1. Contribution from the Davy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London W1X 4BS, U.K., CLRC Daresbury Laboratory, Daresbury, Warrington WA4 4AD, U.K., and Gruppo di Chimica Teorica, Università di Torino, Via Giuria 5, 10125 Torino, Italy