1. Contribution from the Istituto di Chimica Biomolecolare del CNR - Sezione di Catania, Via del Santuario 110, I-95028 Valverde (CT), Italy, and the Department of Chemistry, Mount Allison University, Sackville, NB, Canada E4L 1G8, and National Research Council, Ottawa, ON, Canada K1A 0R6