1. a Department of Chemistry , The Johns Hopkins University , Baltimore, Maryland, 21218
2. d Corporate Research Laboratories , Exxon Research and Engineering Company , P. O. Box 45, Linden, New Jersey, 07036
3. b Applied Physics Laboratory , The Johns Hopkins University , Johns Hopkins Road, Laurel, Maryland, 20810
4. c Corporate Research Laboratory , Exxon Research and Engineering Company , P. O. Box 45, Linden, New Jersey, 07036