Coupled Cluster Theory Determination of the Heats of Formation of Combustion-Related Compounds:  CO, HCO, CO₂, HCO₂, HOCO, HC(O)OH, and HC(O)OOH

Coupled cluster theory through quasiperturbative, connected triple excitations was used to obtain optimized structures, harmonic vibrational frequencies, and heats of formation for seven small molecules important to hydrocarbon oxidation. For the three systems possessing reliable experimental heats of formation, the level of agreement between theory and experiment was excellent. To achieve this level of agreement and to simultaneously minimize the theoretical uncertainty, it was necessary to apply large correlation consistent basis sets (through septuple ζ in some cases) followed by a number of small, but nonnegligible, energetic corrections. For CO, Δ H f 0 (0 K) = −27.0 ± 0.2 (theory) versus −27.20 ± 0.04 kcal/mol (expt). For CO 2, Δ H f 0 (0 K) = −93.7 ± 0.2 (theory) versus −93.97 ± 0.01 kcal/mol (expt). For HC(O)OH (formic acid), Δ H f 0 (0 K) = −88.9 ± 0.4 (theory) versus −88.7 ± 0.1 kcal/mol (expt). For HCO, the experimental and theoretical values are in near perfect agreement, with Δ H f 0 (0 K) = 10.4 ± 0.2 (theory) versus 10.3 ± 2 kcal/mol (expt), although this may be somewhat fortuitous because the experimental value has a large uncertainty. For trans -HOCO, we predict a value of Δ H f 0 (0 K) = −43.9 ± 0.5 kcal/mol, compared to the revised photoionization value of ≥−45.8 ± 0.7 kcal/mol. Theory, however, is in good agreement with the possible experimental value of −42.7 ± 0.9 kcal/mol suggested in the same photoionization experimental analysis. For HCO 2, theory predicts a value of Δ H f 0 (0 K) = −29.3 ± 0.4 versus −30 ± 3 kcal/mol for a recent negative-ion photoelectron measurement. For HC(O)OOH, in which case no experimental data exists, Δ H f 0 (0 K) = −65.6 ± 0.6 kcal/mol. trans -HOCO is only slightly bound (1.1 kcal/mol) with respect to the H + CO 2 asymptote. HCO 2 is 15.7 kcal/mol higher in energy than trans -HOCO and lies above the H + CO 2 asymptote by 14.6 kcal/mol. It is only bound with respect to the OH + CO asymptote by 9.0 kcal/mol. Three widely used parametrized methods (G2, G3, and CBS-Q) were compared to the best coupled cluster heats of formation and found to differ by up to 3.2 kcal/mol.