A quantitative basis for a scale of Na⁺ affinities of organic and small biological molecules in the gas phase

High-pressure mass spectrometric experiments and ab initio calculations have been carried out in order to establish a series of accurate gas-phase sodium ion affinities of organic molecules with a wide variety of functional groups. Ab initio calculations have also been performed on the sodium complexes of three amino acids: serine, cysteine, and proline. A systematic critical evaluation of experimental and computational literature results shows that a significant number require revision. Based on comparisons with accurate experimental measurements, the ab initio procedure used is shown to yield sodium ion affinities with an accuracy of ca. 1 kcal · mol^-1. This enables the construction of the first reliable table of gas-phase Na⁺ affinities for organic and small biological molecules.