SOLID ANGLE AS STERIC PARAMETER OF ACYCLIC SATURATED GROUPS

With the early aim of quantifying steric consequences of chirality, efforts to define a nonempirical steric parameter of chemical groups are reported. Steric hindrance of a reacting center by any acyclic saturated R group has been characterized by a geometric "axial steric parameter": the solid angle of R. When the group is a "symmetric top substituent" (i.e., when all the terminal atoms are equivalent), the solid angle matches the solid angle of a cone envelope of R. The definition of this cone is compared with Tolman's definition of a ligand cone in organometallic complexes. The chemical significance of this parameter is shown by an excellent correlation with the Dubois' experimental steric parameter E's. Modeling steric repulsion by the cone of R, and correcting solid angles for conformational effects, only 3 empirical coefficients are needed to calculate 33 values of E's with less than 10% error. The cone model is suggested to be relevant within the limits of random and independent free rotations about all the bonds in the C-R group. A separation between "axial cone steric hindrance" and other steric effects is proposed. The basic model and the corrections proposed allow the conformational features of esters to be discussed.