1. Theoretische Chemie, Universität Hannover, Am Kleinen Felde 30, 30167 Hannover, Germany, Laboratoire de Chimie Physique, Umr 8000 CNRS, Université de Paris Sud, Bât 490, 91405 Orsay, France, and Department of Organic Chemistry and the Lise-Meitner-Minerva Center for Computational Chemistry, Hebrew University, 91904 Jerusalem, Israel