Microwave spectrum of the C₃S molecule in the vibrationally excited states of bending modes, v₄ and v₅

Rotational spectra of the linear C₃S molecule in the excited bending vibrational states ν₅ = 1, 2, 3, and 4 (CCS bending) and ν₄ = 1 (CCC bending) have been observed in the frequency range 116-383 GHz with a source-modulated microwave spectrometer. Molecular constants of vibration-rotation interaction terms up to higher orders were determined by the least-squares fitting of about 460 transition frequencies. The determined α_t, q_t, and q^J_t constants showed a good coincidence with those of ab initio calculation. The sign and magnitude of γ_ll, the dependence of the rotational constant B_v on vibrational angular momentum, was explained by a symmetric-top-like molecular model. The constant q^K_t, the dependence of the l-type doubling and resonance constant q_t on vibrational angular momentum, was determined experimentally for the first time. The obtained value of q^K_t was a thousand times greater than the theoretical estimate given by Watson′s expression, but the order of magnitude of the experimental value can be understood by an analogy with a slightly asymmetric-top molecule.