The synthesis, vibrational spectra, and structure of 1,3-disilacyclobutane-d₀ and -d₄

Abstract

In order to gain further insight into the forces governing the conformation of cyclobutane and its analogs, the molecules 1,3-disilacyclobutane and 1,3-disilacyclobutane-1,1,3,3-d₄ were prepared and studied for the first time. High resolution far-infrared and low frequency Raman spectra were recorded and tabulated between 200 and 30 cm⁻¹ for the molecules in the gaseous phase. A large number of transitions from excited states of the low frequency ring bending mode were observed, and from them, an accurate one-dimensional potential energy function was determined. Use of a reduced mass model which included the in phase SiH₂ rocking motion gave the best agreement with the observed isotopic shifts in the puckering transitions. The barrier height of the molecules was found to be only 87 cm⁻¹ (0.25 kcal) with the molecule puckered in the ground state by 24 degrees. High resolution infrared and Raman spectra were recorded and tabulated for both molecules between 4000 and 200 cm⁻¹ in gaseous, liquid, and solid phases. The resulting vibrational analysis, supported by valence force field calculations, well characterized all the remaining vibrational modes of the hydride and deuteride. This study clearly showed that 1,3-disilacyclobutanes have a very floppy but puckered structure in gas and liquid phases and are compelled by intermolecular forces to become planar in the solid phase. The resulting change of symmetry when entering the solid was observed to uncouple the ring deformation and in phase rock, a finding which was confirmed also by the force field calculation. In addition, a new synthetic route for the preparation of a large number of new isotopically substituted molecules was outlined and investigated. The far-infrared spectrum of l-silacyclobutane-l-d₁ was recorded.