Formal local charge

Local charge compensation of the formally charged Fe(III) heme, as discussed more fully in a later section, demonstrates a significant modulation (s 210 mV) of the heme reduction potential in a designed heme protein. This scaffold-dependent effect has been shown to be additive to the heme-dependent effect of porphyrin peripheral architecture to demonstrate the modulation of a designed heme protein reduction potential by 450 mV using a single maquette scaffold. 

Little is known of the actual mechanism. A mode of reaction is possible, in which the oxygen atom at the top of the ozone molecule with a formal positive charge (p. 230) reacts with an electron pair, not localized in a bond but on one carbon atom, and in which the ozone therefore reacts by an electrophilic mechanism (Wibaut, Sixma and Kampschmidt). However, in order to explain the differences between the reaction course for ozonization and for other electrophilic reactions, e.g., bromination and nitration with pyrene, these authors assume also an interaction of one of the other oxygen atoms with the adjacent carbon atom. The net result is, however, about the same as that predicted by the bond localization hypothesis. 

In push-pull enamines the groups X1 and X2 are capable of stabilizing a negative charge, while the nitrogen atom, on the second trigonal carbon, bears a positive charge. The structure of 27 clearly implies reduction of the bond order of the formally localized double bond in 26 and a concomitant increase in the orders of the C—N and the C—X1 and C—X2 bonds. 

The application of any xr-matrix also influences the distribution of the formal electrical charge of each atom (localized) resp. DE-system (delocalized). The monitoring of thoses changes during the sucessive applications of the B- and D-functions is done automatically according to the following principles. 

The influence of the various B-functions on the localized formal electrical charge of both atoms that are affected by a B-function may be summarized as follows ... 

The most convincing evidence concerning the relative stabilities of thiophene S,C-ylids, S,N-ylids, and the sulfoxides comes from ab initio restricted Hartree-Foch molecular orbital (MO) calculations, in which the molecular parameters of the ylid 18 and the S-methyl thiophenium salt 19 were compared (84CC859). The ylid (18) has never been isolated, nor would one intuitively expect it to enjoy any real stability. However, it was chosen to simplify the MO calculations. The stability of ylids such as 14 and 15 is almost certainly due to the ability of these systems to delocalize the formal negative charge over several atoms, and clearly this facility is not present in 18. Thus, 18 represents a highly localized ylidic structure. In principle, the salt 19 can be... 

With the charge, ze, transferred by deposition or dissolution of one single atom, a formal local current density, or, for each site of type x or x, respectively, can be defined, taking into account the conjugated pair of reactions (eq. 2.9) ... 

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