Bond Lengths in Conjugated Molecules: The Present Position

Abstract

A survey is given of the present methods for calculating bond lengths in conjugated molecules. Except in simple cases this has to be achieved by combining a calculated bond order with a semi-empirical relation between order and length. There are several definitions of bond order in current use, some of which can be shown to be less valuable than others. Recent accurate X -ray analysis has shown convincingly that the concept of bond order is a valid one, though there are limits to the degree of accuracy that may be claimed. Several possible improvements upon the simple theory are mentioned, most of which indicate alterations in bond lengths calculated from the simple theory, of the order of 0.005 Å. It is concluded that for condensed hydrocarbon molecules, the lengths of individual bonds may be predicted to within about 0.015 Å. For heteromolecules there are still too many additional factors for any reliable estimate to be possible.