THERMOCHEMISTRY OF HOMOLYTIC C-C, C-H, AND C-CL BOND DISSOCIATIONS INPOLYCHLOROETHANES - BENCHMARK ELECTRONIC-STRUCTURE CALCULATIONS

G2 and G2MP2 calculations on the C2H6-nCln, C2H5-nCln., and CH3-nCln. species produce benchmark thermochemical data for homolytic C-C, C-HI and C-Cl bond dissociations in ethane and its chloro derivatives. With out exception, the C-H bonds are found to be stronger than the C-C one s, which turn out to be barely stronger than their C-CI counterparts. The standard enthalpies of the homolytic C-C, C-H, and C-CI bond disso ciations decrease sharply with the increasing number of chlorine atoms , the C-C bonds being affected the most, Wherever available, experimen tal data confirm the reliability of the G2 calculations. The G2MP2 pre dictions are found to follow closely those of the G2 method. At the sa me time, the present study uncovers a very poor performance of the BLY P and B3LYP functionals for systems with strong repulsions between chl orine atoms, Analysis of the computed enthalpies of bond dissociation in terms of substituent contributions leeds to the conclusion that thi s poor performance is caused by a systemic exaggeration of the H ... C l and C ... Cl repulsions. Most likely, this exaggeration is caused by the inability of the currently used density functionals to correctly describe dispersion interactions.