Nitric Acid−Water Complexes:  Theoretical Calculations and Comparison to Experiment

The formation of HNO 3 ·(H 2 O) n complexes is studied both theoretically and experimentally. First principles electronic structure calculations were used to produce minimum structures and harmonic vibrational frequencies of HNO 3 ·(H 2 O) n and DNO 3 ·(H 2 O) n complexes ( n = 0−4). They also provide insight into the ionization of HNO 3 in water, predicting that ionization in isolated clusters occurs for n = 4 or larger. Vibrational absorption spectra of matrix isolated nitric acid/water complexes were obtained using an FTIR spectrometer-based instrument. By incrementally increasing the amount of H 2 O in the matrix, we have been able to study nitric acid/water complexes and the ionization of HNO 3 . New spectral assignments, the first ones for the n = 2 and 3 complexes, are suggested on the basis of the results of the electronic structure calculations.