A high-resolution infrared study of five bands of 1,2,5-thiadiazole in the range 750-1250 cm−1, together with ab initio and DFT studies

The Fourier transform gas-phase IR spectrum of 1,2,5-thiadiazole, C2H2N2S, has been recorded with a resolution of ca. 0.003 cm−1 in the wavenumber region 750-1250 cm−1. Five fundamental bands in this region, ν4 (A1), ν5 (A1), ν11 (B1), ν13 (B1), and ν14 (B2), have been analysed by the Watson Hamiltonian model to yield ground-state rotational and quartic centrifugal distortion constants as well as upper-state spectroscopic constants. A global perturbation of the ν4 level is explained by Fermi resonance with the 2ν15 level which has been located from its resonance effect. Rotational constants, harmonic and anharmonic frequencies have been calculated using a cc-pVTZ basis, at the MP2 and B3LYP methodology levels, and compared with the experimental data.