1. Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701; Guelph-Waterloo Centre for Graduate Work in Chemistry, Guelph Campus, Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario N1G 2W1, Canada; and AT&T Bell Laboratories, 600 Mountain Ave., Murray Hill, New Jersey 07974