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Science 5 November 1999:
Vol. 286. no. 5442, pp. 1119 - 1122
DOI: 10.1126/science.286.5442.1119
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Research Articles

Experimental Determination of the Antiaromaticity of Cyclobutadiene

Ashok A. Deniz, 12* Kevin S. Peters, 2dagger Gary J. Snyder 1dagger ddagger

Photoacoustic calorimetry was used to quantify the antiaromaticity of 1,3-cyclobutadiene (CBD) by measuring the heat release accompanying its formation via photofragmentation of a polycyclic precursor. In combination with quantum yield measurements and thermochemical calculations, this measurement provides an enthalpy of formation for CBD of 114 ± 11 (2sigma ) kilocalories per mole (kcal/mol). The extraordinary reactivity of this prototypical antiaromatic hydrocarbon had previously made its heat of formation inaccessible except by theoretical calculations. Relative to a hypothetical strainless, conjugated diene reference, CBD is destabilized by a total of 87 kcal/mol, 32 kcal/mol of which can be attributed to ring strain and 55 kcal/mol to antiaromaticity (compared with 21 kcal/mol for the aromatic stabilization of benzene). Relative to a reference with isolated double bonds, CBD's antiaromaticity is 48 kcal/mol (compared with 32 kcal/mol for the aromaticity of benzene).

1 Department of Chemistry, University of Chicago, Chicago, IL 60637, USA.
2 Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA.
*   Present address: Department of Chemistry, Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, CA 92037, USA.

ddagger    Present address: Department of Chemistry and Biochemistry, 0303, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA.

dagger    To whom correspondence should be addressed. E-mail: gsnyder{at}ucsd.edu, kevin.peters{at}colorado.edu

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Science. ISSN 0036-8075 (print), 1095-9203 (online)