1. Contribution from the Universität Leipzig, Linnéstr. 5, 04103 Leipzig, Germany, Corporate Strategic Research, ExxonMobil Research and Engineering Company, 1545 Route 22 East, Annandale, New Jersey 08801, Leibniz Universität Hannover, Callinstr. 3-3A, 30167 Hannover, Germany, and Université Montpellier 2, Place E. Bataillon, 34095 Montpellier CEDEX 5, France