1. Department of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, P.O. Box 80083, 3508 TB Utrecht, The Netherlands, Theoretical Chemistry Group, Debye Institute, Utrecht University, P.O. Box 80083, 3508 TB Utrecht, The Netherlands, and ExxonMobil Chemical Europe Inc., European Technology Research Center, Hermeslaan 2, B-1831 Machelen, Belgium