Microwave Spectra and Barrier to Internal Rotation in Cyclopropylmethylsilane

Rotational spectra for 3 silicon isotopologues (28Si, 29Si, 30Si) of cyclopropylmethylsilane (c-C3H5SiH2CH3) have been observed in natural abundance using Fourier-transform microwave spectroscopy, and the dipole moment of the most abundant (28Si) isotopologue has been determined using the Stark effect. The observed rotational constants (A = 8800.5997(9) MHz; B = 2238.6011(3) MHz; C = 2001.0579(3) MHz) and dipole moment components (μ a = 0.195(2) D, μ b = 0.674(11) D, μ c = 0.362(19) D, μtotal = 0.790(13) D) for the 28Si species are consistent with ab initio predictions (MP2/6-311+G(d)) for a gauche conformation about the Si-cyclopropyl bond. All of the observed transitions were split into doublets due to internal rotation of the methyl group, allowing a determination of the V 3 barrier to internal rotation of 6.671(9) kJ mol−1 for the most abundant isotopologue. This barrier will be compared to those for other Si-CH3 containing compounds and will be related to a partial structure determination from the available microwave and ab initio data.