The electronic structure of some polyenes and aromatic molecules IV-The nature of the links of certain free radicals.

Abstract

In the first two papers of this series (1937 a, b ), referred to as I, II, Lennard-Jones has developed a method for investigating the lengths and energies of the links in some unsaturated molecules. For this purpose he used the method of molecular orbitals. In paper III (1937), Penney obtained similar results, using the electron-pair methods of resonance, developed Pauling and others. It is the purpose of the present paper to extend the calculations to chain molecules and radicals in which the number of carbon atom s is odd, an d in which, therefore, there is one electron which does not form a bond, in the usual picture of the chemist. We shall use the method of molecular orbitals, and this work may be said to be a continuation of I and II. The writer would like to than k Professor Lennard-Jones for suggesting this work, and for the opportunity of discussing it with him during the calculations. In general these free radicals with “trivalent” carbon are not stable, and tend to form dimers; but there are certain of them which do exist either as stable substances or in dissociative equilibrium with their dimers. Hückel (1935) has discussed these radicals, on the earlier form of the theory in which all the links were assumed equal and no allowance was made for their compression. His work needs to be extended because there is no reason why the links should be all equal, and in fact, the bond diagrams of the chemist lead one to suspect otherwise, and to believe that there may be one of the carbon atoms (the one on which the unpaired electron is to be found) different from the others (for which all the electrons are paired). On the molecular orbital theory, in which each electron is supposed free to move throughout the whole molecule in an averaged potential field, it is not so easy to see a t once in what way the presence of the odd electron will alter the arrangement of the links. So the first question that we shall ask will be whether in chain molecules, such as C 2 n +1 H 2 n +3 , there is one carbon atom occupying an essentially different situation from all the others. We shall then compute the resonance energy.