Spectroscopic investigation of hydrogen atom transfer in the McLafferty rearrangement

Abstract

In this study, wavelength-selective infrared multiple-photon photodissociation (WS-IRMPD) was used to study the transfer of an H atom in a gas-phase unimolecular rearrangement reaction. The reaction studied was the McLafferty rearrangement, which involves transfer of a γ-position hydrogen atom to a carbonyl oxygen atom through a cyclic intermediate, followed by the elimination of an alkene. The IRMPD experiment allows for the collection of vibrational spectra of discrete gasphase ions, which can then be compared to those predicted using density functional theory calculations to assign accurate composition and structure. For the experiment, the tert-butyl ester of a model peptide, nicotinic acid-glycine, was incubated in a mixture of deuterium oxide and deuterated methanol. This solution was the used to make ions by electrospray ionization. The McLafferty rearrangement was then induced by collision-induced dissociation. Following rearrangement, the product ion, consisting of the model peptide with 2 deuterium atoms and 1 H atom in exchangeable sites, was investigated by IRMPD. The infrared spectrum produced clearly shows that the H atom is transferred to the C-terminal acid group and no migration to amide positions occurs on the time scale of the experiment.