Rotational spectrum, internal dynamics, and molecular structure of methylphenylsilane

The rotational spectra of two isotopologues of methylphenylsilane were measured and assigned by using the supersonic-jet Fourier transform microwave spectroscopic technique in the 2.0-20.5 GHz range. The feature of rotational spectra of methylphenylsilane indicates that the doublets of rotational transitions in methylphenylsilane are contributable to the methyl internal rotation with a V₃ barrier of 559 (25) cm^-1. No splitting has been observed due to the methyl internal rotation in its carbon analog, ethylbenzene, which indicated that the barrier to such motion should be higher than 700 cm^-1. Silicon incorporation of the ethyl group in ethylbenzene leads to a much lower barrier to the methyl internal rotation.