1. Magnetic Resonance Center, CERM, University of Florence, Via L. Sacconi, 6 - 50019 Sesto Fiorentino, Italy, Department of Chemistry, University of Florence, Via della Lastruccia, 3 - 50019 Sesto Fiorentino, Italy, Department of Chemistry and Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and Department of Agricultural Biotechnology, University of Florence, Via Maragliano, 75-77 - 50144 Florence, Italy