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.

The complexes formed by alkali metal cations (Cat ) and glycine (Gly) were studied by means of ab initio quantum chemical methods. Seven types of Gly ± Cat interaction were considered in each case. It was found that in the most stable forms of Gly ± Li and Gly ± Na the metal ion is chelated between the carbonyl oxygen and nitrogen ends of glycine. For Gly ± K an isomer involving complexation with both oxygens of the carboxylic function is found to be degenerate with the above chelate, and becomes slightly more stable for Gly ± Rb and Gly ± Cs. In all cases, interaction of the ion with the carboxylate group of zwitterionic glycine is also low in energy. Computed binding energies (DH 298 , kcal mol À1