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.

Automated summary

This study presents the first structural characterization of protonated cyameluric acid ([CA + H](+)) in the gas phase, revealing that the available O positions on neutral CA are energetically favored protonation sites. Ab initio computations predict different reduced isomer populations of [CA + H](+) in solution and gas phases, indicating the occurrence of isomer interconversion reactions during their transfer into the gas phase.