1. Contribution from the Newman and Wolfrom Laboratory of Chemistry, The Ohio State University, 100 W. 18th Avenue, Columbus, Ohio 43210-1173, Institute for Physical Chemistry, University of Fribourg, Pérolles, CH-1700 Fribourg, Switzerland, and Department of Theoretical Chemistry, Chemical Centre, University of Lund, P.O. Box 124, 2-22100 Lund, Sweden