Infrared Multiple Photon Dissociation Spectroscopy of Protonated Cyameluric Acid

W. OLMEDO; A. CRUZ ORTIZ; L. JIMENEZ; P. MAITRE; G. A. PINO; M. ROSSA

JOURNAL OF PHYSICAL CHEMISTRY A

AMER CHEMICAL SOC

Lugar: Washington; Año: 2021 vol. 125 p. 607 - 614

1089-5639

The infrared multiple photon dissociation (IRMPD) technique was used to probe the structure of gaseous protonated cyameluric acid [(CA+H)+], as produced under tandem mass spectrometry (MS/MS) conditions through electrospray ionization (ESI). IRMPD is most suitable to investigate the diversity of keto-enol isomerism of (CA+H)+, as arising from a number of precursor CA tautomers present under ESI-MS/MS conditions, and the various protonation sites on available N and O positions for any particular CA tautomer. Experimental IRMPD spectra in the fingerprint (990-1900 cm1) and NH/OH stretching (3300-3650 cm1) regions were recorded at the Centre Laser Infrarouge d´Orsay using the free-electron laser and a tabletop optical parametric oscillator/amplifier laser system, respectively. These were compared with the theoretical [B3LYP/6-311++G(d,p)] IR absorption spectra of a number of planar (CA+H)+ isomers, and it was found that the agreement is most satisfactory for the two lowest-energy monohydroxy rotamers. Such a reduced gaseous (CA+H)+ isomer population, as compared to the keto-enol forms potentially produced in the ESI source, suggests that isomer interconversion reactions could be at play during their transfer into the gas phase. The results provide information at the molecular level on the energetically favored protonation sites for neutral CA.