Intramolecular electron delocalization

In the MO theory, the most reliable approach for the study of reaction pathways perhaps is CASSCF [12, 13], but multi-VBSCF is essentially at the same level with CASSCF [14]. Since a VB wave function can be expanded into the combination of numerous Slater determinants that are used to define configurations in the MO theory, the VB theory provides a very compact, accurate description for chemical reactions. While both MO and VB theories have their own advantages as well as disadvantages, in our opinions, there are some areas where the VB theory is particularly superior to the MO theory 1) the refinement of molecular mechanics force field 2) the development of empirical or semi-empirical VB approaches 3) the impact of intermolecular charge transfer or intramolecular electron delocalization on the structure and properties 4) the validation of classical chemical theories and concepts at the quantitative level 5) the elucidation of chemical reactions and excited states intuitively. 

Molecular Metal Complexes Compounds of this type do not form delocalized electronic bands in the sohd state, and their color is due to intramolecular electronic transitions. Many complexes of transition metals with organic ligands belong to this class. complexes with phenanthroline (red/colorless) and Ru + + with 2,2 -... 

These TR results demonstrate that the localized model of Ru(bpy) + is valid on the timescales of electronic motions and molecular vibrations. It is virtually certain that delocalization (via, for example, intramolecular electron transfer or dynamic Jahn-Teller effects) occurs on some longer timescale. 

The intramolecular flexibilities of poly(1,4-phenylene oxide), polyi2,6-dimethyl-1,4-phenylene oxide), poly(2-methyl-6-phenyl-1,4-phenylene oxide), and poly 2,6-diphenyl-1,4-phenylene oxide) are evaluated through estimation of the resistance to rotation about the Cj 4—0 bonds in their backbones. A 6-12 potential is used to account for the van der Waals interactions between nonbonded atoms and groups encountered during the backbone rotations, while the twofold intrinsic potential to rotation about the C14—0 bonds resulting from the -electron delocalization is also included. 

ESR and l70 NMR spectra of le -reduced SiW O o demonstrate that the unpaired electron is weakly trapped on a W atom at low temperatures but undergoes rapid hopping (intramolecular electron transfer) at room temperature (Section II). Anions generated by 2e (and 4e -) reduction are ESR-silent, but 170 and 183W NMR spectra show that the additional electrons are fully delocalized (on the NMR timescale) at room temperature and generate ring currents analogous to those produced by the 7i-electrons of benzene. In contrast, in the case of le -reduced PMoW iO, the electron is localized on a more reducible Mo atom at room temperature (251). 

As described above, both porphyrins and fullerenes have highly delocalized JT-systems suitable for efficient electron transfer, because the uptake or release of electrons results in minimal structural and solvation changes upon electron transfer to afford small reorganization energies of electron transfer. Therefore, extensive efforts have been devoted to study intramolecular electron transfer in elec-... 

Other theoretical activity has centered on the dependence of reaction non-adiabaticity upon the structure of the intervening medium as well as the donor-acceptor separation for intramolecular electron transfer [50], i.e. between donor and acceptor sites contained within a single species such as a binuclear complex. The electron-tunneling probability is predicted to be enhanced substantially by the presence of delocalized electron groups, such as aromatic ligands, between the reacting centers [50]. This is consistent with experimental studies of thermal and optically induced electron transfer within binuclear complexes [51]. 

There are several fundamental reasons why the GMH and adiabatic formulations are to be preferred over the traditionally employed diabatic formulation. The definition of the diabatic basis set is straightforward for intermolecular ET reactions when the donor and acceptor units are separated before the reaction and form a donor-acceptor complex in the course of diffusion in a liquid solvent. The diabatic states are then defined as those of separate donor and acceptor units. The current trend in experimental design of donor-acceptor systems, however, has focused more attention on intramolecular reactions where the donor and acceptor units are coupled in one molecule by a bridge.The direct donor-acceptor overlap and the mixing to bridge states both lead to electronic delocalization, with the result that the centers of electronic localization and localized diabatic states are ill-defined. It is then more appropriate to use either the GMH or adiabatic formulation. 

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