Vibrational Spectroscopy of the Water–Nitrate Complex in the O–H Stretching Region

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 calculations of the anharmonic spectra. 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. A 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 to 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 rocking mode 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