Ab initio 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 strenghts 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 ²Π^--²Σ⁺ splitting in HCC, the ³Z_u⁺-¹Σ_g ⁺ splitting in C₂, and the isomerization energy for the conversion of H₂CC(¹A₁) to HC≡CH(¹Σ_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.