Vibrational spectroscopy of xanthine in superfluid helium nanodroplets

Mid-infrared spectra are employed to identify and characterize the structure and specific vibrational modes of xanthine monomer in helium nanodroplets. The predicted NH vibrational frequencies of xanthine show strong dependence on the level of theory and the basis set used in this study. However, the measurement of the angle between the permanent and corresponding vibrational transition dipole moment, called the vibrational transition moment angle (VTMA), gives unambiguous and straightforward assignments to the specific vibrational bands of xanthine. In this work, the global minimum tautomer of xanthine is observed and characterized using ab initio calculations and VTMAs for the various vibrational modes of the xanthine monomer.