Mass-analyzed threshold ionization spectroscopy of the selected rotamers of hydroquinone and p-dimethoxybenzene cations

Mass-analyzed threshold ionization (MATI) spectroscopy in conjunction with two-color resonant two-photon excitation was used to investigate the ionic properties of the selected rotamers of hydroquinone (HYQ) and p-dimethoxybenzene (PDMB). The observed lower adiabatic ionization energy for the trans conformer is in good agreement with that predicted by ab initio and density functional calculations. With the exception of mode 6a of PDMB, the frequencies of the observed cation vibrations are close to each other for the two rotamers. Detailed analyses on the atomic displacement based on ab initio calculations show that the vibrational frequency of each mode can reflect the extent of the OH and OCH3 substitutents' involvement in the overall molecular motions. The present experiments also demonstrate that a particular conformer can be selected from a chemical sample for recording the vibrational spectra of the cations, In principle, this method can be used not only for species identification but also for separation of molecular conformers.