1. Framework Solids Laboratory, Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India, Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560 064, India, and Davy-Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albermarle Street, London WIS 4BS, U.K.