Solvent-induced electronic polarization

A simple analysis of the influence of the solvent-induced electronic polarization on the magnetic shielding of pyridine in water... 

The merit of the generalized SCRF theory is that it correctly describes polarization effects on the solute subsystem which may be important, e.g. in enzymatic reactions. The fact that the solute nonlinear Schrodinger equation is explicitly solved, allows one to have special solvent induced electronic states, which could appear otherwise as excited states for the isolated system. 

The microscopic mechanism of these reactions is closely related to interaction of the reactants with the medium. When the medium is polar (e.g., water), this interaction is primarily of electrostatic nature. The ionic cores of the donor and acceptor located at fixed spatial points in the medium produce an average equilibrium polarization of the medium, which remains unchanged in the course of the reaction and does not affect the process of electron transfer itself. The presence of the transferable electron in the donor induces additional polarization of the solvent around the donor that is, however, different from polarization in the final state where the electron is located in the acceptor. 

Convincing evidence was found that the majority of acyclic aldo-nitrones exist in the Z-form, by investigating the ASIS-effect (aromatic solvent induced shift effect) (399). However, in some cases, specified by structural factors and solvent, the presence of both isomers has been revealed. Thus, in C -acyl-nitrones the existence of Z -and -isomers was detected. Their ratio appears to be heavily dependant on the solvent polar solvents stabilize Z-isomers and nonpolar, E-isomers (399). A similar situation was observed in a- methoxy-A-tert-butylnitrones. In acetone, the more polar Z-isomer was observed, whereas in chloroform, the less polar E-isomer prevailed. The isomer assignments were made on the basis of the Nuclear Overhauser Effect (NOE) (398). /Z-Isomerization of acylnitrones can occur upon treatment with Lewis acids, such as, MgBr2 (397). Another reason for isomerization is free rotation with respect to the C-N bond in adduct (218) resulting from the reversible addition of MeOH to the C=N bond (Scheme 2.74). The increase of the electron acceptor character of the substituent contributes to the process (135). 

Of the three models that have been proposed to explain the properties of excess electrons in liquid helium, two are considered in detail (1) The electron is localized in a cavity in the liquid (2) The electron is a quasi-free particle. The pseudopotential method is helpful in studying both of these models. The most useful treatment of electron binding in polar solvents is based on a model with the solution as a continuous dielectric medium in which the additional electron induces a polarization field. This model can be used for studies with the hydrated electron. 

These findings indicate the complexity of the solvent polarization effect on the solute charge distribution, which shows differential trends depending on the nature of the solute. In conjunction with the free energy of solvation, the analysis of the solvent-induced changes in the solute s electron density should be valuable to shed light on the influence of solvation on the chemical reactivity of solutes. 

Nine years later, Leonhardt and Weller detected an excimer type emission in solutions containing perylene and dimethylaniline [80]. This first heteroexcimer has become the prototype of an ever expanding area of research. Perhaps the impact of these observations are best illustrated by the monograph dealing with the new phenomenom published only 12 years after the first report [81]. The significance of this research for the proper understanding of photo-induced electron transfer is born out by the first positive identification of a radical anion resulting from the irradiation of a donor-acceptor system in polar solvents (vide infra) [82]. 

A study of chemically induced dynamic electron polarization, CIDEP (see Section 12.3.3) on F and G pairs of radicals formed under photolysis of a common termo- and photoinitiator 2,2 -azobis(2-methylpropionitrile) (AIBN) led to a tentative conclusion that initial spatial separation of 2-cyano-2-propyl radicals does not depend upon viscosity However, it is plausible that the diamagnetic dinitrogen molecule formed under photolysis of AIBN (and is invisible by ESR) separates further from a contact RP under photolysis in solvents of lower viscosity. The problem of initial spatial separation and mutual orientation ofradicals under photolysis still waits experimental elucidation. 

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