Electronic distribution singlet 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. 

The nuclear charges and the fixed charge distribution, the so-called core, which is not affected by any change in the sr-electron distribution. The singlet ground state wave function therefore describes only the n system and is given by a single closed-shell Slater determinant Aq which is constructed from a set of 5r-molecular spin orbitals (SMO) ( la), ( 2 ). etc. 

Distribution of the two additional electrons to 8 required for dianion formation among the degenerate LUMO orbitals of E, symmetry gives rise to four new states, since, within the 5v symmetry group, the direct product Ej Ej may be reduced to a sum of Aj, and 2 irreducible representations. The A2 state represents a triplet, while Aj and 2 are singlet states. 

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. 

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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