DIRECT CALCULATION OF RESONANCE ENERGIES OF CONJUGATED HYDROCARBONS WITH ABINITIO MO METHODS

Resonance energies of conjugated hydrocarbons were calculated as the energy difference between the ab initio SCF energy and the energy expectation value with respect to a model wave function in which the SCF π orbitals were replaced by appropriate nonresonating localized π MOS (reference state). The contributions of electron correlation and of relaxation of σ orbitals on resonance energies were also computed and found to be rather small in most cases. Since all electrons were included in the calculations, vertical and adiabatic resonance energies could be obtained. The adiabatic resonance energy of butadiene, which was computed to be about 9 kcal/mol, has partly to compensate for the unfavorable repulsion of the two nonresonating π MOs in the reference state. The resonance energies of other hydrocarbons were normalized with respect to butadiene and could thus be compared with thermochemical resonance energies. The largest molecules studied were naphthalene and azulene. The SCF calculations on the latter system gave evidence that azulene probably has a structure with bonds of alternate lengths. © 1979, American Chemical Society. All rights reserved.