HOMO-LUMO separation

Dimethyl-2-butene and 2,5-dimethylhexadiene have absorption peaks at 192 and 243 nm in the ultraviolet. Which peak corresponds to which compound What are the approximate HOMO-LUMO separations in electron volts ... 

This indicates that the deviations are due to systematic errors, for example deficiencies of the applied methods and basis sets. DFT-based methods, such as GIAO/DFT calculations are known to overestimate paramagnetic contributions to the chemical shielding. This results, for critical cases with small HOMO/LUMO separations, in overly deshielded competed chemical shifts. Notorious examples for these deficiencies are 29Si or 13C NMR chemical shift computations of silylenes, silylium ions or dienyl cation .(5/-54) Taking into account the deficiencies of the applied method, and bearing in mind very reasonable correlations shown in Figures 4 and 5, the computational results do support the structural characterization of the synthesized vinyl cations. 

C70 also displays a one-electron oxidation with features of chemical reversibility at a potential similar to that of C60. This implies that the HOMO/LUMO separation is roughly similar in the two cases.15... 

These data indicate a HOMO/LUMO separation for C76 of 0.4 eV, compared to the theoretical value of 1.07 eV. 

In butyrronitrile, the present isoporphyrin complex exhibits an oxidation process (E° = +1.09 V, vs. SCE) and two consecutive reduction processes (E° = — 0.29 V and -0.61 V, respectively). As it happens for zinc porphyrins, also in this case the processes are centred on the ligands." It is interesting, however, to note that the HOMO/LUMO separation is 1.38 V, which is considerably lower than that of [Zn(TPP)]. 

Comparison with the redox aptitude of [Zn(TPP)] (Figure 52) points out a minor redox flexibility of phthalocyaninates with respect to porphyrinates, in that in the former either the second reduction or the second oxidation are complicated by degradation of the corresponding congeners. We also note that the HOMO/LUMO separation for [Zn(Pc)] is 1.55 eV, i.e. lower than that of [Zn(TPP)] (2.15 eV). 

Figure 11. Double logarithmic correlation between the HOMO-LUMO separation of the AH3 fragment and dihydrogen and the dissociation energy Dq for all penta-coordinated complexes.

The HOMO-LUMO separation of mesomeric betaines is relatively small. Heteroatoms and conjugated substituents at inactive positions will usually reduce this separation by lowering the energy of the LUMO without perturbing the HOMO. 

The polymers of 4H-cyclopenta[2,l-b 3,4-//Jdi thiophene derivatives showed absorptions in the range 560-590 nm [03MM2705]. Some monomers are based on the nonaromatic (12 n electrons) 4H-cyclopenta[2,l-fr 3,4-l/]dithiophen-4-yl cation 580a (X = CH+) model which was expected to display a reduced HOMO-LUMO separation, compared to related aromatic fused systems (i.e., 580, X = S, O, NF1) (91JCS(CC)752). Polymers of 580 (X = S) showed absorption at 480 nm. Poly(dithieno[3,4-b 3, 4 -4J thiophene) showed an absorption at 590-610 nm (89SM(28)C507 97MI23). 

The above formula is valid for, among others, benzenoid hydrocarbons. 4. The HOMO-LUMO separation... 

What we learn from Eqs. (36) and (37) is that both the reduced ir-electron energy and the reduced HOMO-LUMO separation scale (approximately) according to a simple law. The analogy with the gas equation of state is appropriate here. One can... 

According to PMO theory the introduction of a central nitrogen into the aceindenyl anion (41) leaves the energy of the LUMO unchanged, but lowers that of the HOMO. The introduction of nitrogen atoms into the perimeter also affects the HOMO-LUMO separation. Enhancement of the electronegativity in positions 1, 4>8 >9-2 t and 6 increases this energy difference. 

Since all occupied energy levels fall between 0 and 3 (in ft units), HOMO-LUMO separation decreases with the increase in molecular size. This fact is reflected in the chemical behaviour of polyenes i88> and polyacenes 139). Similarly, conjugated macromolecules are found to be paramagnetic i40>. 

HOMO. Conversely, it will increase the polarisation for the LUMO and hence increase the effectiveness of the interaction of the LUMO of the dipole with the HOMO of the dipolarophile, as in Fig. 6.45b. The difference in energy for the two cases is small enough that firm prediction is not possible. In practice, dipole-HO control appears to be dominant, as shown by the formation of the adduct 6.323 from methyl acrylate, but it only needs the addition of an a-methyl group, for some dipole-LU control to become evident in the formation of some of the adduct 6.324 from methyl methacrylate, in addition to the aziridine 6.326 derived from the normal regioisomer 6.325.858 Perhaps the methyl group has raised the energy of both the HOMO and the LUMO of the dipolarophile, making the HOMO/LUMO separations still more nearly equal. 

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