Medium effects 256 definition

In Section 3.9.2 we review the theory of EET. In Section 3.9.3 definitions of the dielectric constant and the groundwork for a discussion of the role of the host matrix are described. Section 3.9.4 examines the role of local fields and orientation. In Section 3.9.5 we provide a description of medium effects in EET. Finally, Section 3.9.6 concludes the article. 

Step 2 is defined as the primary medium effect and is obviously by definition the same as the free energy of transfer, AG °, of the ion from the infinitely dilute solution in one solvent to the infinitely dilute solution in another solvent. 

Over the last decade, developments in high-resolution NMR techniques for solids have been extended to mercury nuclei. Solid-state NMR studies provide more definitive characterization of mercury complexes since their interpretation is not compromised by exchange processes or solvent coordination. While medium effects on spectra are not absent in the solid-state, they are generally more defined and therefore studied more readily. Furthermore, the chemical shift anisotropy, coupling constant anisotropy, and dipolar coupling constants obtained by solid-state NMR provide an additional probes into structure and bonding. Comparison of solid-state NMR spectra of structurally characterized complexes with solution-state NMR spectra promises to reveal significant differences between the solution and solid-state structures. 

Magnetic moment, 153, 155, 160 Magnetic quantum number, 153 Magnetization, 160 Magnetogyric ratio, 153, 160 Main reaction, 237 Marcus equation, 227, 238, 314 Marcus plot, slope of, 227, 354 Marcus theory, applicability of, 358 reactivity-selectivity principle and, 375 Mass, reduced, 189, 294 Mass action law, 11, 60, 125, 428 Mass balance relationships, 19, 21, 34, 60, 64, 67, 89, 103, 140, 147 Maximum velocity, enzyme-catalyzed, 103 Mean, harmonic, 370 Mechanism classification of. 8 definition of, 3 study of, 6, 115 Medium effects, 385, 418, 420 physical theories of, 405 Meisenheimer eomplex, 129 Menschutkin reaction, 404, 407, 422 Mesomerism, 323 Method of residuals, 73 Michaelis constant, 103 Michaelis—Menten equation, 103 Microscopic reversibility, 125... 

According to Owen s definitions, the AG term in 2.11.11 is related to the total medium effect, AG to the primary medium effect and the logarithmic term is the secondary medium effect. It is evident therefore that the primary medium effect (or simply the medium effect) reflects differences in ion-solvent interactions, and the secondary medium effect (or salt effect or concentration effect ) reflects differences in ion-ion interactions and solvation effects the former quantity is, of course, independent of concentration whereas the latter quantity, which is usually several orders of magnitude smaller, is defined for a constant (finite) concentration. The standard free energy of transfer is defined for the transfer of 1 mole of substance from water to the organic solvent, i.e. 

In the continuum approach to the surrounding medium one has by definition, m = 0. Medium effects are therefore presented by a reaction field term in Eq. (24). Three types of environment can be represented in this framework i) an anisotropic medium without spatial dispersion, where the permitivity tensor is defined with the ansatz c(r — r ) = c(r) (5(r — r ), that leads to a distance dependent dielectric system dependance ii) an isotropic medium which is characterized by s(r — r ) = e(r)1 (r — r ) hi) a homogeneous and isotropic medium, the permitivity tensor is the unit tensor multiplied by the static dielectric constant Thus the effective Schrodinger equation for each case is obtained from Eq. (24) after integration of the r -variable with the corresponding ansatz for the permitivity tensor. 

A competition experiment was carried out to determine whether the presence of added hercynine would decrease the incorporation of histidine-2-C into ergothioneine by N. crassa. No effect could be found. It was possible that this was due to impermeability of the cell wall to hercynine. Since W. Feldman in our laboratory had found that N. crassa in growing cultures is relatively impermeable to ergothioneine, but becomes more permeable when it is starved, the same technique was tried with hercynine. When the competition experiment was carried out with mycelium suspended in water rather than growth medium, a definite effect was observed, with a marked decrease in the uptake of histidine- -Ci in the presence of hercynine (Fig. 5). 

D. G. Hall, J. Chem. Soc. Faraday Trans. II, 68, 25 (1972). A definition of the primary medium effect for individual ionic species in solution. 

The possibility that the lack of piperidinolysis of 14 in the presence of 0.01 M CTABr might be due to preferential micellar incorporation of only one reactant (i.e., relatively more hydrophobic 14 or 14") has been ruled out based upon experimental evidence. The values of k bs at 100% v/v CHjCN, obtained in the absence of CTABr, indicate small but definite nucleophilic reactivity of piperidine toward 14. Thus, the absence of apparent nucleophilic reactivity of Pip toward 14 in the presence of 0.01 M CTABr cannot be attributed to merely micellar medium effect. 

If we now transfer our two interacting particles from the vacuum (whose dielectric constant is unity by definition) to a hypothetical continuous isotropic medium of dielectric constant e > 1, the electrostatic attractive forces will be attenuated because of the medium s capability of separating charge. Quantitative theories of this effect tend to be approximate, in part because the medium is not a structureless continuum and also because the bulk dielectric constant may be an inappropriate measure on the molecular scale. Eurther discussion of the influence of dielectric constant is given in Section 8.3. 

Another pertinent observation is the fact that the reaction proceeded twice as fast in -butyraldehyde (polar) as in benzene (nonpolar), even though the catalyst concentration was reduced to only one-third the comparable level. A graphic illustration of this effect is given in Fig. 9. The rate of gas uptake is plotted as a function of time for a reaction conducted in benzene and again for a second reaction conducted in butyraldehyde. The rate of reaction in the polar solvent was initially fast and decreased with time. The rate in the nonpolar benzene was initially slow, became faster as the solvent became more polar with the presence of product aldehyde, and then subsequently diminished with time. When the data were replotted as the log of unreacted olefin vs. time, the polar medium reaction showed first-order dependence on olefin concentration, whereas the nonpolar solvent reaction showed no definite order, owing to the constantly changing polarity. 

The effective molarity (EM) is formally the concentration of the catalytic group (RCOO- in [5]) required to make the intermolecular reaction go at the observed rate of the intramolecular process. In practice many measured EM s represent physically unattainable concentrations, and the formal definition is probably relevant only in reactions (which will generally involve very large cyclic transition states) where the formation of the ring or cyclic transition state per se is enthalpically neutral, or in diffusion-controlled processes. For the formation of small and medium-sized rings and cyclic transition states the EM as defined above contains, and may indeed be dominated by, the enthalpy of formation of the cyclic form. This topic has been discussed briefly by Illuminati et al. (1977) and will be treated at greater length in a future volume in this series. 

Since this type of result shows that sonication is definitely not just another method of providing agitation of a medium, but exhibits its own peculiarities, it stands to reason that it should obey some rules of its own. An examination and classification of published material led to an empirical systemisation of sonochemistry [28, 30). This classification concentrates on the chemical effects in sonochemistry but it should also be recognised that in some cases ultrasound does act in a mechanical sense achieving remarkable results through super agitation. Sometimes the mechanical and chemical effects occur together. 

Temperature effect on ion-radical stability and the very possibility of ion-radical organic reactions have already been discussed in the preceding chapters. Flowever, one topic of the problem deserves a special consideration, namely, the effect of solvent temperature on dynamics of IRPs. In a definite sense, IRPs are species close to CTCs. As known, the lower the medium temperature, the higher is the stability of CTCs. And what about IRPs ... 

---