VIBRATIONAL ENERGY-DISTRIBUTIONS OF REACTING CH2 A-1(1) FROM 4358,3660, AND 2537 A-PHOTOLYSES OF CH2N2 AND UPON REACTION WITH CYCLOBUTANE - CHEMICAL ACTIVATION STUDY

The decomposition of chemically activated methylcyclobutane formed by the photolyses of ketene and diazomethane in the presence of excess cyclobutane was studied. A model incorporating a distribution of initial energies available to the chemically activated methylcyclobutane and stepwise collisional stabilization was found to satisfactorily describe both the ketene and diazomethane photolysis data. The distribution of chemically activated methylcyclobutane initial energies is the result of a rather narrow thermal Boltzmann distribution for the reactants at 300 K combined with, in the case of diazomethane photolyses, a broad vibrational energy distribution for the reacting CH2(1A1) upon reaction with cyclobutane. An approximate energy level population model for the vibrationally excited CH2(1A1) is developed in order to describe this latter distribution. Chemically activated methylcyclobutane was assumed to be formed with a thermal Boltzmann distribution of initial energies above Emin in the 3340 Å photolyses of ketene/cyclobutane/oxygen mixtures. The decomposition data for this system were fitted over an extended pressure range by the calculational model with a collisional deactivation step size of 4 ± 1 kcal/mol. Utilizing this 4 kcal/mol step size the calculational model was fitted to the diazomethane photolysis data at 4358, 3660, and 2537 Å by different vibrational energy distributions for the reacting CH2(1A1). These 4358, 3660, and 2537 Å distributions were represented by “shifted” Gaussians characterized by maxima at 8.4, 10.0, and 1.0 kcal/mol above Emin and by values of 4a of 5.6, 8.5, and 18.4 kcal/mol, and giving average reacting CH2(1A1) vibrational energies of 8.8, 11.3, and 14.4 kcal/mol, respectively. The differences in these reacting CH2(1A1) vibrational energy distributions for different wavelength photolyses are interpreted as reflecting differences in the initial CH2(1A1) vibrational energy distribution from excess energy partitioning during photodecomposition of diazomethane. No evidence suggesting the participation of CH2(1B1) in the 2537 Å photolysis of diazomethane was found. A brief discussion of the interpretative problems presented to ketene photochemistry by the energetics based on a recent spectroscopic determination of the CH2(1A1) ↔ CH2(3B1) splitting is given. © 1978, American Chemical Society. All rights reserved.