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

Abstract

A 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 calculs.tions 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 h Y 0.2, and for the concerted ring opening of 1,l-dichlorocyclopropane h 'v 0.4 must be assumed. The isotope effects for the decomposition of CD3CDzCl and CH3CDzCl 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 h 'v 1.0. This different behavior of concerted and biradica.1 reactions against falloff calculations can hardly be attributed to experimental uncertainties in the Arrhenius parameters and/or the collision frequency alone.

Intramolecular isotopic effect in the pyrolysis of ethyl‐2d1 chloride

“…Furthermore this agreement suggests that the assigned vibrational frequencies for the ground and the transition states of the CH2DCHZC1 molecule are consistent with the experimental results. The collisional efficiency was found to be X = 0.3, in close agreement with the value of 0.2 obtained by the RRKM calculations of Holbrook and Marsh [18] and the Forst calculations of Dill and Heydtmann [19] for the ethyl chlorides.…”