1. Theory and Computation Group, Department of Chemistry, Centre for Synthesis and Chemical Biology, Conway Institute of Biomolecular and Biomedical Research and Computing Centre, University College Dublin, Belfield, Dublin 4, Ireland, and Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”, Sez. Chimica, Politecnico di Milano, via L. Mancinelli 7, 20121 Milano, Italy