Benzocyclobutadiene: The question of structures, magnetic shieldings, and aromatic character*

The importance of electron correlation for structure and magnetic shielding of benzocyclobutadiene is studied using the [2,2]-CASSCF, the [8,8]-CASSCF, and the MP2 approaches. The study aims at the question of applicability of the terms aromatic or antiaromatic for classification of this system and at the assignment of the experimental hydrogen chemical shifts. As reference, results for benzene are also included. Optimized structures are reported, including the two, respectively, three roots at the two complete active space self-consistent field (CASSCF) levels. For the ground state, electron correlation plays about the same role for structure and energy as it does for benzene, while the magnetic properties are sensitive to correlation, and are not entirely resolved at the present levels. The higher CASSCF roots correspond to inherently multiconfigurational states, and the magnetic results for the second molecular state indicate the necessity for higher level computations. The chemical shifts computed for the ground state are compatible with the experimental data, in contrast to a previous assignment associating the shifts with a molecular structure representing the second CASSCF root. Applying a criterion based on the perpendicular component of the shielding field, the ground state of benzocyclobutadiene qualifies for neither a global aromatic nor a global antiaromatic assignment. In a dichotomic classification, the six-membered ring again qualifies for neither an aromatic nor an antiaromatic assignment, leaving open an antiaromatic assignment for the four-membered ring, in accord with the reactivity of benzocyclobutadiene. We compare and discuss the assignments proposed here and in the literature in the light of a prevailing feeling of elusiveness shading the concept of aromaticy. © 2002 John Wiley & Sons, Inc. Int J Quantum Chem, 2002