Infrared multiple-photon dissociation spectroscopy of cationized glycine: effects of alkali metal cation size on gas-phase conformation

Abstract

The gas-phase structures of cationized glycine (Gly), including complexes with Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺, are examined using infrared multiple-photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser, in conjunction with ab initio calculations. To identify the structures present in the experimental studies, measured IRMPD spectra are compared to spectra calculated at B3LYP/6-311+G(d,p) for the Li⁺, Na⁺, and K⁺ complexes and at B3LYP/def2TZVP for the Rb⁺ and Cs⁺ complexes. Single-point energy calculations were carried out at the B3LYP, B3P86, and MP2(full) levels using the 6-311+G(2d,2p) basis set for Li⁺, Na⁺, K⁺ and the def2TZVPP basis set for Rb⁺ and Cs⁺. The Li⁺ and Na⁺ complexes are identified as metal cation coordination to the amino nitrogen and carbonyl oxygen, [N,CO]-tt, although Na⁺(Gly) may have contributions from additional structures. The heavier metal cations coordinate to either the carbonyl oxygen, [CO]-cc, or the carbonyl oxygen and hydroxy oxygen, [CO,OH]-cc, with the former apparently preferred for Rb⁺ and Cs⁺ and the latter for K⁺. These two structures reside in a double-well potential and different levels of theory predict very different relative stabilities. Some experimental evidence is provided that MP2(full) theory provides the most accurate relative energies.