Multistep Collisional Deactivation of Chemically Activated Dimethylcyclopropane

Article abstract of DOI:10.1021/j100242a024

The decomposition of chemically activated cis- and trans-dimethylcyclopropane formed by the photolysis of ketene (at 334 nm) or diazomethane (at 436 and 366 nm) in the presence of excess cis-butene-2 and small amounts of oxygen was studied.Apparent first-order decomposition rate coefficients for the chemically activated molecules were measured and found to be pressure dependent over the extended pressure range from 0.1 to 40 kPa.The turnup observable at low pressure demonstrates the occurence of multistep deactivation of the chemically activated molecules,while the pressure dependence at high pressure is indicative of a wide initial vibrational energy distribution of cis-dimethylcyclopropane.The theoratical approach used RRKM theory to calculate the energy-dependent decomposition rate coefficients and assumed a stepladder model to describe collisional transition probabilities.The initial energy distributions of cis-dimethylcyclopropane were approximated by shifted Gaussian-type functions.The experimental results could be fit over the entire pressure range investigated with a theoratical model utilizing a collisional deactivation step size of about 14-22 kJ mol^-1 and initial energy distributions whose widths increased with increasing energy of chemically activated molecules.It has been concluded that the wide distributions obtained in the diazomethane photolysis systems were mainly due to the dispersion originating from energy partitioning in the photolytic event producing singlet methylene.