Ab initio calculations on hono: Energies, geometries and force fields on different levels of theory

Abstract

The geometries of the electronic ground states of cis and trans HONO have been optimized on several levels of theory, and the frequencies and force fields calculated on 5 different levels (including HF, MP2 and CID calculations). The force fields were reduced and then averaged. An estimate of 1302 cm −1 was obtained for the wavenumber of the HON bending band of cis HONO. The geometry of the torsional maximum was optimized on a few levels; its frequencies were calculated using the basis 4-31G. The trans—cis torsional barrier height is estimated as 44 kJ mol −1 (the activation energy of this process being 39 kJ mol −1). Improving the level of calculation did not automatically lead to improved values for the cis—trans energy difference. The possibility of an intramolecular interaction in cis HONO is discussed.