ELECTRON-DENSITY MAPS OF BENT BONDS IN NON-CYCLIC MOLECULES

Much structural evidence supports the crude notion that a chemical bond behaves much like a flexible spring and is readily bent by unbalanced 1 ... 3 steric repulsions. Electron-density maps, derived experimentally (X-ray diffraction) or theoretically (extended-basis SCF calculations), show that such bending is expressed in a deformation-density peak slightly displaced off the bond axis. This is demonstrated in 2-cyanoguanidine, hydrazoic acid, cyanogen azide, formic acid, and diimide. Besides accounting for tilted methyl groups, as in methanol, and numerous similar violations of local symmetry, the flexible-spring model rationalizes a variety of conformational effects, such as the preferred syn-planar conformation of carboxylic acids, where the axis of internal rotation does not coincide with the internuclear vector. © 1979.