1. Center for Heterocyclic Compounds, University of Florida, Gainesville, Florida 32611-7200, Institute of Chemical Physics, University of Tartu, 2 Jakobi Street, EE2400 Tartu, Estonia, Department of Chemistry, University of Warsaw, ul. L. Pasteura 1, 02093 Warszawa, Poland, and Theoretische Chemie, Universität Hannover, D-30167 Hannover, Am Kleinen Felde 30, Germany