1. Department of Chemistry, University of Utah, 315 South 1400 East, RM, 2020, Salt Lake City, Utah 84112, Department of Chemistry and The Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, and W. M. Keck FT-ICR Mass Spectrometry Laboratory and Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695 stang@chem.utah.edu