1. Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, 2575 Sand Hill Road, MS 69, Menlo Park, California 94025, Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada, ExxonMobil Research and Engineering Company, Route 22 East, Annandale, New Jersey 08801, and Department of Chemistry, State University of New York at Albany, Albany, New York 12222