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 ΔνOH of the hydrogen-bonded O-H stretches, 671 cm-1 (HOH···NC-) and 812 cm-1 (HOH···CN-), confirm the universal correlation of ΔνOH with the corresponding proton affinities.