Structures and spectra of the isomers HNCO, HOCN, HONC, and HCNO from ab initio quantum mechanical calculations

Abstract

Ab initio calculations are reported on the equilibrium geometries, dipole moments, rotational constants, and force constants of the isomers HOCN, HNCO, HCNO, and HONC. The most accurate calculations on the unknown isomer HOCN lead to a predicted geometry of R OH = 0.96 ± 0.005 A ̊ , R OC = 1.302 ± 0.007 A ̊ , R CN = 1.153 ± 0.007 A ̊ , 〈HOC = 109 ± 1°, 〈OCN = 177 ± 1° in a planar trans conformation. This structure has rotational constants B = 10.64 ± 0.12 GHz and C = 10.47 ± 0.12 GHz, accurate enough to encourage a radio search for this species in dense interstellar clouds as well as experiments designed for terrestrial identification. In much less accurate calculations, not extensive enough to provide error bars, an approximate structure for the other unknown isomer, HONC, is R HO = 1.05 A ̊ , R ON = 1.14 A ̊ , R NC = 1.26 A ̊ , 〈HON = 109°, 〈ONC = 172.5°, in a planar trans conformation. The calculated structures of HNCO and HCNO are in good agreement with experiment. The nonlinearity of the heavy-atom axis is firmly established for HOCN, HNCO, and HONC. In all of the molecules the angle of the hydrogen-bending motion is strongly coupled to the angle formed by the heavy atoms. These hydrogen-bending motions are discussed in detail, especially the potential for inversion in HNCO and the quasi-linear potential in HCNO.