Experimental Determination of the Antiaromaticity of Cyclobutadiene

Abstract

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).

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

“…In the third step not only partial π restoration occurs with resonance energy 17.9 kcal/mol [48] but in addition a relaxation term must be considered due to the decrease of the two effects. All recently reported values of angular strain are between 32 and 35 kcal/mol [64,[70][71][72]. ∆E rel is estimated to be largely negative, −59 kcal/mol from the strain energy of 34 kcal/mol [64] and the repulsion energy between π bonds of 25 kcal/mol c. Then, ∆E 3 = −QM RE(R) + ∆E rel = −76.9 kcal/mol.…”

The Heat of Formation of Cyclobutadiene

“…Unfortunately, these quantities are unavailable, and there is not enough information in the literature to estimate them by using additivity approaches such as the Bensons group equivalents method. [11] To solve this problem, Deniz et al [10] computed the geometries and energies of 2-4 by using molecular mechanics (namely, the MM3 force field) and semiempirical AM1 calculations, respectively. These data enabled DH f (1) = (477 AE 46) kJ mol À1 to be derived.…”

“…(1)]. [10] The resulting reaction enthalpy can be directly converted into the heat of formation of 1 if the energies of 2-4 are known. Unfortunately, these quantities are unavailable, and there is not enough information in the literature to estimate them by using additivity approaches such as the Bensons group equivalents method.…”

Cyclobutadiene: The Antiaromatic Paradigm?

“…In view of all the above it is not surprising that recent opinions vary enormously on how much CBD is thermochemically destabilized solely by virtue of the cyclic conjugation of its four p electrons; estimations range from 230 kJ mol À1 (Deniz et al [24] ) to 44 kJ mol À1 (Mo and Schleyer [13] ). If even this lower number is attributed to p repulsion, one is left with the prediction that CBD, in spite of its enormous destabilization, must be considered as essentially nonaromatic, as based on its thermochemical properties.…”