Relative to their neutral precursors, highly charged multi-tritylium ions show typical downfield 'H-NMR shifts, whereas the corresponding phenylfluorenylium ions show upfield shifts. A Hijckel based model is presented that accounts for this contrasting behavior in terms of antiaromatic ring current in the five-membered ring of the fluorenyl system with essentially null ring currents in the two six-membered rings.
In a recent study of the UV/VlS-and 'H-NMR-spectroscopical properties of multitritylium ions we showed that the mono-through tetracations 1-4 not only reveal interesting variations in their colors (i.e., their longest wavelength electronic absorptions), but also characteristic charge-related deshieldings relative to their carbinol-ether precursors.' The downfield shifts of the p-protons was quite marked. In striking contrast, the corresponding phenylfluorenylium ions 8-8 experience strong uofield shifts of their fluorenylium protons relative to the carbinol-ether precursors. The contrasting behavior of the fluorenylium and tritylium ions could be taken as evidence for considerable paratropic ring current contributions in the fluorenylium systems2.
It was of interest, therefore, to investigate whether these contrasting NMR shifts could be reproduced by a model that has proved quite successful in correlating the ring-current dominated chemical shifts of neutral aromatic and antiaromatic hydrocarbons3. Appropriate modifications of the model would have to be made