1. a Research and Technology Department , ICI Chemicals and Polymers Ltd. , P.O. Box 8, The Heath, Runcorn, Cheshire , WA7 4QD
2. c Department of Theoretical Chemistry , University of Bristol , Cantocks Close, Bristol , BS8 1TS
3. b Department of Chemistry , University of Liverpool , P.O. Box 147, Liverpool , L69 3BX