1. Department of Chemistry, University of California, Berkeley, California 94720, Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, Department of Chemistry, IFM & NIS Centre of Excellence, University of Torino, Via Quarello 11 I-10135 Torino, Italy, and Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899