The Spin-coupled Description of Aromatic, Antiaromatic and Nonaromatic Systems

The spin-coupled description of aromatic, antiaromatic and nonaromatic systems 

David L. Cooper8, Joseph Gerratt1 and Mario Raimondi8 

Organic chemists tend to be pragmatists when faced with rival MO and VB descriptions of molecular electronic structure. Many will use whichever model seems most convenient for the problem at hand. MO descriptions are widely employed in frontier orbital approaches, as in the Woodward-Hoffmann rules, and tend to be favoured when predicting excited states or photoelectron spectra. On the other hand, it is customary to represent reaction mechanisms in terms of resonance between classical VB structures with single, double etc. bonds (plus any unpaired electrons or lone pairs) and then to indicate by means of curly 

For each system, we take account of electron correlation for the n electrons, but not for the a framework. However, the n orbitals for aromatic systems lie to a considerable extent within the space of the a orbitals and not well outside it, as is commonly assumed. This might seem to bring into question the fundamental concept of ct-ji separation. However, the n orbitals are a great deal more polarizable than the a orbitals, and so it can be argued that the n system provides a large proportion of the response of the system to chemical and other influences. As a result, much of the chemistry of aromatic systems can be understood by considering only the n electrons. 

An important outcome of all these spin-coupled calculations is the consistency of the descriptions. In particular, a simple and highly-visual model emerges for the behaviour of correlated n electrons in all of the aromatic molecules that we have studied. These 7t-electron systems are well described in terms of fairly localized, nonorthogonal, singly-occupied orbitals. The special stability of such systems arises in the spin-coupled model from a profoundly quantum mechanical 

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D.L. Cooper, J. Gerratt and M. Raimondi,The spin-coupled description of aromatic, antiaromatic and nonaromatic systems, in Pauling s legacy Modem modelling of the chemical bond, Vol. 6, ed. Z. B. Maksic and W. J. Orville-Thomas (Elsevier, Amsterdam, 1999). 

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