IR spectroscopy of isolated neutral and protonated adenine and 9-methyladenine

IR spectroscopy is employed to study isolated adenine and its derivative 9-methyladenine in both their neutral and protonated forms. The IR spectra of neutral adenine and 9-methyladenine are measured in a molecular beam expansion via IR-UV ion-dip spectroscopy in the 525 to 1750 cm^-1 region. For adenine, UV excitation selects the 9H tautomer to give a conformer-selective IR spectrum. For 9-methyladenine, only one tautomer exists because of the methyl substitution at the N(9) position. The experimental spectra agree closely with spectra computed for these tautomers at the B3LYP/6-311++G(df,pd) level of theory. These spectra complement previous tautomer-specific IR spectra in the hydrogen stretching range. The 9H-adenine spectrum obtained is compared to a previously recorded FTIR spectrum of adenine at 280 °C, which shows close agreement, although the 7H tautomer cannot be excluded from contributing. Protonated adenine and 9-methyladenine are generated by electrospray ionization and studied via IR multiple-photon dissociation (IRMPD) spectroscopy. Comparison of the experimental spectra with computed spectra allows identification of the protonation site, which suggests that the 1-9 tautomer is the dominant contributor to the spectra. The adenine tautomer puzzle: The IR spectra of isolated adenine and 9-methyladenine obtained in the 500 to 1750 cm^-1 region are in excellent agreement with the theoretical calculations of 9H-adenine and 9-methyladenine, respectively. Good agreement with a previously recorded FTIR spectrum is observed for measurements in the NH stretching region (see picture). Tautomeric dominance of protonated adenine is investigated by IR multiple-photon dissociation spectroscopy.