Theoretical thermochemistry of homolytic C-C and C-Cl bond dissociations in unbranched perchloroalkanes

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 Delta H(C-C)degrees 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 Delta H(C-C)degrees and Delta H(C-Cl)degrees 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, Delta H(C-C)degrees 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 Delta H(C-C)degrees derived from the CBS-4 standard enthalpies with the E-CBS energy term excluded.