EFFECT OF HALOGEN SUBSTITUENTS ON THE GAS-PHASE ACIDITIES AND ELECTRON-AFFINITIES OF METHYLENES AND METHYLIDYNES - CALCULATION OF HEATS OF FORMATION

Ab initio molecular orbital calculations are reported for methylenes CXY, methylidynes CX, and anions CXY-and CX-, where X and Y are all possible combinations of H, F, and Cl. Structures were optimized at SCF/6-311++G(d,p), and single point calculations are reported at both MP4SDTQ/6-311++G(2dfp) and, for the smaller molecules,at MP4SDTQ/6-311++G(3df,3pd). For anions containing chlorine, optimization at SCF/6-31++G(d,p) gave unrealistically long bond lengths, and more acceptable structures were achieved by inclusion of electron correlation at MP2/6-31++G(d,p). At the highest level of theory, Bronsted acidities are calculated within 1 kcal/mol of the experimental values, electron affinities are within 0.06 eV, and heats of formation are within 2.4 kcal/mol. The calculated DELTAH(f)degrees(298) of 105.1 kcal/mol for CCl supports the literature ''estimated'' value of 104 kcal/mol, but the calculated value for CHF (33.9 kcal/mol) is 8 kcal/mol higher than the experimental value.