On the Effects of Intramolecular van der Waals Forces

Evidence for the existence of strong nonbonded repulsions between atoms bonded to a given atom is cited. It is observed that bond angles in ethylene derivatives and related molecules can be correlated with a simple ``hard sphere'' model in which atomic positions are presumed to be governed by the sizes of spherical atoms packed around a given atom. Estimations of the magnitudes of the nonbonded interactions are presented. These indicate that the shortening of carbon-carbon single bonds in unsaturated hydrocarbons may be principally attributable to the relaxation of nonbonded repulsions across digonal or trigonal bonds as compared with tetrahedral bonds. It is suggested that intramolecular van der Waals forces may be even more important than effects of hybridization and conjugation or hyperconjugation in governing bond angles, bond distances, energies of isomerization and hydrogenation, and bending vibrations of molecules.