Calculated OH-Stretching Vibrational Transitions of the Water−Nitric Acid Complex

We have calculated fundamental and overtone OH-stretching vibrational band frequencies and intensities of the water−nitric acid complex. The calculations use the simple harmonically coupled anharmonic oscillator (HCAO) local-mode model with local-mode parameters obtained from scaled ab initio calculations and ab initio-calculated dipole moment functions. The ab initio calculations were performed at the HF, B3LYP, and QCISD levels of theory predominantly with the 6-311++G(2d,2p) basis set. We have compared our results for the water−nitric acid complex with results for the water dimer and the nitric acid and water molecules. The results show that the water−nitric acid complex is more strongly bound, and the changes in spectroscopic properties compared to the individual molecules are more significant than those for the water dimer. The total OH-stretching intensity of the water−nitric acid complex is significantly higher than the sum of the intensities for the individual molecules in the fundamental and higher overtone regions. The transitions associated with the hydrogen-bonded OH bond show a very large red shift compared to the OH-stretching transitions of the nitric acid molecule. These red-shifted bands provide likely spectral regions for the detection of the water−nitric acid complex in the near-infrared. The effect of the water−nitric acid complex on atmospheric OH radial productions and absorption of solar radiation is discussed.