1. Contribution from the Department of Chemistry, University of Wisconsin, 1101 University Avenue, Madison, Wisconsin 53706-1396, Instituto Superior Técnico, Centro de Química Estrutural, Química Orgânica, Av. Rovisco Pais, 1049-001 Lisboa, Portugal, and Computer-Chemie-Centrum, Friedrich-Alexander-Universität Erlangen-Nürnberg, Nägelsbachstrasse 25, 91052 Erlangen, Germany