1. Contribution from the Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel, Faculty of Chemical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel, Carmel Olefins Ltd., P.O.B. 1468, Haifa 31014, Israel, and Chimie de la Matiere Condensee, Universite Paris 6 - T54-E5, 4 Place Jussieu, 75252 Paris Cedex 05, France