Accurate proton affinities: Ab initio proton binding energies for N2, CO, CO2, and CH4

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Abstract
A set of large‐scale ab initio molecular orbital calculations on the title molecules and their protonated forms has been performed. The aim of the present study has been to help establish very accurate absolute proton affinities for each of these molecules. For each molecule a series of calculations was performed using increasingly larger atomic natural orbital (ANO) one‐particle spaces. The energetics of protonation were then evaluated using four methods. These include self‐consistent‐field (SCF), second‐order perturbation theory (MP2), the singles and doubles coupled‐cluster (CCSD) ansatz, and the CCSD(T) method, which includes a perturbational estimate of connected triple excitations. At each of these levels of theory the incompleteness of the one and N‐particle spaces was ascertained by an evaluation of the basis set superposition error (BSSE) for the protonation reaction. We believe that the final proton affinities all attain chemical accuracy in that they contain <1 kcal/mol error. Our results are in extremely good agreement and consistent with the recently revised absolute proton affinity scale.