Unimolecular Decomposition in Nonequilibrium Systems. sec-Butyl and sec-Butyl-d1 Radicals Produced by Chemical Activation at Different Levels of Vibrational Excitation

Abstract

Results of earlier studies on the decomposition of activated sec-butyl radicals formed in nonequilibrium systems by the addition of H or D atoms to trans-butene-2, to cis-butene-2, and to butene-1 are summarized together with new data. The work constitutes a program of study in which butyl radicals have been produced at six different levels of minimum internal excitation energy. Two choices of ambient temperature (300° and 195°K), together with pressure variation over the (ideal) limiting range of zero to infinity, give rise in these systems to a further increase in the number of values of the average energy of the reacting radicals; these energies correspond to a range from 7.2 to 14.2 kcal mole—1 for the average excess energy of the radicals with respect to decomposition. Twenty-one of such average energy states out of the possible twenty-four values, corresponding to two pressure extremes at each of two temperatures, have been obtained experimentally. These data permit a more consistent test of the theoretical models which were presented in the earlier work. A comparison and discussion of the observed rates is given. The agreement is satisfactory. An ``energy'' isotope effect which arises upon replacement of reactant H atom by D is well accounted for. It is concluded that the merits of the chemical activation technique for clarifying the nature of unimolecular reaction, at high levels of vibrational excitation of polyatomic species, are well borne out. The present evidence, together with other work, shows the usefulness of the Marcus—Rice rate expression in all systems where the assumptions may apply, when accurate calculations are made with quantum statistical models. The effect of variation of thermochemical and molecular parameters on the calculations is described. Not only are all vibrational degrees of freedom to be taken as active, as the best a priori assumption, but there is evidence that where they exist, internal rotations and possibly one of the over-all rotations should be so treated also.