Calculation of Arrhenius Parameters by the Rrkm Method for the Carbon–Hydrogen Bond Fission Reaction of Fluorobenzene

Abstract

The present study provides quantitative results for the rate of the unimolecular hydrogen–carbon bond fission reaction of fluorobenzene at elevated temperatures up to 2000 K. The potential energy surfaces for each C–H bond fission reaction (in the ortho, meta, and para sites) of fluorobenzene were investigated using ab initio calculations. The geometry and vibrational frequencies of the species involved in these reactions were optimised at the unrestricted MP2 level of theory, using the cc-pVDZ basis set. Since the C–H bond fission channel is a barrier-less reaction, we have used variational Rice–Ramsperger–Kassel–Marcus (RRKM) theory to predict rate constants. The difficulty in the application of the RRKM method to molecules bigger than benzene is discussed and a method is offered to solve the problem. Using calculated rate constants at different temperatures, the activation energies and exponential factors were determined. The Arrhenius expressions for the C–H bond fission reactions of fluorobenzene at the ortho, meta and para sites were obtained as k( T) = 6.1 × 1016 e−57328/ T, k( T) = 1.8 × 1017 e−59080/ T and k( T) = 1.3 × 1017 e−59600/ T respectively. Moreover, the effect of the fluorine atom including electron attraction and resonance with the benzene ring, on molecular rotation and the tunnelling effect on the rate expression have been discussed.