1. Contribution from the Center for Polymers and Organic Solids, Department of Chemistry and Biochemistry, University of California−Santa Barbara, Santa Barbara, California 93106; School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332; Los Alamos National Laboratory, Theoretical Division, Los Alamos, New Mexico 87545; and Department of Chemistry, University of Arizona, Tucson, Arizona 85721