Ab initio study of structure and vibrational spectra of SN⁺, SN, and S₂N₂

Using large Gaussian basis sets ab initio calculations on the molecules SN⁺, SN, and S₂N₂ have been performed at the SCF level. Additionally, electron correlation effects were studied for SN ⁺ and S₂N₂ by the PNO-CI and CEPA-PNO methods. The following equilibrium geometries were obtained: SN⁺, r _SN^e =2.64 (SCF), 2.72 a.u. (CEPA); SN, r_SN ^e=2.78 a.u. (SCF); S₂N₂, D_2h symmetry, r_SN^e=3.07 (SCF), 3.16 a.u. (CEPA), ≮ NSN=90.2° (SCF), 90.5° (CEPA). Complete harmonic force fields of S ₂N₂ and the corresponding vibrational frequencies were computed. The assignments of the experimental ir and Raman spectra for S ₂N₂ are discussed critically on the basis of the theoretical results.