The torsional dependence of the dipole moment operator in methyl silane

Abstract

The torsional dependence of the dipole moment operator μ in methyl silane has been in vestigated in detail. To represent the dependence of μ on torsional state to the order corresponding to quadratic distortion, four torsion or torsion-rotation operators enter involving four dipole distortion parameters: μ 0 T , μ 2 T , μ | T , and ( μ | T - μ ⊥ T ). The Stark effect has been studied for the microwave transitions ( J = 1 ← 0) for states with torsional quantum number v = 0 and 2. From the differences in the effective dipole moments, determinations were made of μ 0 T and of an upper limit to μ 2 T . The weak, perturbation-allowed torsional absorption spectrum has been observed from 150 to 380 cm −1 at a resolution of 0.15 cm −1 using Fourier transform spectroscopy. The spectrum was dominated by five bands for which the torsion-rotation structure was largely resolved: v = 1 ← 0, 2 ← 1, 2 ← 0, 3 ← 1, and 4 ← 2. An intensity analysis yielded values for μ ⊥ T , ( μ | T - μ ⊥ T ), and a linear combination of μ 0 T and μ 2 T . By comparing the present results with earlier precision molecular beam measurements of the effective constants μ ∼ K and μ ∼ J Q that characterize the rotational dependence of the dipole moment for matrix elements diagonal in J, it has been shown that these two constants arise from torsional effects, rather than centrifugal distortion. Only three dipole expansion parameters are required to account for the molecular beam measurements in the ground torsional state, the microwave Stark shifts, and the far-infrared torsional intensities. These are: μ 0 T = 0.0297(10) D, μ ⊥ T = 25.1(1.0) μD, and ( μ | T - μ ⊥ T ) = −93.3(1.1) μD.