Application of Forst's method to the calculation of thermal unimolecular reaction rates and isotope effects in the falloff region

Abstract

AbstractA method proposed in 1972 by W. Forst is used to calculate the experimentally accessible pressure dependence of thermal unimolecular rate constants. The specification of an activated complex always employed in RRKM calculations is avoided. This allows for a more consistent comparison between the results obtained by the application to various unimolecular processes. In order to bring experimental and calculated curves into agreement, fourcenter eliminations of hydrogen halides from alkyl halides require the formal introduction of a collision efficiency factor λ ≃ 0.2, and for the concerted ring opening of 1,1‐dichlorocyclopropane λ ≃ 0.4 must be assumed. The isotope effects for the decomposition of CD3CD2Cl and CH3CD2Cl have been studied, and the pressure dependence of kH/kD is reported. Studying the biradical ring opening of oxetan, cyclobutane, and cyclopropane, the falloff curves and isotope effects are predicted within the experimental uncertainty by the use of λ ≃ 1.0. This different behavior of concerted and biradical reactions against falloff calculations can hardly be attributed to experimental uncertainties in the Arrhenius parameters and/or the collision frequency alone.