POTENTIAL-ENERGY SURFACES FOR THE PT2+H2 REACTION

Potential energy surfaces for the Pt2 + H-2 reaction are obtained using a complete active space multiconfiguration self-consistent field (CAS-MCSCF) method followed by multireference singles + double CI (MRSDCI) calculations. Several approached of H-2 such as parallel, perpendicular, collinear, end-on with respect to Pt2 are considered. In addition, out-of-plane twist motions of hydrogens relative to the Pt-Pt bond are considered. The parallel approach was found to be most reactive in the 1A1 electronic state, which forms a cis Pt2H-2 saddle point after surmounting a barrier of approximately 20 kcal/mol. The saddle point thus formed spontaneously transforms to a trans Pt2H-2, 1A(g) ground state through an out-of-plane twist motion. The dissociation of H-2 in the parallel mode of collision was found to be brought about primarily through the interaction of the d(delta) orbitals of the two Pt atoms with the H-2 1-sigma-g and 1-sigma-u* orbitals. The spin-orbit effects were studied using a relativistic CI (RCI) method and found to be significant for Pt2H-2. Spin-orbit coupling was found to induce an avoided crossing. This destabilizes the Pt2H-2 (1A(g)) molecular state with respect to the dissociated Pt2 + H-2. The energy separation between the Pt2H-2 1A(g) trans minimum and the cis saddle point was calculated at the MRSDCI level as 3 kcal/mol. We find that the reactivity of Pt2 with H-2 varies as a function of electronic state and orientation.