HOMO-LUMO energy gaps dianions

Examples of non-symmetric and electronically flexible ligands are shown in Fig. 7, including neutral monoanionic as well as dianionic ligands. The ligands possess HOMO/LUMO energy gap values of <8.5 eV for neutral ligands and <7.0 eV for anionic ligands, at the MNDO-PM3 level calculations. 

Table 16. HOMO-LUMO Energy Gap, Line Broadening, and Charge Density of the Heterocyclic Dianions n9).

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... 

Relationship Between the HOMO-LUMO Energy Gap and the XH Line Shape of 4n tt Paratropic Carbocyclic and Heterocyclic Dianions. 

Table III. NMR Faratropic Shifts and HOMO-LUMO Energy Gaps of Polybenzenoid Dianions...

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). 

Fig. 11. H NMRparatropic shifts of 4nrt polybenzenoid dianions vs. LUMO-HOMO energy gaps 361...

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. 

Table 9. LUMO-HOMO Energy Gaps Estimated (to 3 Method) for Dianions Derived from Benzenoid Polycycles. ...

From electrochemical investigation of 50 the authors concluded that reversible reductions to the radical anion and the dianion and a reversible oxidation to the radical cation are possible [57]. The electrochemically determined HOMO and LUMO values are —4.93 and —3.61 eV, respectively. The electrochemical energy gap of 1.32 eV is in agreement with the optical gap (1.35 eV from the miset of absorption, /l ax = 870 nm) [57]. In summary, the four CF3 groups considerably stabilize the heptacene by decreasing the energies of the frontier orbitals. 

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