Electronic distribution triplet state

Indazoles have been subjected to certain theoretical calculations. Kamiya (70BCJ3344) has used the semiempirical Pariser-Parr-Pople method with configuration interaction for calculation of the electronic spectrum, ionization energy, tt-electron distribution and total 7T-energy of indazole (36) and isoindazole (37). The tt-densities and bond orders are collected in Figure 5 the molecular diagrams for the lowest (77,77 ) singlet and (77,77 ) triplet states have also been calculated they show that the isomerization (36) -> (37) is easier in the excited state. 

Molecules with two or more unpaired electrons may be divided into two classes by far the most common examples are molecules where the unpaired electrons are contained in a set of degenerate atomic or molecular orbitals with qualitatively similar spatial distributions, e.g., an octahedral Cr(m) (4A2g) or Ni(n) (3A2g) complex, a ground state triplet molecule like 02, or the excited triplet states of naphthalene or benzophenone. 

The two molecules that have one- or three-electron n bonds show triplet ground states. This conforms to Hund s rule in atoms where one has unpaired electrons distributed among degenerate orbitals to produce the highest possible multiplicity. The other molecules all have electron pair bonds or unshared pairs and are in singlet states. 

The obtained data clearly show that the g-anisotropy of the triplet states is larger than that of the respective cation-radical. A similar effect has been observed for the triplet states of the primary donors in PS II231 and in the bacterial RC.111112114 This can be explained by the fact that the triplet electrons probe the spin distribution in two different orbitals (HOMO and LUMO), and the latter has a rather large spin density at the nitrogens and the central magnesium (cf. references 216, 218), by which the spin-orbit coupling and the g-anisotropy is increased. 

In the past few years the ESR technique has been applied to demonstrate the triplet nature of many highly reactive organic biradicals and of various metastable photoexcited states, to estimate the rates of their decay, and to evaluate their electronic distribution. These metastable states are normally produced and observed either frozen in glassy matrices at 77°K or aligned in a host crystal. Spectra have also been obtained of triplet species dissolved in a translucent plastic. 

Figure 3.8(b) shows an example of a crossing of singlet and triplet states of different electron distributions resulting from different singlet-triplet splittings. 

This question of equilibration of the protonation and deprotonation processes leads to another fundamental problem in the case of excited state reactions between which states can a protolytic equilibrium be at all established A molecule has only one ground state, so there can be no ambiguity about the thermal protolytic equilibrium which connects of course the ground states of the acid and base forms. However, there are many excited states of both these forms, excited states which can differ greatly in electron distribution (e.g. mr and 7T7T states) or even in multiplicity (e.g. singlet and triplet states). 

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