Cryogenic Ion Vibrational Spectroscopy of Protonated Valine: Messenger Tag Effects

We report the infrared photodissociation spectrum of tagged protonated valine in the range 1000-1900 cm^-1, prepared in a cryogenic ion trap. Comparison of experimental results with calculated infrared spectra based on density functional theory shows that the hydroxyl group of the carboxylic acid functionality and the protonated amine group adopt a <i>trans</i> configuration. Nitrogen and methane molecules were used as messenger tags with optimal tagging temperatures of 30 K for N₂ and 60 K for CH₄. While the calculated infrared spectra of the tagged ion suggest only a weak influence of the messenger tag on the frequency positions of ValH⁺, the measured intensities for N₂-tagged ValH⁺ appear strongly suppressed for all but the highest frequency feature at 1773 cm^-1. We trace this behavior to the binding energy of the N₂ tag, which is significantly higher than that of CH₄, based on density functional and coupled cluster calculations and rate estimates for photoinduced unimolecular dissociation from statistical theory.