LXXVI. The bond lengths of unsaturated hydrocarbons

Summary The standard excited state approximation for an unsaturated hydrocarbon molecule is used to determine the equilibrium configuration of the nuclei of the molecule. Each bond length of the molecule is shown to depend, to an approximation, on a scalar quantity, called bond order, which is characteristic of the bond. This concept of bond order is numerically equal to that of π-bond order used by Coulson and others. The quantum mechanical formulation gives the form of the bond order-bond length relation, but its actual shape is determined from the known bond lengths of naphthalene and anthracene.