Broadband microwave spectroscopy of cyclopentylsilane and 1,1,1-trifluorocyclopentylsilane

Abstract

The broadband microwave spectra of two related organosilicon molecules, cyclopentylsilane and 1,1,1-trifluorocyclopentylsilane, have been measured and assigned. Both molecules exhibit a Cs-symmetric “envelope” structure, which is confirmed by a determination of the heavy atom backbone rs substitution structures using isotopic substitution in natural abundance, as well as the presence of strong a-type transitions, weaker c-type, and a non-detection of b-type transitions, indicative of a mirror plane perpendicular to the prolate symmetry axis of the molecule. Both molecules appear to have significant zero-point vibrational averaging of their ring twisting and puckering coordinates, which lead to deviations between the ground state experimental structure and the predicted equilibrium structures at the B3LYP-D3/def2-TZVP level of theory. Additionally, we observe an excited vibrational state associated with this large amplitude motion and we use a simple perturbation theory argument to confirm the identity of these excited state carriers.