Vibrational assignment of isoxazole aided by rovibrational data and density functional theory

IR and Raman spectra of isoxazole and D₃-isoxazole are reassigned with the aid of density functional theory calculations at the B3LYP/6-311+G(d,p) level. Harmonic wavenumber values of all non-CH stretch modes give an r.m.s. deviation from experimental values of 3 cm⁻¹ when they are uniformly scaled by 0.9818. Anharmonic values obtained using a second-order perturbative approach show an r.m.s. deviation of 7.6 cm⁻¹. Results of equal quality are obtained for D₃-isoxazole. Predicted Raman and IR band strengths match experimental spectra very closely. Detailed re-examination of rovibrational constants and inertial defect data from high resolution IR spectra, in comparison with values from the anharmonic frequency calculation, provides confirmation of the vibrational assignments. ‘Dark state’ fundamentals ν₁₂ (900.2 cm⁻¹) and ν₁₅ (866 cm⁻¹) are detected through the perturbations they cause in nearby bands.