1. Contribution from the Magnetic Resonance Center (CERM), University of Florence, Via Luigi Sacconi 6, 50019 Sesto Fiorentino, Italy, Department of Chemistry, University of Florence, Via della Lastruccia 3, 50019 Sesto Fiorentino, Italy, Department of Agricultural Biotechnology, University of Florence, via Maragliano 75-77, 50144 Florence, Italy, and Istituto per le Applicazioni del CalcoloSezione di Firenze, Polo ScientificoCNR, Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy