An accurate quartic force field and vibrational frequencies for HNO and DNO

Abstract

An accurate ab initio quartic force field for HNO has been determined using the singles and doubles coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations, CCSD(T), in conjunction with the correlation consistent polarized valence triple zeta (cc-pVTZ) basis set. Improved harmonic frequencies were determined with the cc-pVQZ basis set. Fundamental vibrational frequencies were determined using a second-order perturbation theory analysis and also using variational calculations. The N–O stretch and bending fundamentals are determined well from both vibrational analyses. The H–N stretch, however, is shown to have an unusually large anharmonic correction, and is not well determined using second-order perturbation theory. The H–N fundamental is well determined from the variational calculations, demonstrating the quality of the ab initio quartic force field. The zero-point energy of HNO that should be used in isodesmic reactions is also discussed.