Pure Rotational Spectrum and Structural Determination of 1,1-Difluoro-1-silacyclopentane

The ground state, pure rotational spectrum of 1,1-difluoro-1-silacyclopentane has been studied using chirped-pulse, Fourier transform microwave (CP-FTMW) spectroscopy and observed in the 6-20.3 GHz region of the electromagnetic spectrum. This spectrum was acquired leveraging the deep averaging capability of the technique. The parent species, 13C, 29Si, and 30Si singly substituted isotopologues were observed in natural abundance and are reported. Only one conformer, the C2 conformer (half-chair), was observed. This is confirmed with a determined CCCC dihedral angle of -48.1(11)°. The spectrum is comprised of entirely a-type transitions in accordance with quantum chemical calculations. Multiple split transitions are present in the spectrum which have been attributed to a ring-twisting of the carbon atoms attached to the silicon atom in the ring. This motion has the carbons crossing the a-axis in the bc-plane leading to an inversion potential. Potential energy surfaces for the ring-twisting motion were undertaken and the experimentally determined energy level difference observed in comparison to these surfaces is reasonable. A Kraitchman analysis of the experimentally determined, singly substituted isotopologues is in agreement with the optimized, twisted (nonplanar) equilibrium structure. This structure has been compared to other similar silicon-containing ring molecules using second moment arguments and these comparisons are discussed.