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a dense, CO2-ruch early Martian atmosphere. Science, under review. Tian, F. and J.F. Kasting (2008) Invariance of the total atmosphere escape rate from the atmosphere of early Mars. GRL, in preparation. Walker, J.C.G. (1977) Evolution of the atmosphere. Macmillan, New York. E-mail: feng.tian@colorado.edu Prebiotic Syntheses A Prebiotic Surface-Catalysed Formation of Alkyl Imines Nigel Aylward School of Physical Adenosine and Chemical Sciences Queensland University of Technology George St., Brisbane, Queensland 4000 AUSTRALIA Alkynes such as ethyne form weak charge-transfer, η 1 -alkenyl (vinyl) complexes (Collman et al., 1987) with surface catalysts such as Mg.porphin in which the alkenyl group has a net positive charge, and the conjugated porphin has a negative charge. The enthalpy change for the complex formation is small (−0.002 h). This neutral complex is polarised and undergoes a nucleophilic addition reaction with ammonia at the carbene carbon to form Mg.2-amino ethenyl (vinyl).porphin with a small enthalpy change (0.016 h).