Annulene Story

There is no question but that the agreement of the QM calculations with the experiment is only consistent with the molecule having alternating bond lengths in the gas phase and in solution. Again, this is the same conclusion that was reached from the ultraviolet spectra in 1969, namely the alternating bond structure is the stable one in the gas phase and in solution. Chemical shift data of the molecule in the crystal are not available at this writing. So does this really mean that the structure of the molecule 

TABLE 10.1. NMR Chemical Shifts for Proton Absorptions in [18]Annulene  

If the two structures were accurately drawn and placed side by side, the difference is too small to see by just looking at them. They are really similar but not identical. 

So the thermochemical definition of aromaticity is not met by [18]annulene, and by that definition, the compound is not aromatic. But note also that it is not significantly antiaromatic either. It is just a polyene, and the chemical behavior of the compound is consistent with this statement. For example, it reacts with air and polymerizes (turns brown) on standing. 

Turning to the ring current definition of aromaticity, the chemical shift calculations for different structures for [18]annulene can now be carried out in a straightforward manner by quantum mechanics. These calculations predict quite a large ring current, and the chemical shifts are substantially larger for the inner protons if there is bond alternation than if the bonds were to be equivalent. (By equivalent here is meant similar to those in benzene and to each other, and the quantum mechanical stationary 

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CONTENTS List of Contributors. Introduction to the Series An Editor s Forward, Albert Padwa. Preface, Randolph P. Thummel. Cyclooctatetraenes Conformational and ii-Elec-tronic Dynamics Within Polyolefinic [8] Annulene Frameworks, Leo A. Paquette. A Compilation and Analysis of Structural Data of Distorted Bridgehead Olefins and Amides, Timothy G. Lease and Kenneth J. Shea. Nonplanarity and Aromaticity in Polycyclic Benzenoid Hydrocarbons, William C. Herndon and Paul C. Nowak. The Dewar Furan Story, Ronald N. Warrener. Author Index. Subject Index. 

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