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.

The ν12 Band of CH3SiD3

“…Here the coefficient a 12 (E) is the torsional energy in the state (v 12 ϭ 1, v 6 ϭ 0) for a given (arbitrary) value of K averaged (unweighted) over . A similar treatment was found to be necessary for CH 3 SiH 3 (2), CH 3 CD 3 (5), and CH 3 CH 3 (6). From the results of (6).…”

“…A similar treatment was found to be necessary for CH 3 SiH 3 (2), CH 3 CD 3 (5), and CH 3 CH 3 (6). From the results of (6).…”

“…Second, F 0,6J is no longer needed; a similar result was obtained for CH 3 SiH 3 when the Coriolis-type interactions were introduced (2). For the state v 12 ϭ 1, nine constants were used; the parametrization was similar to that in the minimal model used for similar molecules (2,6,5). The values for D 12,K , D 12,Jm , D 12,SJ , F 12,3J , and F 12,3K were constrained to equal their counterparts for the ground vibrational state and A 12,m was set equal to A 12,F ϭ A 12 /(1 Ϫ ) as was done in Ref.…”

“…For example, in the ground torsional state v 6 ϭ 0 of the ground vibrational state (gs) of CH 3 SiD 3 , the torsional splitting is on the order of 200 MHz (1). However, in the zero-point torsional state of the lowest frequency vibrational fundamental, unusually large -splittings were observed for CH 3 SiH 3 (2)(3)(4), CH 3 CD 3 (5), and CH 3 CH 3 (6). This effect was attributed to Coriolis-type coupling between the stacks of torsional levels in the two different vibrational states.…”

Relative high-resolution absorption cross sections of C2H6 at low temperatures

“…This is, in part, due to the ν 4 torsional mode near 35 µm (290 cm −1 ) [28,29,30,31] which produces numerous low frequency hot bands, extensive perturbations and a very dense line structure. The ν 9 mode near 12 µm (830 cm −1 ), often used in remote sensing [32,33], has been the focus of comprehen-sive analyses that have significantly improved line assignments [34,35,36].…”