The aromatic character of annulenes and dicupraannulenes from current density calculations†
We have investigated the aromatic properties of seven low-lying isomers of annulene and of the recently synthesized dicupraannulene compounds that were crystallised with two or four lithium counterions (Wei et al., J. Am. Chem. Soc., 2016, 138, 60–63). The molecular structures of the annulene conformers and the dicupraannulenes with bulky trimethylsilyl (TMS) and phenyl groups, as well as the corresponding unsubstituted dicupraannulenes were optimised using density functional theory, employing a semi-empirical dispersion correction to consider van der Waals interactions. The structures of the hydrocarbon annulenes were subsequently optimised at the SCS-MP2/def2-QZVPD level. Single-point coupled-cluster calculations with explicit treatment of the electron correlation CCSD(F12)(T) were performed to obtain the relative energies of the hydrocarbon annulenes. Four of the conformations lie close in energy relative to each other. Three substituted and three unsubstituted dicupraannulene structures with either four, two or no Li+ counterions were investigated. Magnetically induced current densities calculated using the GIMIC program were used for the assessment of the aromatic properties of the studied molecules. The conformations of annulene with lowest energies are non-aromatic. The calculations revealed that the electron donation of the lithium atoms to the dicupraannulene core significantly affects the electronic and molecular structures of the dicupraannulenes. The annulene ring is non-planar for all studied dicupraannulenes except for the unsubstituted one with four Li+ counterions, which was also found to be the only molecule that sustains a strong diatropic ring current around the dicupraannulene ring. The other five dicupraannulenes sustain very weak net ring currents and can be considered non-aromatic.