Detailed kinetic study of the ring opening of cycloalkanes by CBS-QB3 calculations

TL;DR

This study uses CBS-QB3 quantum chemical calculations to analyze the decomposition pathways of cycloalkanes, providing detailed thermochemical data, rate constants, and insights into the role of conformer rotation and ring strain in their decomposition.

Contribution

It offers a comprehensive theoretical analysis of cycloalkane decomposition, including thermochemical data, rate constants, and the significance of conformer rotation and ring strain effects.

Findings

Rotational barriers significantly influence decomposition rates.

Main ring strain is released during activation.

Calculated rate constants agree with experimental data.

Abstract

This work reports a theoretical study of the gas phase unimolecular decomposition of cyclobutane, cyclopentane and cyclohexane by means of quantum chemical calculations. A biradical mechanism has been envisaged for each cycloalkane, and the main routes for the decomposition of the biradicals formed have been investigated at the CBS-QB3 level of theory. Thermochemical data (\delta H^0_f, S^0, C^0_p) for all the involved species have been obtained by means of isodesmic reactions. The contribution of hindered rotors has also been included. Activation barriers of each reaction have been analyzed to assess the 1 energetically most favorable pathways for the decomposition of biradicals. Rate constants have been derived for all elementary reactions using transition state theory at 1 atm and temperatures ranging from 600 to 2000 K. Global rate constant for the decomposition of the cyclic alkanes in molecular products have been calculated. Comparison between calculated and experimental results allowed to validate the theoretical approach. An important result is that the rotational barriers between the conformers, which are usually neglected, are of importance in decomposition rate of the largest biradicals. Ring strain energies (RSE) in transition states for ring opening have been estimated and show that the main part of RSE contained in the cyclic reactants is removed upon the activation process.