The aromaticity/antiaromaticity continuum. 1. Comparison of the aromaticity of the dianion and the antiaromaticity of the dication of tetrabenzo[5.5]fulvalene via magnetic measures.
The aromaticity of the dianion (2) and the antiaromaticity of the dication (3) of tetrabenzo[5.5]fulvalene have been evaluated through magnetic criteria, (1)H NMR shifts, nucleus-independent chemical shifts, NICS, and magnetic susceptibility exaltation, Lambda. The sum of the NICS values, using the GIAO (gauge-independent atomic orbital) method, for 2 is -35.2; that of 3 is +38.2, indicating the aromaticity of 2 and the antiaromaticity of 3. Calculation of magnetic susceptibility exaltation using the CSGT (continuous set of gauge transformations) method gives a similar result, with Lambda of -81.8 ppm cgs for 2 and 95.8 ppm cgs for 3. The general validity of these values is supported by excellent agreement between the NMR shifts calculated by the GIAO and CSGT methods with experimental shifts. Comparison of 1H NMR shifts with those of model compounds allows evaluation of the magnitude of the diatropic shift in 2 and paratropic shift in 3 and supports their assignment as aromatic/antiaromatic, respectively. The agreement between calculated and experimental 1H NMR shifts is excellent for 3 in the absence of counterions but much better for 2 when counterions are included. Inclusion of counterions in the evaluation of diatropic shift for 2 gave a smaller shift than in the absence of counterions, suggesting a decreased aromaticity. When counterions were included in the calculation of Lambda, the value was also decreased, suggesting a decreased aromaticity. This observation has important consequences in the use of experimental data for the evaluation of aromaticity, and presumably antiaromaticity, of anions since, in most cases, there will be close interaction with counterions.
σ-Aromaticity and σ-antiaromaticity
Abstract The concepts of σ-aromaticity and σ-antiaromaticity have been examined in the light of their connection with the type of the electronic shell of skeletal bonds. Electron count rules for σ-skeletal electrons have been formulated for both small saturated and unsaturated cyclic compounds. The electronic shells of the conjugated compounds are classified into 4 types by their properties of gp- and σ-aromaticity and/or π- and gs-antiaromaticity depending on the type of filling up of the π- and σ-skeletal subsystems. Experimental and calculational data regarding rearrangements and ways of stabilization of a variety of carbocyclic compounds have been systematized on the basis of approaches developed to the concepts of π- and σ-aromaticity and antiaromaticity.
LCAO MO CALCULATIONS ON SATURATED HYDROCARBONS AND THEIR SUBSTITUTED DERIVATIVES
The simple semiempirical molecular orbital method was applied to saturated hydrocarbons in three different approximations. Electronic charge distribution diagrams were obtained. Characteristic diff...