Isolating the Isomeric Hydrogen Bonding Signatures of the Cyanide–Water Complex by Cryogenic Ion Trap Vibrational Spectroscopy

The vibrational spectroscopy of the cyanide–water complex and its fully deuterated isotopologue is studied in the spectral range from 800 to 3800 cm –1 . Infrared/infrared double-resonance population labeling spectroscopy of the cryogenically cooled, messenger-tagged complexes isolates the spectral signature of the two quasi-isoenergetic, singly hydrogen-bonded isomers HOH···NC – and HOH···CN – . The infrared photodissociation spectra are assigned based on a comparison to simulated anharmonic spectra. Infrared multiple photon dissociation spectra in the temperature range from 6 to 300 K confirm the stability of the two isomers at lower temperatures and provide evidence for a considerably more dynamic structure, also involving doubly hydrogen-bonded configurations, at higher internal energies. The observed red shifts Δν OH of the hydrogen-bonded O–H stretches, 671 cm –1 (HOH···NC – ) and 812 cm –1 (HOH···CN – ), confirm the universal correlation of Δν OH with the corresponding proton affinities.