Electron charge polarization

The discussion above has been more or less empirical and descriptive. However, considerable effort has been made to interpret 3-SCS on a more physical basis. Electric-field effects (71-75) were invoked to explain signal shifts of 3-carbon atoms induced by protonation of amines (157,158) (cf. Section II-B-3). This approach was later extended to other functionalities by Schneider and coworkers, who assumed that the SEF component (E2) rather than inductive properties of the substituents should be responsible for 3-SCS (113). They found fairly linear correlations of 3-SCS(X ) and 3-SCS(X ) in cyclohexyl derivatives (76) and attributed the difference between these for a given X to a widening of the C -Cp-Cv bond angle by 2.2° in the axial conformer (114,159). The decrease of 3-SCS in the order primary Cp —> secondary Cp — tertiary Cp — quaternary Cp was explained by electron-charge polarization in the Cp-C" bond(s) induced by the LEF component of the C -X dipole, which is already of significance at this distance, though ( 2) still dominates (160). Such an electron flow toward the 3 carbon is expected to be much more pronounced in C-C than in C-H bonds because of the polarizability difference (aCH = 0.79 acc = 1.12) (150,151,160). 

As discussed in Section X, the most relevant application of this idea is the possibility of measuring nonlinear optical responses (which are properties based, at least in principle, on mechanisms that involve electronic charge polarization), through the measurement of infrared intensities and Raman cross sections. 

Partial charges Polarity indices Topological electronic index Multipoles... 

Compare the electronic charges at chlorine in chlorocy-clohexane and chlorobenzene on Learning By Modeling to verify that the C—Cl bond is more polar in chlorocyclohexane. 

The large charge transfer from one atom to the other in the formation of a predominately ionic molecule is accompanied by a polarization of the electronic charge of the atoms in a... 

It is important to understand that the atomic charges refer to atoms that are not spherical. Consequently the centroid of electronic charge of an atom does not in general coincide with the nucleus, and each atom therefore has an electric dipole moment—or, more generally, an electric dipolar polarization (since only the dipole moment of electrically neutral atoms is origin independent). 

Some simple global descriptors are molecular weight, number of atoms present in a molecule (e.g., number of chlorine atoms), number of double bonds, etc. Other descriptors represent the ramification of the molecule. Certain descriptors take into consideration the electronic charge on a certain atom, or its polarity. 

We start by recalling that the framework of diahatic states depicts a competition in solution between the electronic resonance coupling [5 — which tends to delocalize the solute electronic charge — and the solvent polarization — which tries to localize it, to better solvate the reaction sys-... 

I consider there to be a sharp distinction between the most polar form of a molecule and its ionically dissociated form. The reason for this is empirical An ion is defined as a species carrying a charge equal to an integral multiple of the electronic charge, and this definition implies that it will have a characteristic predictable electronic spectrum and, under suitable conditions, mobility in an electric field. There is so far no evidence which would compel one to abandon this definition, and I think it is important to distinguish clearly in this context between reaction intermediates (chain carriers, active species) of finite life-time, and transition states. 

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