1. Institut Charles Sadron, CNRS-ULP, 6 rue Boussingault, 67083 Strasbourg, France, Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, New Jersey 07974, and Dipartimento di Chimica, Università degli Studi di Napoli Federico II, Via Mezzocannone 4, 80134 Napoli, Italy