AN ACCURATE QUARTIC FORCE-FIELD AND VIBRATIONAL FREQUENCIES FOR HNO AND DNO

An accurate ab initio quartic force field for HNO has been determined using the singles and doubles coupled-cluster method that includes a p erturbational estimate of the effects of connected triple excitations, CCSD(T), in conjunction with the correlation consistent polarized val ence triple zeta (cc-pVTZ) basis set. Improved harmonic frequencies we re determined with the cc-pVQZ basis set. Fundamental vibrational freq uencies were determined using a second-order perturbation theory analy sis and also using variational calculations. The N-O stretch and bendi ng fundamentals are determined well from both vibrational analyses. Th e H-N stretch, however, is shown to have an unusually large anharmonic correction, and is not well determined using second-order perturbatio n 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.