1. Department of Organic Chemistry, Stockholm University, S-106 91 Stockholm, Sweden, Department of Chemical Physics, Lund University, P.O. Box 124, S-22100 Lund, Sweden, Department of Quantum Chemistry, Uppsala University, Box 518, S-751 20 Uppsala, Sweden, Department of Physical Chemistry, Uppsala University, Box 579, S-751 23 Uppsala, Sweden, and Department of Analytical Chemistry, Uppsala University, Box 599, SE-751 24 Uppsala, Sweden