1. Contribution from the Laboratory of Molecular Spectroscopy, Institute of Chemistry, Eötvös University, H-1518 Budapest 112, P.O. Box 32, Hungary, Institute of Organic Chemistry, Justus-Liebig University, Heinrich-Buff-Ring 58, 35392 Giessen, Germany, and Center for Computational Chemistry, Department of Chemistry, University of Georgia, Athens, Georgia 30602