Structure and hydrogen-bond vibrations of water complexes of azaaromatic compounds: 7-(3'-pyridyl)indole.

Complexes with water have been studied in the regime of supersonic jet isolation for 7-(3'-pyridyl)indole, a bifunctional molecule possessing hydrogen bond donor and acceptor groups. Two rotameric forms, syn and anti, are possible, of which only the former is able to form cyclic hydrogen bonds with protic solvents. Infrared-induced ion depletion spectroscopy was used to obtain vibrational patterns for 1:1 and 1:2 complexes in the hydrogen bond stretching region. The analysis of the spectra, supported by DFT calculations, revealed that for both stoichiometries the dominant forms correspond to cyclic, doubly or triply hydrogen-bonded species. The frequencies of NH...O, OH...N, and OH...O stretching vibrations were compared with the literature data to assess the strength of single vs multiple hydrogen bonds. Several new assignments and reassignments were proposed.