Covalent delocalization

Several methods exist for calculating g values. The use of crystal field wave functions and the standard second order perturbation expressions (22) gives g = 3.665, g = 2.220 and g = 2.116 in contrast to the experimentaf values (at C-band resolution) of g = 2.226 and g 2.053. One possible reason for the d screpancy if the use of jperfXirbation theory where the lowest excited state is only 5000 cm aboye the ground state and the spin-orbit coupling constant is -828 cm. A complete calculation which simultaneously diagonalizes spin orbit and crystal field matrix elements corrects for this source of error, but still gives g 3.473, g = 2.195 and g = 2.125. Clearly, covalent delocalization must also be taken into account. 

Two mechanisms contribute to the decrease of Racah B parameters. First, lone pairs of electrons from the ligand may penetrate the 3d shell of the transition metal and screen its 3d electrons from the nucleus, thereby decreasing the effective nuclear charge experienced by the electrons and expanding the 3d shell. This mechanism is termed central field covalency. In the second mechanism, referred to as symmetry restricted covalency, delocalization of the trans-... 

In the A-B fragment development we similarly define the following average contributions of both constituent atoms to the diatomic covalency (delocalization) entropy ... 

Low-barrier hydrogen bonds (LBHB). Some enzymes have been proposed to catalyze their reactions by forming a so-called low-barrier, partially covalent (delocalized) hydrogen... 

If D is less than zero, must be greater than k, where the k terms allow for anisotropic covalent reduction of the spin-orbit coupling. The ki values directly relate to differences in covalent delocalization of the different d orbitals due to formation of metal-ligand bonds. ... 

For the late TM diborides the 5(M, M ) 0.15, which is comparable with the corresponding values in elementary metals. Thus, the bonding picture in these diborides can smoothly vary from covalent delocalized bonding inside the boron sheets combined with ionic one between these sheets and metals to the covalent bonding between M and B as well as between metal atoms in all three spatial directions, which explains remarkable chemical flexibility of these diboride structure type. The weakening of the B-B bonds within the sheet for the late transition metals can explain the appearance of distorted boron sheets, observed in the crystal structures of some 4d-metal diborides. Enhanced interaction between metal atoms in two and three directions is likely related with the... 

Delocalization (Section 1 9) Association of an electron with more than one atom The simplest example is the shared electron pair (covalent) bond Delocalization is important in conjugated tt electron systems where an electron may be associated with several carbon atoms... 

Semiconductor materials are rather unique and exceptional substances (see Semiconductors). The entire semiconductor crystal is one giant covalent molecule. In benzene molecules, the electron wave functions that describe probabiUty density ate spread over the six ting-carbon atoms in a large dye molecule, an electron might be delocalized over a series of rings, but in semiconductors, the electron wave-functions are delocalized, in principle, over an entire macroscopic crystal. Because of the size of these wave functions, no single atom can have much effect on the electron energies, ie, the electronic excitations in semiconductors are delocalized. 

In aqueous alkaline conditions with chloroacetic acid the pyrido[4,3- f]pyrimidinethione (80) undergoes facile ring opening, attributed to the resonance stabilization of a delocalized covalent hydrate dianion intermediate (81) (82). Pyrido[2,3- f]pyrimidine-4-thiones (and... 

Aromatic compound (Section 11.3) An electron-delocalized species that is much more stable than any structure written for it in which all the electrons are localized either in covalent bonds or as unshared electron pairs. 

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