ACCURATE SPECTROSCOPIC CONSTANTS FOR THE CD(S-1, P-3, P-1)-H-2 VAN-DER-WAALS COMPLEXES - A THEORETICAL-STUDY

Very accurate interaction energies and equilibrium geometries are obtained for the C-2 upsilon Cd(S-1, P-3, P-1)-H-2 van der Waals (VDW) complexes using extensive ab initio multireference configuration interaction plus second order multireference Moller-Plesset calculations. Gaussian basis sets are of triple-xi plus polarization quality and a relativistic effective core potential for cadmium has been used. For the interaction energies the Boys and Bernardi counterpoise (CP) correction has been used. The ground state Cd(S-1)-H-2 van der Waals complex was found to be very weakly bound with a 108 cm(-1) well at 8.4 a.u., as compared with the estimated experimental values of D-e congruent to 50 cm(-1) at 8.3 a.u. The computed (D-e=1580 cm(-1), R(e)=4.9 a.u.) constants for the B-1(1) Cd(P-1)-H-2 exciplex are in good agreement with the recent experimentally derived values (D,=1713 cm(-1), R,=4.7 a.u.) obtained by Wallace et al. [J. Chem. Phys. 97, 3135 (1992)]. The well depth and equilibrium geometry for the B-3(1) Cd ((3)p) -H-2 exciplex were computed as 420 cm(-1) and 5.7 a.u. while those of the B-3(2) exciplex are 650 cm(-1) and 4.75 a.u. The B-1(2) surface did not lead to a VDW complex but instead it produced a strongly bound (D-e congruent to 10 kcal/mol) bent H-Ch-H molecule responsible for the experimentally observed CdH+H and Cd+H+H reaction channels. The (3,1)A(1) states arising from the Cd(P-3,P-1)+H-2 asymptotes were found to be totally repulsive. In all cases the CP correction was a non-negligible fraction of the interaction energy. Finally, the energetic position and the geometry of the B-1(1)/(3)A(1) surface crossing, crucial to explain the observed Cd(B-1(1),upsilon')-H-2 predissociation lifetimes, are accurately determined.