ABINITIO THERMOCHEMISTRY FOR UNSATURATED C2 HYDROCARBONS

The first systematic study of the sequential bond dissociation energies for ethylene and acetylene, using generalized valence bond (GVB) theory and the correlation-consistent configuration interaction (CCCI) method is presented. The ab initio GVB/CCCI sequential C-H bond strengths in ethylene and acetylene are: D0(H2CCH-H) = 109.5 (110.9 +/- 2) kcal/mol, D0(H2CC-H) = 70.8 (74.4 +/- 3) kcal/mol, D0(HCCH-H) = 37.1 (40.0 +/- 3) kcal/mol, D0(CCH-H) = 86.3 (91.3 +/- 5) kcal/mol, D0(HCC-H) = 129.7 (131.1 +/- 2) kcal/mol, and D0(CC-H) = 95.6 (102.5 +/- 3) kcal/mol. The values in parentheses represent our best estimates for the actual bond strengths, taking into account known systematic and random errors. New predictions for C = C double bond strengths are D0(H2C = CH) = 162.5 (168.2 +/- 6) kcal/mol and D0(H2C = C) = 149.5 (155.5 +/- 5) kcal/mol, while predictions for C = C triple bond strengths are: D0(HC = C) = 160.4 (168.9 +/- 13) kcal/mol and D0(C = C) = 134.7 (129.2 +/- 15) kcal/mol. The adiabatic singlet-triplet splittings for HCCH and H2CC, the 2-PI--2-SIGMA+ splitting in HCC, the 3-SIGMA-u+-1-SIGMA-g+ splitting in C2, and the isomerization energy for the conversion of H2CC(1A1) to HC = CH(1-SIGMA-g+) are predicted to be 83.9, 48.1 +/- 2.5, 14.8, 42.2, and -44.3 kcal/mol, respectively. GVB-PP (perfect pairing) equilibrium geometries and harmonic vibrational frequencies of the above species are also presented. The qualitative trends in adiabatic bond strengths can be well rationalized in terms of two simple quantities: intrinsic (diabatic) bond strengths and final-state relaxation effects.