Theoretical Thermochemistry of Homolytic C−C and C−Cl Bond Dissociations in Unbranched Perchloroalkanes

Abstract

Proper description of the dispersion interactions that attenuate the closed-shell repulsions among chlorine atoms in n-perchloroalkanes requires an accurate treatment of long-range electron correlation effects. The presently known density functionals, which do not correctly account for such effects, grossly underestimate thermodynamic stability of these molecules, yielding the standard enthalpy of C−C bond dissociation in n-C4Cl10 as low as 20 kcal/mol. In contrast, the predictions of the CBS-4 scheme fail to reproduce the weakening of the C−C bonds that is experimentally observed in the higher members of the CnCl2n+2 homologous series. For the CCl4 and C2Cl6 the most reliable estimates of and are provided by the G2 and G2MP2 methods. The MP2/6-311G** level of theory (without ZPEs and finite-temperature corrections) appears at present to be the only viable, though quite inaccurate, theoretical approach to theoretical thermochemistry of larger chlorocarbons. At that level of theory, is predicted to decrease by ca. 10 kcal/mol upon the addition of each −CCl2− unit, convincingly explaining the thermal lability of higher n-perchloroalkanes. Similar conclusions are reached by examining the estimates of derived from the CBS-4 standard enthalpies with the ECBS energy term excluded.