1. Contribution from the Department of Chemistry, University of Florence, Via Gino Capponi 9, 50121, Florence, Italy, Centro di Risonanze Magnetiche, University of Florence, Via Luigi Sacconi 6, 50019, Sesto Fiorentino, Italy, and Department of Agricultural Biotechnology, University of Florence, P. le delle Cascine 24, 50144, Florence, Italy