Paratropic shift, HOMO-LUMO energy

The correlations between the HOMO-LUMO energy gap as deduced from self-consistent field molecular orbital (SCF MO) calculations (29-32) and the extent of the paratropic shifts revealed by the carbocyclic dianions are shown in Table III (11). (NOTE For Table III, refer to Tables I and II... 

Figure 6. Correlation between the H NMR paratropic shifts of 4n 7r poly-benzenoid dianions and the HOMO-LUMO energy gaps obtained by SCF MO...

Minsky, Meyer, and Rabinovitz studied doubly charged systems derived from fused benzenoid polycyclic compounds and found a linear relationship between the paratropic H NMR shifts Ad and the HOMO—LUMO energy gap. Indeed, they found for dications and dianions... 

Table 8. H NMR Paratropic Shifts of Polybenzenoid Dianions vs. LUMO-HOMO Energy Gaps 36). 

The dependence of NMR patterns upon states of solvation should not be overlooked. This aspect of polyanion chemistry has been discussed (Sect. 4). The influence of the counter cation upon the dianion though unaccounted for by simple calculations is supposed to vary from system to system. The different topologies of the anions and different modes of delocalization may be responsible for the deviations from a linear dependence of the correlation. From Table 9 one can arrive at conclusions on the relative antiaromaticity of the various anions. For example, benzanthracene dianion 49 ) is less antiaromatic than chrysene dianion (542 ), and dibenzo-anthracene dianion (J/2-) is more antiaromatic than the pentacene dianion (472 ). It is also proposed that H NMR paratropic shifts may be used to calibrate the LUMO-HOMO energy gap. 

---