ENERGY PARTITIONING IN PHOTOCHEMICAL REACTIONS . PHOTOLYSIS OF 4-METHYL-1-PYRAZOLINE

Methylcyclopropane produced by the 3130-Å photolysis of 4-methyl-1-pyrazoline contains enough of the light energy absorbed by the reactant and the exothermicity of the reaction to undergo subsequent isomerization to the butene isomers. The pressure dependence of the magnitude of the experimental unimolecular rate constant for the isomerization of the vibrationally excited methylcyclopropane formed in the primary process is compared to theoretical rate constants calculated for this reaction using the RRKM formulation. The results indicate that, although the hydrocarbon fragment contains a broad distribution of internal energies, the fraction of the available energy that it receives is less than would be predicted by a statistical model for energy partitioning. Consideration of the structural changes that occur when 1-pyrazoline decomposes to a trimethylene intermediate and nitrogen offers a qualitative explanation of the partitioning of energy between these two products of this photodecomposition reaction.