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Effect on Ring Current of the KekuleĢ Vibration in Aromatic and Antiaromatic Rings
Derivative currentādensity maps are used to follow the changes in ring-current (and hence, on the magnetic criterion, the changes in aromaticity) with the KekuleĢ vibrations of the prototypical aromatic, antiaromatic, and nonaromatic systems of benzene, cyclooctatetraene (COT), and borazine. Maps are computed at the ipsocentric CHF/6-31G**//RHF/6-31G** level. The first-derivative map for benzene shows a growing-in of localized bond currents, and the second-derivative map shows a pure, paratropic āantiring-currentā, leading to the conclusion that vibrational motion along the KekuleĢ mode will reduce the net aromaticity of benzene, on average. For planar-constrained <i>D</i><sub>4<i>h</i></sub> COT, the KekuleĢ mode (positive for reduction of bond-length alternation) increases paratropicity at both first and second order, indicating an average increase in antiaromaticity with zero-point motion along this mode. On the ring-current criterion, breathing expansions of benzene and <i>D</i><sub>4<i>h</i></sub> COT reduce aromaticity and increase antiaromaticity, respectively