1. Contribution from the Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan, Instituto de Química Física, “Rocasolano”, C. S. I. C. E-28006 Madrid, Spain, and Departamento de Química Física, Universidad de Alcalá, E-28871, Alcalá de Henares (Madrid), Spain