Quantum-Mechanical Prediction of the Averaged Structure, Anharmonic Vibrational Frequencies, and Intensities of the H 2 O···trans-HONO Complex and Comparison with the Experiment.

The geometrical parameters, the frequencies, and absolute intensities of vibrational transitions of H2 O···trans-HONO hydrogen-bonded complex are calculated using the approach earlier tested in calculations of isolated molecules of nitrous acid and complexes of this acid with ammonia. The equilibrium nuclear configuration and potential energy and dipole moment surfaces are calculated by the MP2/aug-cc-pVTZ method with the basis set superposition error taken into account. The fundamental transition frequencies and intensities of the complex are first obtained in the harmonic approximation, and then the energy values, vibrational wave functions, and transition frequencies and intensities are determined from variational solutions of one- to four-dimensional anharmonic equations. The results obtained are compared with the data calculated in the same approximation for an isolated trans-HONO molecule and the NH3 ···trans-HONO complex. The average discrepancy between the anharmonic frequency values and five available experimental data is 15 cm-1 . Three absorption bands of trans-HONO with the highest intensity are recommended for detecting the presence of H2 O···trans-HONO.