SiC2: A molecular pinwheel

Abstract

We present the results of a combined experimental and theoretical study of the large-amplitude motion in SiC2 in which the C2 fragment undergoes hindered internal rotation. Stimulated emission pumping (SEP) is used to obtain rovibrational term energies for levels with up to 14 quanta of excitation in the large-amplitude vibration. We analyze the SEP data, as well as other available experimental data, using a semirigid bender model that allows for complete internal rotation within a triatomic molecule. From the least-squares fitting of this model to the data, we determine the potential energy along the minimum energy path of the large-amplitude vibration, the harmonic energies of the small-amplitude vibrations, and the variations of these energies and of the molecular geometry with the large-amplitude coordinate. The fitting is aided by results obtained from ab initio calculations we perform on the triangular and linear configurations of the molecule. The current data set is consistent with a large-amplitude potential energy function in which the energy difference between the triangular and linear configurations is 1883 cm−1. The statistical error on this energy difference is 22 cm−1, but we estimate the physical uncertainty to be about 200 cm−1. This result is in excellent agreement with the energy difference of 1819 cm−1 we obtain in our best ab initio calculations. The semirigid bender fitting and our best ab initio calculations are also both consistent with a potential energy function having no local minimum at linearity.