ORGN 392 |
| Geometry is one of the primary and most direct indicators of aromaticity and antiaromaticity: a regular structure with delocalized double bonds (e.g., benzene) is symptomatic of aromaticity whereas a distorted geometry with localized double bonds (e.g., 1,3-cyclobutadiene) is characteristic of antiaromaticity. Here, we present an MO model of aromaticity that explains, in terms of simple orbital-overlap arguments, why this is so. Our MO model is based on accurate Kohn-Sham DFT analyses of the bonding in benzene, 1,3-cyclobutadiene, cyclohexane and cyclobutane, and how the bonding mechanism is affected if these molecules undergo geometrical deformations between regular, delocalized ring structures and distorted ones with localized double bonds. We show that the propensity of the π electrons is always, i.e., in both the aromatic and antiaromatic molecules, to localize the double bonds, against the delocalizing force of the σ electrons. More importantly, we show that the π electrons nevertheless decide about the localization or delocalization of the double bonds.
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Physical Organic Chemistry: Calculations, Mechanisms, Photochemistry, and High Energy Species
8:00 AM-11:40 AM, Tuesday, August 21, 2007 BCEC -- 254 A/B, Oral
Division of Organic Chemistry |
