Proton affinities of candidates for positively charged ambient ions in boreal forests

Abstract. The optimized structures and proton affinities of a total of 81 nitrogen-containing bases, chosen based on field measurements of ambient positive ions, were studied using the CBS-QB3 quantum chemical method. The results were compared to values given in the National Institute of Standards and Technology (NIST) Chemistry WebBook in cases where a value was listed. The computed values show good agreement with the values listed in NIST. Grouping the molecules based on their molecular formula, the largest calculated proton affinities for each group were also compared with experimentally observed ambient cation concentrations in a boreal forest. This comparison allows us to draw qualitative conclusions about the relative ambient concentrations of different nitrogen-containing organic base molecules.