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The gas-phase structures of alkali-metal cation complexes of serine (Ser) are examined using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser, in conjunction with ab initio calculations. Spectra of Li+(Ser) and Na+(Ser) are similar and relatively simple, whereas Cs+(Ser) includes distinctive new IR bands, and K+(Ser) and Rb+(Ser) exhibit intermediate behavior. Measured IRMPD spectra are compared to spectra calculated at a B3LYP/6-311+G(d,p) level to identify the structures present in the experimental studies. On the basis of these experiments and calculations, the only conformations accessed for the complexes to the smaller alkali-metal cations, Li+ and Na+, are charge-solvated structures involving tridentate coordination to the amine and carbonyl groups of the amino acid backbone and to the hydroxyl group of the side chain, M1[N,CO,OH]. For the cesiated complex, a band corresponding to a zwitterionic structure, ZW[CO2-], is clearly visible. K+(Ser) and Rb+(Ser) exhibit evidence of the charge-solvated analogue of the zwitterions, M3[COOH], in which the metal cation binds to the carboxylic acid group. Calculations indicate that the relative stability of the M3[COOH] structure is very strongly dependent on the size of the metal cation, consistent with the range of conformations observed experimentally.

Experimental cross sections for charge exchange between a negative ion (TCNQF{sub 4}{sup -}, perfluorinated tetracyanoquinodimethane anion) and a xenon atom to produce a dianion (TCNQF{sub 4}{sup 2-}) exhibit a delayed threshold and pronounced oscillations as a function of collision energy. These features are explained by a post-collision-interaction theory which includes phase interferences between the two final state surfaces involving the two Xe{sup +} fine structure levels. In support of this interpretation, charge-exchange cross sections between TCNQF{sub 4}{sup -} and sodium are found to smoothly decline as a function of energy from 5 to 100 keV.