Infrared Multiple Photon Dissociation Spectroscopy of Protonated Cyameluric Acid

The present study reports the first structural characterization of protonated cyameluric acid ([CA + H]+) in the gas phase, which paves the way for prospective bottom-up research on the condensed-phase chemistry of CA in the protonated form. A number of [CA + H]+ keto-enol isomers can a priori be produced as a result of protonation at available N and O positions of precursor neutral CA tautomers, yet ab initio computations predict different reduced [CA + H]+ isomer populations dominating the solution and gas phases that are involved in the ion generation process (i.e., electrospray ionization). Infrared multiple photon dissociation spectra were recorded in the 990–1900 and 3300–3650 cm–1 regions and compared with theoretical [B3LYP/6-311++G­(d,p)] IR absorption spectra of several [CA + H]+ isomers, providing a satisfactory agreement for the most stable monohydroxy form in the gas phase, [1358a]+, yet the contribution of its nearly isoenergetic OH rotamer, [1358b]+, cannot be neglected. This is indicative of the occurrence of [CA + H]+ isomer interconversion reactions, assisted by protic solvent molecules, during their transfer into the gas phase. The results suggest that available O positions on neutral CA are energetically favored protonation sites in the gas phase.