1. Department of Organic Chemistry and Centro CNR Meccanismi di Reazioni Organiche, University of Padova, Via L. Marzolo, 1 35131 Padova, Italy, Department of Chemical Sciences, Univeristy of Trieste, Via L. Giorgieri, 1 34127 Trieste, Italy, and Department of Physical Chemistry, University of Padova, Via L. Loredan, 2 35131 Padova, Italy