Molecular hydrogen messengers can lead to structural infidelity: A cautionary tale of protonated glycine

Abstract

The effects of tagging protonated glycine with either He or between 1 and 14 H2 molecules on the infrared photodissociation spectra and the ion structure were investigated. Differences in the IR spectra with either a single He atom or H2 molecule attached indicate that even a single H2 molecule can affect the frequencies of some vibrational bands of this simple ion. The protonation site is the preferred location of the tag with He and with up to two H2 molecules, but evidence for H2 attachment to the hydrogen atom of the uncharged carboxylic acid is observed for ions tagged with three or more H2 molecules. This results in a 55 cm(-1) red shift in the carboxylic acid OH stretch, and evidence for some structural isomers where the hydrogen bond between the protonated nitrogen and the carbonyl oxygen is partially broken; as a result H2 molecules attached to this site are observed. These results are supported by theory, which indicates that H2 molecules can effectively break this weak hydrogen bond with three or more H2 molecules. These results indicate that large spectral shifts as a result of H2 molecules attaching to sites remote from the charge can occur and affect stretching frequencies as a result of charge transfer, and that tagging with multiple H2 molecules can change the structure of the ion itself.

Identification of Active Sites and Structural Characterization of Reactive Ionic Intermediates by Cryogenic Ion Trap Vibrational Spectroscopy

“…The influence of the nature of the tagging species has been discussed in quite some detail; see for example the extensive literature on the influence of tagging species on protonated, metal, and metal oxide clusters. Tagging, in the simplest case, can lead to spectral shifts and changes in IR transition intensities.…”

“…[68] There are two alternative, elegantI Ra ction techniques that allow probing in the linear absorption regime without tagging, namely by way of laser induced reaction( LIR) [21] and laser-induced inhibition of complexg rowth (LIICG). [22,69] The influence of the nature of the tagging speciesh as been discussed in quite some detail;s ee for example the extensive literatureo nt he influenceo ft agging species on protonated, [70][71][72][73][74][75][76] metal, [77,78] and metal oxide [49] clusters. Ta gging, in the simplestc ase, can lead to spectral shifts and changes in IR transition intensities.…”

Protonated glycine supramolecular systems: the need for quantum dynamics

“…To point out clearly the importance of quantum mechanical effects in vibrational spectra influenced by proton dynamics, we consider that, recently, Masson, Williams and Rizzo published a series of very interesting IRMPD spectra, where protonated glycine, GlyH + , was tagged by an increasing and controlled number of hydrogen molecules. 3 We focus particularly on two of these investigations. The first one regards protonated glycine solvated by a single hydrogen molecule (GlyH + H 2 ) + , while in the other instance three H 2 molecules are involved (GlyH + 3H 2 ) + .…”

“…4 . 3 Moving to classical simulations, we calculated the classical spectrum, I cl , by means of eqn (2) and report it in panel (c) separately for each mode for a better comparison with the experiment. The main spectroscopic features are reproduced, even if the signal corresponding to the NHa and NHb bands is quite broad.…”

“…4 and 5 lies in the appearance of a second OH stretching band, located at 3491 cm –1 , and labeled as OHr. Masson et al 3 suggested that this band is the result of a configuration where one of the three H 2 molecules interacts with the carboxylic group. Indeed, the peak is red-shifted by about 55 cm –1 with respect to the free OH stretching (the OHb band).…”