1. Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States
2. Johns Hopkins University, Baltimore, Maryland 21218, United States
3. University of Girona, Campus Montilivi (Ciències), Plaça de Sant Domènec, 17004 Girona, Spain
4. ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain