A HIGH-LEVEL COMPUTATIONAL STUDY ON THE THERMOCHEMISTRY OF VINYL AND FORMYL HALIDES - HEATS OF FORMATION, DISSOCIATION-ENERGIES, AND STABILIZATION ENERGIES

The heats of formation, Delta H-f 298, of vinyl and formyl halides cal culated using G2 theory are -145.9 (CH2-CHF), 21.8 (CH2CHCl), 79.6 (CH 2CHBr), 147.1 (CH2CHI), -343.7 (HF-CO), -192.7 (HClCO), -133.9 (HBrCO) , and -66.2 kJ mol(-1) (HICO), which agree with the available experime ntal data. While the covalent radii of sp(3) and sp(2) hybridized carb ons are 0.762 and 0.735 Angstrom, respectively, at the MP2/6-31G(d) le vel, the C-X bond lengths in formyl bromide and iodide are 0.016 and 0 .045 Angstrom, respectively, longer when compared with those in methyl bromide and iodide. However, the C-F bond in formyl fluoride is 0.040 Angstrom shorter than the C-F bond in methyl fluoride. The C-X bond d issociation energies (DC-X) for vinyl, formyl, and methyl halides show good linear correlations with the Mulliken and Pauling electronegativ ites of the halogens. The DC-X values for formyl bromide and iodide ar e smaller than those calculated for methyl bromide and iodide. The sta bilization energies (SE) yielding estimates of the stabilization of th e C=C and C=O double bonds by halogens decrease from X = F to X = I an d show reasonable linear correlations with the Mulliken and Pauling el ectronegativities of the halogens. While the Dc-x energies for formyl halides are smaller than those for vinyl halides, the SE(formyl) value s are larger than the corresponding SE(vinyl). The elimination of HX f rom formyl halides is exothermic, in contrast to the endothermic elimi nation of HX from vinyl halides.