Bond relaxation and non-bonded distances in gem-dihalides and related molecules

Experimental structure determinations for gem-disubstituted difluorides and dichlorides, both acyclic and alicyclic, show that the non-bonded distances d(Y ? Y) (Y = F, Cl) are essentially constant. It is proposed that d(Y ? Y) represents the limiting non-bonded contact distance and determines the bonded C-Y distance, for a given angle ∠YCY: as the skeletal angle ∠CCC increases, the angle ∠YCY decreases, and the constant Y?Y distance causes d(CY) to increase; and conversely. This hypothesis is supported by both MINDO/3 and Molecular Mechanics calculations.