The ro-vibrational spectrum of propylene oxide in the methyl rocking fundamental band ν17 = 1 is reported for the first time, based on high-resolution infrared measurements at room temperature and under supersonic jet-cooled conditions. The band origin and symmetry-resolved molecular parameters were determined with high precision. Unexpectedly large splittings into A- and E-symmetry components are observed in the jet-cooled spectrum, indicating perturbative coupling of the ν17 state with nearby torsionally excited dark states. The spectra were analysed using an effective Hamiltonian approach implemented in the newly developed General Fitting Code (GFC), which enables a consistent treatment of rovibrational and torsional interactions. In addition, previously published microwave, millimetre- and submillimetre-wave spectra of the ground torsional state and the first torsionally excited state ν24 = 1 were reanalysed using refined data sets. A simultaneous treatment of both torsional states allows an independent determination of the internal rotation constant F0 and the threefold torsional barrier height V3, as well as improved values for higher-order barrier terms.
Vávra et al. (Thu,) studied this question.