Effect of Incident Translational Energy on the Surface-Induced Isomerization of Cyclopropane

Article abstract of DOI:10.1021/j100294a038

A supersonic free jet of cyclopropane molecules in the free molecular flow regime was intercepted by a heated mica surface.The resulting interaction isomerized a small fraction of the incident molecules to propylene.Previous studies had shown that reaction probability depended strongly on the temperature of the source gas, which determined the extent of vibrational excitation in the incident cyclopropane molecules.There was also a somewhat weaker dependence of reaction probability on surface temperature.We report here some experimental results on the effect of incident translational energy on reaction probability.With source temperature fixed the translational energy of the cyclopropane was varied by aerodynamic acceleration with helium or deceleration with argon or krypton.At each of five translational energies from 9 to 65 kJ/mol of cyclopropane we determined the surface temperature dependence of reaction probability to arrive at an apparent "surface activation energy" from the slope of an Arrhenius plot.It emerged that the sum of the translational and apparent surface activation energies had a constant value of about 105 kJ/mol.One model that explains these results assumes a high barrier to adsorption that is surmounted most effectively by vibrational energy in the incident cyclopropane molecule.Incident translational energy determines the sticking probability and thus affects the reaction rate.The resulting intermediates become transformed into a product propylene molecule and desorbed from the surface.