Triplet-state aromaticity of 4npi-electron monocycles: analysis of bifurcation in the pi contribution to the electron localization function.

The pi contribution to the electron localization function (ELF) is used to compare 4npi- and (4n+2)pi-electron annulenes, with particular focus on the aromaticity of 4npi-electron annulenes in their lowest triplet state. The analysis is performed on the electron density obtained at the level of OLYP density functional theory, as well as at the CCSD and CASSCF ab initio levels. Two criteria for aromaticity of all-carbon annulenes are set up: the span in the bifurcation values DeltaBV(ELF(pi)) should be small, ideally zero, and the bifurcation value for ring closure of the pi basin RCBV(ELF(pi)) should be high (> or = 0.7). On the basis of these criteria, nearly all 4npi-electron annulenes are aromatic in their lowest triplet states, similar to (4n+2)pi-electron annulenes in their singlet ground states. For singlet biradical cyclobutadiene and cyclooctatetraene constrained to D4h and D8h symmetry, respectively, the RCBV(ELF(pi)) at the CASSCF level is lower (0.531 and 0.745) than for benzene (0.853), even though they have equal proportions of alpha- and beta-electrons.