Computational Infrared Spectroscopy of HNO Beyond the Double Harmonic Approximation

This work explores, for the first time, the calculation of HNO molar absorptivities using methods that go beyond the double harmonic approximation. Accurate molar absorptivities of HNO are essential for the kinetic interpretation of recent experiments that studied tunneling-mediated chemistry of H + NO reactions at low temperatures. This work will show that the double harmonic approximation for computing molar absorptivities is not sufficient to obtain accurate molar absorptivities for HNO. Since the only published molar absorptivities for HNO are computed using the double harmonic approximation, they cannot be used to interpret the experimental data.First, we compute the molar absorptivity values using the double harmonic approximation with several combinations of basis sets and theories. With the double harmonic approximation, we also calculate the vibrational frequency shifts with isotopic substitutions of the individual atoms within the HNO molecule. After comparing double harmonic results to experimental results, we consider extensions to this approximation. By including anharmonic effects, we first consider electrical anharmonicity alone, and then consider the combination of electrical and mechanical anharmonicity. With the inclusion of both forms of anharmonicity, the molar absorptivity values change greatly from the double harmonic molar absorptivity values. Lastly effects of mechanical coupling are investigated. Results from this investigation encourage future work that would include mechanical coupling, and also consider electrical coupling, in accurate calculations of HNO molar absorptivities.This work will show that large basis sets are required for the theoretical calculation of the molar absorptivities of HNO, and that the double harmonic approximation is unreliable for this molecule. In particular, the NH stretching mode is very anharmonic. The computed isotopic shifts in the vibrational frequencies of HNO are found to be very sensitive to the way in which electron-electron correlation is treated. This indicates that very high level electron correlation models may be important for accurate theoretical studies of HNO. Finally, the extension beyond the double harmonic approximation and treatment with anharmonic effects are proven to be extremely valuable in describing the true mechanical and electrical properties of HNO.