The electronic structure of some polyenes and aromatic molecules. VII. Bonds of fractional order by the molecular orbital method

Abstract It is known that in aromatic molecules the individual links have a length which is neither that of a pure single bond, nor of a pure double bond, but which is, in general, intermediate between the two. In earlier papers of this series (Lennard-Jones 1937; Penney 1937) two methods have been outlined for calculating these lengths and the energies associated with them. An approximate description of a particular link can be given if we assign to it an order. This order will be the sum of two parts, the one from the σ-bonds which is assumed to have the value unity in all cases, and the other from the π-bonds. We may call this latter contribution the “mobile order” p, since it arises from the motion of those electrons which Lennard-Jones (1937) calls “mobile electrons”. For a pure single bond such as ethane, the total order is 1; for a pure double bond such as ethylene, the total order is 2; for a pure triple bond such as acetylene, the total order is 3. The corresponding values of p are 0, 1 and 2 respectively. For other links, as for example in benzene, p will have a fractional value.