The structures of proton-bound dimers of glycine with phenylalanine and pentafluorophenylalanine

Infrared multiple photon dissociation spectra of the proton-bound heterodimers of phenylalanine/glycine (Phe·H⁺·Gly) and pentafluorophenylalanine/glycine (F₅-Phe·H⁺·Gly) have been acquired in the 975-1975 cm^-1 region. Exhaustive basin hopping molecular dynamics searches and subsequent DFT calculations predict four low energy intermolecular binding motifs. The spectrum of F₅-Phe·H⁺·Gly is assigned predominantly to isomers exhibiting a Nsbnd H⁺⋯O intermolecular interaction between nitrogen-protonated Gly and the carbonyl oxygen atom of F₅-Phe. In contrast, the spectrum Phe·H⁺·Gly is found to consist of a mixture of isomers exhibiting the Nsbnd H⁺⋯O between nitrogen-protonated Gly and the carbonyl oxygen of neutral Phe, and isomers exhibiting a Nsbnd H⁺⋯N interaction between nitrogen-protonated Phe and the neutral Gly moiety. Cation-π effects also seem to influence the cluster geometries, especially in the case of the Phe·H⁺·Gly global minimum structure where the phenyl ring orients with the site of protonation so as to maximize charge-quadrupole interactions.