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This work reports a theoretical study of the gas phase unimoleculardecomposition of cyclobutane, cyclopentane and cyclohexane by means of quantumchemical calculations. A biradical mechanism has been envisaged for eachcycloalkane, and the main routes for the decomposition of the biradicals formedhave 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 bymeans of isodesmic reactions. The contribution of hindered rotors has also beenincluded. Activation barriers of each reaction have been analyzed to assess the1 energetically most favorable pathways for the decomposition of biradicals.Rate constants have been derived for all elementary reactions using transitionstate theory at 1 atm and temperatures ranging from 600 to 2000 K. Global rateconstant for the decomposition of the cyclic alkanes in molecular products havebeen calculated. Comparison between calculated and experimental results allowedto validate the theoretical approach. An important result is that therotational barriers between the conformers, which are usually neglected, are ofimportance in decomposition rate of the largest biradicals. Ring strainenergies (RSE) in transition states for ring opening have been estimated andshow that the main part of RSE contained in the cyclic reactants is removedupon the activation process.

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