Probing the structure and dynamics of the heterocyclic PAH xanthene and its water complexes with infrared and microwave spectroscopy.

To assess the presence of oxygen-containing polycyclic aromatic hydrocarbons (OPAHs) in the interstellar medium and understand how water aggregates on an OPAH surface, we present a comprehensive gas-phase spectroscopy investigation of the OPAH xanthene (CHO) and its complexes with water using IR-UV ion dip spectroscopy and chirped-pulse Fourier transform microwave spectroscopy. The infrared spectrum of xanthene shows weak features at 3.42, 3.43, and 3.47 μm, which have been suggested to partly originate from vibrational modes of PAHs containing sp hybridized carbon atoms, in agreement with the molecular structure of xanthene. The high resolution of rotational spectroscopy reveals a tunneling splitting of the rotational transitions, which can be explained with an out-of-plane bending motion of the two lateral benzene rings of xanthene. The nature of the tunnelling motion is elucidated by observing a similar splitting pattern in the rotational transitions of the singly-substituted C isotopologues. The rotational spectroscopy investigation is extended to hydrates of xanthene with up to four water molecules. Different xanthene-water binding motifs are observed based on the degree of hydration, with O-H⋯π interactions becoming preferred over O-H⋯O interactions as the degree of hydration increases. A structural comparison with water complexes of related molecular systems highlights the impact of the substrate's shape and chemical composition on the arrangement of the surrounding water molecules.