Comparative studies of geometric and quasielastic characteristics of PP and SS bonds

It is demonstrated that density functional theory in B3PW91/aug-cc-pVTZ formulation provides a reasonable estimate of the geometrical and vibrational parameters of various PP and SS bonds. Calculated quasielastic characteristics of SS bonds, such as relaxed force constants f(SS), are approximately twice as large as f(PP). This means much higher elasticity of the PP bond, compared to the SS bond. From the dependence of the relaxed force constants on the formal bond order, the various PP bonds studied represent single bonds. By contrast, the more than twice as high rigidity of the SS bonds implies multiple SS bonding. Pronounced shortening of the SS bond on passing from less electronegative to more electonegative substituents at the sulfur atoms can be ascribed to the corresponding increase of the SS pi interactions. Essentially single PP bonds are not affected by electronic effects of this kind, which is the reason for the small variability of PP bond lengths in the gas phase. At the same time, intermolecular forces acting in crystals may strongly influence the length of highly elastic PP bonds.