A gas-phase electron diffraction and quantum chemical investigation of the molecular structure of 1-bromosilacyclobutane

The gas-phase electron diffraction experiment has shown that 1-monobromosilacyclobutane (MBSCB) exists in two conformational forms, the axial and equatorial with a significantly higher prevalence of the latter form (73(6)%). Various quantum mechanical procedures have been applied to investigate the thermodynamic equilibrium of the two conformers as well as the geometrical parameters of MBSCB. Among these methods was MP2/6-311++G(2df,2pd). This level of theory provided an axial to equatorial ratio of 29:71% and values for the geometrical parameters that are in good agreement with the experimental values except for the C–C bond length, which is by 0.02 Å shorter than in the experiment. The main geometrical parameters obtained from the experiment are: ( r a Å, ∠ a°): Si–C 1.872(3); C–C 1.583(6); Si–Br 2.225(3); ∠ CSiC 79.3; dihedral angle φ 28.8(52) ( axial) and 39.9(2) ( equatorial). Natural bond orbital (NBO) and atoms in molecule (AIM) analyses have been performed. Both, the donor–acceptor (Lewis and non-Lewis) orbital interactions and the topological properties of the charge density at the critical points ρ( r) have consistently confirmed the experimental results and facilitated their interpretation. For the purpose of comparison and systematic investigation, we optimized the geometries and analyzed the NBOs and the topological properties of silacyclobutane, 1-monofluorosilacyclobutane (MFSCB), and 1-monochlorosilacyclobutane (MCSCB). Simple relationship has been found between the puckering angle θ and the puckering amplitude q, which allows for the prediction of either θ or q for mono- and dihaloginated silacyclobutanes. Additionally, NBO deletion analysis comprising NOSTAR, NOVIC, and NOGEM deletion algorithms have been conducted. Interesting conclusions regarding structure and conformational stability of the studied monohalogenated silacyclobutanes could be drawn from this analysis.