1. Dipartimento di Chimica e I.S.T.M.-C.N.R., Universita' di Perugia, 06123, Perugia, Italy, Department of Physical Chemistry, University of Geneva, 30 Quai Ernest Ansermet, CH-1211 Geneva, Switzerland, and Section Theoretical Chemistry, Faculty of Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands