Naprężenie kątowe w molekule benzenu?

The stabilization energies (often called the aromatic stabilization energies) of π-electron molecules play important role in discussing the properties of these systems. The so-called homodesmotic reactions (involving the π-electron molecules of interest, and a set of reference molecules), treated with reliable methods of computational chemistry, B3LYP/6-31G(d) or B3LYP/6-311+G(2d,p) level of theory, provide a convenient general theoretical framework for calculating the stabilization energies. The present paper is a case study of the benzene molecule, as created from a set of nonstrained linear polyenes. We calculate the corresponding reaction energies, and show how the limit corresponding to the infinite polyene can be obtained. Moreover, in this limit the benzene molecule may be seen as ,,budded off'' from an infinite cyclic polyene (a cyclic model of polyacetylene). A similar treatment can be applied to some analogs of benzene - small cyclic polyenes, thus providing a platform for comparing the corresponding stabilization energies. At this point one faces the problem of strain in (small) cyclic polyenes - in a planar molecule the C-CH-C valence angles are different from 120o . The corresponding angular strain contributes to the calculated reaction energy - and we argue that the strain energy should be somehow ,,filtered out'', in order to arrive at the ,,true'' stabilization energy. Seemingly the benzene molecule is free from the angular strain. But our calculations for linear polyenes indicate that the corresponding C-CH-C valence angles are greater than 124o . Therefore, by going from the reference molecule (a cyclic model of polyacetylene) to the benzene molecule, some energy must be pumped in. Finally, we find that the stabilization energy for benzene equals: - (the reaction energy, including the angular strain energy) + (the estimated angular strain energy) = 17,25 + 3,24 = 20,49 kcal/mol. Thus, the strain energy correction contributes 19% to the final benzene stabilization energy.