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Abstract

The vibrational spectroscopy of the nitrate-water isotopologues is studied in the O-H and O-D stretching regions using temperature-dependent infrared multiple photon dissociation spectroscopy combined with anharmonic electronic structure calculations. At a temperature of 15 K a series of discrete peaks is observed in the IRMPD spectra of [(NO₃^-)(H₂O)], [(NO₃^-)(HDO)] and [(NO₃^-)(D₂O)]. This structure is considerably more complex than predicted by harmonic calculations. Signal is only observed in the hydrogen-bonded O-H (O-D) stretching region, characteristic of a double hydrogen-bond donor binding motif. With increasing temperature the peaks broaden, leading to a quasi-continuous absorption from 3150-3600 cm^-1 (2300-2700 cm^-1) for [(NO₃^-)(H₂O)] ([(NO₃^-)(D₂O)]) and, above 100 K, an additional band in the free O-H (O-D) stretching region, suggesting the population of complexes containing only a single hydrogen-bond at higher internal energies. Vibrational configuration interaction calculations confirm that much of the structure observed in the IRMPD spectra derives from progressions in the water rock resulting from strong cubic coupling between the O-H (O-D) stretch and water rock degrees of freedom. The spectra of both [(NO₃^-)(H₂O)] and [(NO₃^-)(D₂O)] display a strong peak that does not derive from the water rock progression but results instead from a Fermi resonance between the O-H (O-D) stretch and H-O-H (D-O-D) bend overtone. Additional insight into the nature of the O-H (O-D) stretch and water rocking coupling in these complexes is provided by an effective Hamiltonian that allows for the cubic coupling between the O-H stretch and water rock degrees of freedom.