1. Contribution from Central Research & Development, E. I. DuPont de Nemours and Company, Inc., Experimental Station, Wilmington, Delaware 19880-0328, School of Engineering and Physical Sciences, William H. Perkin Building, Heriot-Watt University, Edinburgh EH14 4AS, U.K., and Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842-3012