Proton Affinity and Enthalpy of Formation of Formaldehyde

The proton affinity and the enthalpy of formation of the prototypical carbonyl, formaldehyde, have been determined by the first-principles composite focal-point analysis (FPA) approach. The electronic structure computations employed the all-electron coupled-cluster method with up to single, double, triple, quadruple, and even pentuple excitations. In these computations the aug-cc-p(C)VXZ [X = 2(D), 3(T), 4(Q), 5, and 6] correlation-consistent Gaussian basis sets for C and O were used in conjunction with the corresponding aug-cc-pVXZ (X = 2-6) sets for H. The basis set limit values have been confirmed via explicitly correlated computations. Our FPA study supersedes previous computational work for the proton affinity and to some extent the enthalpy of formation of formaldehyde by accounting for (a) electron correlation beyond the gold standard CCSD(T) level; (b) the non-additivity of core electron correlation effects; (c) scalar relativity; (d) diagonal Born-Oppenheimer corrections computed at a correlated level; (e) anharmonicity of zero-point vibrational energies, based on global potential energy surfaces and variational vibrational computations; and (f) thermal corrections to enthalpies by direct summation over rovibrational energy levels. Our final proton affinities at 298.15 (0.0) K are Delta H-pa degrees (H2CO) = 711.02 (704.98) +/- 0.39 kJ mol(-1). Our final enthalpies of formation at 298.15 (0.0) K are Delta H-f degrees (H2CO) = -109.23 (-105.42) +/- 0.33 kJ mol(-1). The latter values are based on the enthalpy of the H-2 + CO -> H2CO reaction but supported by two further reaction schemes, H2O + C -> H2CO and 2H + C + O -> H2CO. These values, especially Delta H-pa degrees (H2CO), have better accuracy and considerably lower uncertainty than the best previous recommendations and thus should be employed in future studies. (C) 2009 Wiley Periodicals, Inc. Int J Quantum Chem 109:2393-2409, 2009