C3V VERSUS C2V CD(1S,3P,1P)-CH4 VANDERWAALS COMPLEXES - A VARIATIONALAND PERTURBATIONAL MULTIREFERENCE CONFIGURATION-INTERACTION STUDY

The Cd(1S,3,1P)-CH4 interaction is studied for the C3v vertex-on Cd(1S ,3,1P)-HCH3 and face-on Cd(1S,3,1P)-H3CH approaches as well as the C2, Cd(1S,3,1P)-H2CH2 edge-on approach of the metal atom to methane using extensive ab initio multireference configuration interaction plus sec ond order multireference Moller-Plesset calculations. The ground state Cd(1S)-CH4 is totally repulsive for the C3v vertex-on and C2v edge-on approaches, Only a stable face-on Cd(1S)-H3CH complex was found. Alth ough vertex-on and edge-on minima were found for the Cd(3P)-CH4 excite d complexes, they are less attractive than the corresponding Cd(3P)-H3 CH face-on exciplexes. The optimal orientation of the face-on occupied 5p orbital of Cd(5s5p) is that which minimizes the overlap with C-H b onds leading to a ''3PI(y)'' exciplex. For the Cd(1P)-CH4 interaction, two face-on and two edge-on very stable complexes (all with D(e) > 13 00 cm-1) were found. The Cd(1S)-H3CH (1A1) complex was found to have a 185 cm-1 well at 8.03 a.u. Both excited P states present shorter equi librium distances-the Cd (3P)-H3CH (3E) exciplex has a 799 cm-1 well a t 6.08 a.u., while the Cd(1P)-H3CH exciplex (1E) has a 1337 cm-1 well at 5.57 a.u. These results are in good agreement with the recent exper imental values X 1O+ (1S)-(D(e) = 121 cm-1, R(e) = 7.9 a.u.); A 3O+ (3 P)-(D(e) = 677 cm-1, R(e) = 5.80 a.u.); and 1P-(D(e) congruent-to 1300 cm-1, R(e) = 5.59 a.u.) obtained by Wallace and Breckenridge [J. Chem . Phys. 97, 2318 (1992)]. The present work confirms the C3, facial nat ure of the observed (1S) ground and (3P) excited complexes. The presen t results are inconclusive as to the facial or edge-on nature of the C d(1P)-CH4 exciplex, but they preclude the C3, vertex-on possibility.