Isodesmic and homodesmotic stabilization energies of [ n]annulenes and their relevance to aromaticity and antiaromaticity: is absolute antiaromaticity possible?

Abstract

Isodesmic stabilization energies (ISEs) for conjugated molecules, which are determined by a balance of the stabilizing π-electron non-cyclic delocalization and either stabilizing aromatic or destabilizing antiaromatic π-electron cyclic delocalization, do not yield a reasonable estimation of aromaticity or antiaromaticity. Homodesmotic stabilization energies (HSEs) should be used for this purpose. The ISEs for [ n]annulenes are larger by the ( n 2 ) ISE( s-trans-1,3-butadiene) value than the corresponding HSEs. The experimental value of the ISE( s-trans-1,3-butadiene) is 59.4 ± 4.1 kJ mol −1 at 298 K. While cyclooctatetraene has only the relative antiaromatic character, cyclobutadiene features both relative and absolute antiaromaticity. Since the HSE and ISE of cyclobutadiene, corrected for its strain energy, are still negative (−174.0 and −55.2 kJ mol −1, respectively, at the MP4SDTQ/6-31G (d,p)//MP2/6-31G(d,p) + ZPE(HF/6-31G(d)) level), π-antiaromaticity rather than σ-strain is at the origin of the remarkable instability of cyclobutadiene. At the MP4SDTQ/6-31G(d,p)//MP2/6-31G(d,p) + ZPE(HF/6-31G(d)) level, the antiaromatic destabilization of cyclobutadiene corrected for the ring-strain and normalized per π-electron is −43.5 kJ mol −1, a value that is larger (in absolute value) than the antiaromatic destabilization of planar D 4 h cyclooctatetraene (−14.9 kJ mol −1 per π-electron) and the aromatic stabilization of benzene (17.7 kJ mol −1 per π-electron). In contrast to cyclobutadiene, planar D 4 h cyclooctatetraene has a positive ISE (118.8 kJ mol −1) indicating that the stabilizing π-electron non-cyclic delocalization overbalances the destabilizing antiaromatic π-electron cyclic delocalization. Nevertheless, planar cyclooctatetraene is assigned to the class of antiaromatic molecules since its HSE is −118.8 kJ mol −1 at the MP4SDTQ/6-31G(d,p)//MP2/6-31G(d,p) + ZPE(HF/6-31G(d)) level.