Interactions in symmetric top molecules between vibrational polyads: rotational and rovibrational spectroscopy of low-lying states of propyne, H3C–C≡CH

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More than 700 new pure rotational transitions (Δν=0, ΔJ=1, Δk=0) pertaining to the vibrational states ν 10=1, 2 and ν 9=l have been recorded between 85 and 922GHz corresponding to 4≤J″≤53. In the case of ν 10=1, high-K transitions could be recorded up to Kl 10=−17 and +19, respectively, only limited by the vanishing intensities of these lines involving highly excited states. Similarly, the ν 9=1+1 subbranch could be followed up to K=18. Investigations of the other subbranches were limited to K values below those severely perturbed by Fermi interactions with vibrational states of the next tetrad approximately 350cm−1 higher in vibrational energy. Moreover, perturbations in the K=8 stack of ν 5=1 could be traced to a intervibrational q 12 resonance with ν 10=2−2, K=9. Several highly perturbed transitions, including transitions between the two vibrational states, have been recorded in the submillimetre range and fit to within experimental uncertainties. The intervibrational transitions also permitted one to improve the determination of the K level structure in the ground vibrational state. The analysis of a previously recorded high resolution infrared spectrum of the ν 10 region around 331cm−1 has been extended to high-K values for the cold ν 10 band and to a substantial portion of the 2ν 10−ν 10 hot band. Furthermore, new high resolution infrared spectra of propyne have been recorded around 650, 1000 and 1350cm−1. Assignments for the ν 9 and ν 5 bands with origins at 639 and 930cm−1 encompass those of previous studies. The 2ν 10, l=0 subband at 651cm−1 has been assigned for the first time. The global fit of pure rotational and rovibrational data obtained in the present study, combined with previous data for the ν 10=1 state and the ν 5=1, ν 10=ν 9=1, ν 10=3 and ν 8=1 tetrad, has resulted in the first quantitative descriptions of the Fermi-type interaction between ν 9=1 and ν 10=ν 9=1 as well as between ν 10=2 and ν 10=3 and of Coriolis type interactions between ν 10=2 and ν 9=1 and ν 5=1, respectively. In addition, the combined analysis yielded improved spectroscopic parameters for the Fermi resonances between ν 10=1 and v 10 =2 and between ν 9=1 and ν 10=2, as well as for the Coriolis interaction between ν 10=1 and ν 9=1.