  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.
