Structure and stability of a bridged biphenyl: an X-ray and theoretical study

Structure of an axially chiral biphenyl, (S,M)-dibenzo[d,f][1,3,2]dioxaphosphepin-6-amine-1,11- dimethoxy-4,8-dimethyl-N-(1-phenylethyl)-6-sulfide was determined by X-ray diffractometry and by theoretical (semiempirical AM1 and ab-initio RHF) calculations in the gas phase. A relevant feature of the ortho-substituted biphenyls is the extent of molecular distortion around the inter-ring bond, which can be quantified through the interplanar dihedral angle. If the distortion of no-bridged biphenyls is mainly due to overcrowding between the ortho substituents, in the presence of a single or double bridge between the ortho positions, the interplanar dihedral angle is considerably influenced by the nature of the bridge(s). In the solid state, crystal-packing forces may also contribute to the molecular distortion. In both ortho unsubstituted and substituted biphenyls with no bridges, intermolecular interactions play generally a substantial role in determining the geometry of the molecule. In the title compound, the value of the interplanar dihedral angle, 49.93(9)°, was reproduced within a few degrees at all levels of theory, indicating a weak influence of the lattice forces on the biphenyl distortion. The stability of the title compound was analyzed at theoretical level by calculating the torsional potential around the inter-ring bond and by locating the transition state for the isomerization process. A relatively low rotation barrier of 23.3 kcal/mol was obtained.