Some observations on the stereospecificity of weakly bound complexes of organic molecules

Vinyl bromide-benzene/p-xylene and acrylonitrile-benzene/p-xylene complexes have been studied by measuring the ¹H chemical shifts induced in the vinyl protons by the aromatic molecules. Evidence is presented that indicates that the complexes are 1:1, and values for the equilibrium quotient, K_x, and the full induced shift Δ_c, are deduced for each proton; similar values of K_x are obtained for each of the three viny] protons. It is shown that the molecular interactions are specific. However, the complexes cannot be considered to have fixed geometry, but molecules involved take up a variety of relative orientations.