The infrared spectrum of isothiazole in the range 600–1500 cm^–1, including a high-resolution study of the v₁₁ (A') band at 818 cm^–1 and the v₁₆ (A'') band at 727 cm^–1, together with ab initio studies of the full spectrum

The gas-phase high-resolution infrared spectrum of isothiazole in the range 600-1500 cm(-1) has been recorded, and revised band centres obtained for a number of vibrations. An analysis of the nu(11)(A') band at 818 cm(-1) and the nu(16)(A") band at 727cm(-1) has been performed, using the Watson Hamiltonian, A-reduction, IIIr representation. These were combined with previous microwave spectral data to provide combined analyses for rotational constants and quartic centrifugal distortion constants Delta(J), Delta(JK), Delta(K), delta(J) and delta(K). These extend the knowledge derived from previous microwave and IR spectral studies. The equilibrium structures and the derived harmonic frequencies were calculated by ab initio methods, using a variety of basis sets with MP2, MP4 and CCSD(T) methods, and a comparison of these with experimental data is discussed. At a pragmatic level, the closest correlation of the rotational constants with experiment is not obtained with the most sophisticated methodology. Similarly, the vibration frequencies and intensities also vary strongly with the procedure. In particular, we found that the cc-pVTZ + MP2 results probably provide the best numerical comparison with experimental IR data for this molecule.