Experimental determination of the antiaromaticity of cyclobutadiene.

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 (2final sigma) 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).