A combined analysis of the ν9 band and the far-infrared torsional spectra of ethane

Abstract

Fourier transform measurements of the lowest frequency degenerate fundamental band of CH3CH3 (v9=1←0) in the 12-μm region together with far-infrared torsional spectra have been analyzed to investigate vibration–torsion–rotation effects in a symmetric top molecule. Several spectra of the ν9 band were recorded under different experimental conditions with apodized slit functions of about 0.002 cm−1. Although the intrinsic tunneling splitting in each (J′←J″) doublet in the ν9 band is predicted to be of the order of 0.002 cm−1, in some cases the observed splitting for an intermediate J″ of 20 is several times this value. In extreme cases, splittings of the order of 0.25 cm−1 have been observed. These splittings are caused primarily by the Coriolis interaction between the torsional stack of levels v4=0,1,2,…, for v9=1 and the corresponding stack for the ground vibrational state. Because of a near-degeneracy between the upper level in the ν9 band and its interacting partner (v9=0, v4=3), the (l=−1;K=17,σ=0) torsion–rotation series is resonantly perturbed. For this case, perturbation-allowed v4=3←0 torsional transitions have been identified. Here σ=0, 1, 2, or 3 labels the torsional sublevels. Measurements from the ν9 and 3ν4 bands, frequencies from the far-infrared torsional spectra in the ground vibrational state, and lower state combination differences from ν9+ν4−ν4 band were fitted to within experimental uncertainty using a symmetry adapted effective Hamiltonian which has been used for analyses of similar spectra in methyl silane and CH3CD3. Two Coriolis parameters were determined: the experimental value of ζ̃9z=0.2610(12) is in good agreement with the calculated value of 0.25, whereas the experimental value of ζ̃4,9x=0.2267(20) is about 3 times smaller than the calculated value of 0.60. The theoretical treatment presented here makes use of standard symmetric top formalism and the G36† double-group formalism.