The structures of three proton-bound dimers (Met 2 H + , MetTrpH + , and Trp 2 H + ) are investigated in the gas phase with infrared multiple photon disassociation (IRMPD) spectroscopy in combination with quantum chemical calculations. Their IRMPD spectra in the range of 600−1850 cm −1 are obtained experimentally using an FT-ICR mass spectrometer and the CLIO free electron laser as an IR light source. The most abundant conformers are elucidated by comparing the IRMPD spectra with harmonic frequencies obtained at the B3LYP-GD3BJ/6-311++G** level of theory. Discrepancies between the experimental and theoretical data in the region of 1500−1700 cm −1 are attributed to the anharmonicity of the amino bending modes. We confirm the result of a previous IRMPD study that the structure of gas-phase Trp 2 H + is charge-solvated but find that there are more stable structures than originally reported (Feng, R.; Yin, H.; Kong, X. Rapid Commun. Mass Spectrom. 2016, 30, 24−28). In addition, gas-phase Met 2 H + and MetTrpH + have been revealed to have charge-solvated structures. For all three dimers, the most stable conformer is found to be of type A. The spectrum of Met 2 H + , however, cannot be explained without some abundance of type B charge-solvated conformers as well as salt-bridged structures.

Proton‐bound homochiral and heterochiral dimers, X‐H+‐X, of five amino acids (X = Ser, Ala, Thr, Phe, and Arg) are investigated theoretically using quantum chemical density functional theory (DFT) calculations and molecular dynamics simulations with the aim to unveil diastereomer‐specific mid‐infrared (mid‐IR) absorption bands in the spectral range of 1000 to 1800 cm−1. The theoretical calculations performed in this work imply that all systems, except Ala2H+, have distinct mid‐IR absorption bands in homochiral and heterochiral configurations, which make them appropriate systems to be studied experimentally with mid‐IR spectroscopy. We show that intermolecular interaction with the side chain, in the form of hydrogen bonding or cation‐π interaction, is necessary for chiral effects to be present in the mid‐IR spectra of proton‐bound dimers of amino acids. We also report new conformers for Ala2H+, Thr2H+, Phe2H+, and Arg2H+, which were not found in earlier studies of these dimers.

We investigate gas-phase structures of homo- and heterochiral asparagine proton-bound dimers with infrared multiphoton dissociation (IRMPD) spectroscopy and quantum-chemical calculations. Their IRMPD spectra are recorded at room temperature in the range of 500–1875 and 3000–3600 cm –1 . Both varieties of asparagine dimers are found to be charge-solvated based on their IRMPD spectra. The location of the principal intramolecular H-bond is discussed in light of harmonic frequency analyses using the B3LYP functional with GD3BJ empirical dispersion. Contrary to theoretical analyses, the two spectra are very similar.

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