Flash Photolysis of Hydrogen Iodide in the Presence of Deuterated Hydrocarbons

Abstract

The reactions of hot hydrogen atoms in the range 0.8–2.4 eV with perdeuterated and partially protonated hydrocarbons have been studied. The hot atoms were produced in C2D6, C3D8, n-C4D10, and various isotopically mixed ethanes by flash photolyzing HI-RHD mixtures. Graphs of product ratio H2/HD vs the initial reactant ratio HI/RD were characteristically linear. These and similar graphs for mixtures containing a rare gas were analyzed using the kinetic theory of hot-atom reactions developed by Estrup and Wolfgang and also by a simpler approach. Where comparison was possible, the results of the two methods of analysis showed good agreement. The following parameters were evaluated: average relative reaction probabilities for H*–HI and H*–RD collisions (where the asterisk denotes the hot species), the fraction of H* moderated to that reacting hot in pure RD, and β, the average fractional energy loss by H* is collision with RD. The most important conclusions which can be drawn from the results are: (a) a simple interpretative scheme employing average reaction and moderation probabilities serves to give nearly quantitative estimates of the parameters involved; (b) in all cases the probability of H* reaction with HI is considerably larger than with RD, although several times smaller than the corresponding reaction probabilities of thermal hydrogen atoms; (c) deuterium abstraction by H* from the six partially deuterated ethanes is simply statistical; (d) collisions between H* and RD are quite inelastic, the values for β being greater than 4, 6, and 12 times the respective rigid-sphere values for perdeuterated ethane, propane, and n-butane.