1. Contribution from the Laboratory of Thermochemistry, Department of Chemistry, University of Surrey, Guildford, Surrey GU2 7XH, UK, Instituto de Física de São Carlos, Universidade de São Paulo, C. P. 369, 13560 São Carlos (SP), Brazil, and Departamento de Física, Facultad de Ciencias Exactas, Universidad Nacional de La Plata and Instituto IFLP (CONICET − UNLP), C. C. 67, 1900 La Plata, Argentina