Theoretical study of the nitrous acid conformers:: comparison of theoretical and experimental structures, relative energies, barrier to rotation and vibrational frequencies

The experimental and theoretical literature data for the trans and cis conformers of nitrous acid (HONO) are augmented by additional Hartree-Fock (HF), Moller-Plesset second-order perturbation (MP2), Moller-Plesset fourth-order perturbation (MP4) and density functional (B3LYP) computations. The latter yield optimized theoretical parameters and vibrational frequencies that are closest to the best experimental values. Adding diffuse functions to a given basis set lowers the energy of the trans conformer relative to the cis at all levels of theory utilized in this work. There have been no convincing assignments of infrared (IR) spectral bands to the O-N=O (v(3)) and H-O-N (v(5)) bending modes of cis-HONO, both of which are predicted to have very low intensities. Although IR spectral features about 30 cm(-1) below v(3) for trans-MONO has been tentatively assigned to v(3) of cis-MONO, calculations with unsealed and scaled quantum-mechanical force fields invert this order. If these predictions are correct, v(3) of cis-HONO would fall in the spectral region around 1300 cm(-1), where a number of other compounds containing N and O atoms have IR bands (N2O, N2O4, etc.) and make it difficult to observe. The frequency calculations for cis-HONO, with all the unsealed HF force fields, yield v(5) > v(6) (the torsional mode). The situation is reversed for all the frequency calculations with the unsealed B3LYP and MP force fields. Transferring the scale factors obtained for the trans-MONO conformer HF/6-311G** force field to the corresponding force field of cis-HONO yields v(5) = 590 cm(-1) and v(6) = 673 cm(-1), a very good indication that v(5) is indeed at a lower frequency than v(6) for the cis conformer.