When the nucleus independent chemical shifts, NICS(1)zz, for a set of aromatic and antiaromatic hydrocarbons are summed, they show an excellent linear relationship with the magnetic susceptibility exaltation, Lambda, for neutral, cationic, and monoanionic species. Aromatic and antiaromatic dianions show a similar relationship but with a different slope. However, when both Lambda and the summation of NICS(1)zz are divided by the area of the ring squared, the vast majority of the aromatic and antiaromatic species fall on the same line, indicating that both NICS and Lambda are affected by the size of the ring. The species that deviate slightly from the line are a few of the anionic compounds, which may be a result of the difficulties of calculating magnetic properties of anions. This is the first report of the relationship of NICS to ring area. In addition, the excellent correlation between Lambda and the summation of NICS(1)zz demonstrates that summation of NICS(1)zz values for individual ring systems of polycyclic ring systems to give a measure of the aromaticity of the entire system is justified. By extension, the excellent correlation also serves to demonstrate that the NICS(1)zz values for individual ring systems are reliable measures of local aromaticity/antiaromaticity. Finally, the excellent correlation between experimental shifts and the 13C NMR shifts calculated with density functional theory, B3LYP/6-311+g(d,p), serves as an indirect validation of the accuracy of the NICS(1)zz calculated by the same method and basis set.

Oxidation of tetrabenzofulvalene derivatives 1-5 containing fluorenylidene and 5-7-membered rings resulted in the formation of dications which are effectively fluorenyl cations with perpendicular cyclic substituents. The observed paratropicity of the fluorenyl cation in these systems is attributed to an antiaromatic ring current, after evaluation of the effects of geometry, charge density, and polarity of the medium, and is dependent on the geometry and electronic character of the cyclic substituent. A linear relationship between the 1 H shifts of appropriate protons of the fluorenyl system and 13 C shift of carbon a suggests that the effect of the substituent is transmitted through cross-hyperconjugation.

No abstract

The NICS values of the dication of tetrabenzo[5.5]fulvalene (1) show substantial antiaromaticity. Substituted fluorenyl cations possess antiaromatic five-membered rings but the calculated antiaromaticity (NICS) of the six-membered rings depends on the calculational level. Through calculation of magnetic susceptibility exaltation (Λ), 1 is antiaromatic while fluorenyl cations are not. The paratropic shift seen in the 1H NMR spectra of 1 and the substituted fluorenyl cations is linearly related to NICS (six-membered ring) and to Λ. NICS values suggest that electron delocalization in the fluorenyl cations occurs to maintain the aromaticity of the benzene subunits and to localize the positive charge in the five-membered ring. In contrast, electron delocalization in 1 results in delocalization of a positive charge throughout each fluorenyl system.

An anionic three-way switch A bipyridyl bisurea-based anion receptor that is highly selective for dihydrogen phosphate demonstrates spectroscopically distinct anion bound conformations toward halides and select oxoanions. 1H NMR studies show the differing anion induced conformations are reversible allowing this system to function as a three-way molecular switch.