The lowest frequency parallel fundamental band ν5 of CH3SiH3 near 700 cm−1 has been measured at a resolution of 0.004 cm−1 with Fourier transform spectroscopy to investigate vibration–torsion–rotation interactions in symmetric tops. The torsional splittings in the spectrum are increased from ∼0.005 cm−1 to ∼1 cm−1 by Fermi-type vibration–torsion interactions between the torsional stack (v6=0,1,2,…) in the ground vibrational state and the corresponding stack for v5=1. Resonant interactions were observed between the states (v5=1,v6=0) and (v5=0,v6=5) for the rotational series with (k=±1,σ=∓1), where σ labels the torsional sublevels. In this resonance, the two unperturbed states are near opposite limits for torsional motion: (v5=0,v6=5) involves nearly free rotation, while (v5=1,v6=0) involves small amplitude torsional oscillation. For the (k=±1,σ=∓1) rotational series, perturbation-allowed transitions in the high overtone (v6=5←0) were observed. Over 750 frequencies measured here have been analyzed together with more than 2500 measurements involved in the recent analysis of the lowest-lying degenerate fundamental band ν12 given by Moazzen-Ahmadi et al. [J. Mol. Spectrosc. 175, 54 (1996)]. A fit to within experimental error was achieved using 41 parameters, an increase of only 4 when the new band is added. The analysis shows that the inclusion of the Fermi-type interactions leads to a considerable simplification of the Hamiltonian for the ground vibrational state. For example, both the second and third terms (V0,6,V0,9) in the Fourier expansion of the hindering potential as well as the torsional flexing term (F0,m) vanish in the ground state. The changes in the leading terms in the torsional Hamiltonian have been quantitatively explained by a contact transformation. The large perturbations produced by the interaction matrix elements off-diagonal by 5 units in v6 have serious implications for vibrational relaxation in molecules undergoing internal rotation.

No 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.

No abstract

No abstract