted mode. The tr ipler-singlet intersystem crossing occurs at the tight chemical interaction stage for cluster reactions in contrast to activation by a bare atom. (C) 1999 Elsevier Science B.V. All rights reserved.