Perchlorovinylsilane (Cl2CCCl−SiCl3): Conformational Structure, Vibrational Analysis, and Torsional Potential Determined by Gas-Phase Electron Diffraction, ab Initio Calculations, and Variable Temperature Raman Spectroscopy

Abstract

The molecular structure, conformation, vibrational spectra, and torsional potential of perchlorovinylsilane (PCV), Cl2CCCl−SiCl3, have been studied by using gas-phase electron diffraction (GED) data at 100 °C and variable temperature Raman spectroscopy, together with ab initio molecular orbital calculations. The GED data were treated by using a dynamic theoretical model. This involves fitting a chosen two-term potential function to the experimental data, thereby obtaining values for both a 3-fold and a 6-fold potential constant (V3 and V6) in the series V(φ) = 1/2ΣiVi[1 − cos i(180−φ)], where φ is the value of the torsional angle CCSiCl. According to the GED refinements, this molecule exists in the gas phase at 100 °C as a mixture of two minimum-energy conformers, syn (torsional angle φ(CCSiCl) = 0° or 120°) and anti (torsional angle φ(CCSiCl) = 180°), where the anti form predominates, occupying approximately 80% of the gas composition. Relevant structural parameters are as follows (anti): Bond lengths (rg): r(Si−C) = 1.863(13) Å, r〈(Si−Cl)〉 = 2.020(3) Å (average value), r(CC) = 1.349(12) Å. Bond angles (∠α): ∠〈CSiCl〉 = 111.1(15)°, ∠CCSi = 124.0(12)°. Error limits are given as 2σ (σ includes estimates of uncertainties in voltage/height measurements and correlation in the experimental data). The estimated experimental conformational energy difference obtained from GED is ΔE°A-S = −1.04(±0.58) kcal/mol, based on the refined value of the V3 potential constant. From the variable temperature Raman study, two corresponding energy differences obtained from two separate pairs of doublets in the liquid phase are ΔE°A-S = −0.30(1) and ΔE°A-S = −0.43(5) kcal/mol. The ab initio value (HF/6-31G(d)) was ΔE°A-S = −1.43 kcal/mol. All results suggest that anti is the low-energy form. Full geometry optimizations were performed for seven pseudoconformers (including 120° and 180° forms), which were employed in the dynamic GED model, by using the ab initio MO HF/6-31G(d) level of theory. Scaled HF zero-point vibrational energy corrections were estimated from frequency calculations. The theoretical results are compared with experimental observations.