1. Contribution from the Laboratoire Francis Perrin, CEA/DSM/DRECAM/SPAM - CNRS URA 2453, CEA Saclay, F-91191 Gif-sur-Yvette, France, Gaussian, Inc., 340 Quinnipiac St. Bldg 40,Wallingford, Connecticut 06492, Dipartimento di Chimica, Universita Federico II, Complesso Universitario Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy, and Istituto Biostrutture e Bioimmagini/CNR, V. Mezzocannone 6-80134 Napoli, Italy