Anchoring the Absolute Proton Affinity Scale

Converged first-principles proton affinities (PA) of ammonia and carbon monoxide have been determined by the focal-point analysis (FPA) approach, thus fixing the high and low ends of the molecular proton affinity scale. The electronic structure computations employed the all-electron (AE) coupled-cluster (CC) method up to single, double, triple, quadruple, and pentuple excitations. Aug-cc-pCVXZ [X = 2(D), 3(T), 4(Q), 5, and 6] correlation-consistent (cc) Gaussian basis sets for C, N, and O were used in conjunction with the corresponding aug-cc-pVXZ (X = 2−6) sets for H. Our FPA study supersedes previous computational work by accounting for (a) electron correlation beyond the “gold standard” CCSD(T) level; (b) the nonadditivity of core electron correlation effects; (c) scalar relativity; (d) diagonal Born−Oppenheimer corrections (DBOC); (e) anharmonicity of zero-point vibrational energies, based on accurate AE-CCSD(T)/cc-pCVQZ internal coordinate quartic force fields and fully 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 Δpa H°(NH3) = 852.6(846.4) ± 0.3 kJ mol−1 and Δpa H°(CO) = 592.4(586.5) ± 0.2 kJ mol−1. These values have better accuracy and considerably lower uncertainty than the best previous recommendations and thus anchor the proton affinity scale of molecules for future use.