1. Department of Chemistry, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, IL 61801
2. Current address: Department of Chemistry and Biochemistry, University of Texas, Austin, TX 78731
3. Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, IL 61801