Molecules medium effect

Sn2 reactions with anionic nucleophiles fall into this class, and observations are generally in accord with the qualitative prediction. Unusual effects may be seen in solvents of low dielectric constant where ion pairing is extensive, and we have already commented on the enhanced nucleophilic reactivity of anionic nucleophiles in dipolar aprotic solvents owing to their relative desolvation in these solvents. Another important class of ion-molecule reaction is the hydroxide-catalyzed hydrolysis of neutral esters and amides. Because these reactions are carried out in hydroxy lie solvents, the general medium effect is confounded with the acid-base equilibria of the mixed solvent lyate species. (This same problem occurs with Sn2 reactions in hydroxylic solvents.) This equilibrium is established in alcohol-water mixtures ... 

Seliskar C, Brand L (1971) Electronic spectra of 2-aminonaphthalene-6-sulfonate and related molecules. II. Effects of solvent medium on the absorption and fluorescence spectra. JACS 93 5414-5420... 

Reasonably reliable pATbh+ values for the protonation of weak bases or of weakly basic substrates can be obtained via equation (17), together with m slope parameters that can be used to classify basic molecules as to type, and for an estimate of the solvation requirements of the protonated base. Measurements at temperatures other than 25°C can be handled using equation (22), and enthalpies and entropies for the protonation can be obtained. Protonation-dehydration processes are covered by equation (26). Medium effects on the... 

It is frequently necessary to subtract from the total absorption a part owing to the un-ionized molecule in measuring the amount of ion present. If a given solute exists in two forms, such as a molecule and an ion, it is quite likely that the absorption curves of the two forms will intersect at some point. The intersection is known as an isobestic point, and at the wave length of the isobestic point the extinction depends only on the total amount of solute and not on the proportion of the two forms.166 One investigator who had the bad luck to pick the isobestic wave length for all his experiments came to the mistaken conclusion that it was not possible to determine the position of his particular equilibrium by optical means. A medium effect will cause a family of absorption curves to intersect in a small region rather than in a point, but failure to show an isobestic point even approximately means that the solute exists in more than two forms. 

One of the most important characteristics of micelles is their ability to enclose all kinds of substances. Capture of these compounds in micelles is generally driven by hydrophobic, electrostatic and hydrogen-bonding interactions. The dynamics of solubilization into micelles are similar to those observed for entrance and exit of individual surfactant molecules, but the micelle-bound substrate will experience a reaction environment different from bulk water, leading to kinetic medium effects308. Hence, micelles are able to catalyse or inhibit reactions. The catalytic effect on unimolecular reactions can be attributed exclusively to the local medium effect. For more complicated bimolecular or higher-order reactions, the rate of the reaction is affected by an additional parameter the local concentrations of the reacting species in or at the micelle. 

Whenever possible, we will attempt to identify the effects of interactions between relatively few frontier orbitals, which describe the distribution of relatively few electrons of a molecule. We effectively ignore the vast numbers of the rest of the electrons in the system as well as the nuclear charges. In some cases this neglect is not justified and may lead to misleading results. When a molecule or parts of it are charged, coulombic interactions may dominate. The electrostatic energy of interaction of each pair of centers, i and j, with net charges, qt and qj, respectively, separated by a distance Ry in a medium with dielectric constant em has the form... 

In addition, medium effects play an important role through the interaction of solvent molecules with p and a as well as with each other thus the two partners should present geometrically matched hydrophobic/hydrophobic or hydrophilic/hydro-philic (solvophobic, solvophilic, more generally) domains. 

The medium may have a marked effect on the shape of receptor molecules itself. Shape modifications could strongly influence their substrate binding properties, for instance in the case of amphiphilic cyclophane receptors subjected to hydrophobic-hydrophilic factors in aqueous solution. Such medium effects in action are visualized by the solid state structures of two different forms of the water-soluble hexasodium salt of the macrobicyclic cyclophane 66, which could be crystallized in two very different shapes an inflated cage structure 71 building up cylinders disposed in a hexagonal array and a flattened structure 72 stacked in molecular layers separated by aqueous layers in a lamellar arrangement [4.73]. These two... 

Solvolytic reactions, medium effects on the rates and mechanisms of, 14, 1 Spectroscopic detection of tetrahedral intermediates derived from carboxylic acids and the investigation of their properties, 21, 37 Spectroscopic observations ofalkylcarbonium ions in strong acid solutions, 4, 305 Spectroscopy, 13C NMR in macromolecular systems of biochemical interest, 13, 279 Spectroscopy of substituted phenylnitrenes, kinetics and, 36, 255 Spin alignment, in organic molecular assemblies, high-spin organic molecules and,... 

Results for improved by introducing the surrounding 4 water molecules into the cavity, but still only leads to 45% of the gas-to-liquid shift for the 170 nucleus (97). Likewise, this method fails to account for all of the gas-to-liquid shift of 19F in fluoromethanes (99) and of 77Se in H2Se (100). Clearly, medium effects can not be treated accurately by using a reaction field model. The major problem with the above two approaches is that only the electric polarization effects are included in the model. 

Medium Effects on NMR Chemical Shift Most MO calculations of nuclear shielding relate to the case of a molecule in a vacuum. For nuclei forming the molecular skeleton, such as 13C, and nuclei with small shielding ranges, such as H, this may not be an unreasonable approximation. This is particularly true if comparison of the theoretical results is to be made with the experimental data taken on a molecule dissolved in an inert solvent. 

The second explanation for the solvent isotope effect arises from the dynamic medium effect . At 25 °C the rotational and translational diffusion of DjO molecules in D20 is some 20% slower than H20 molecules in H20 (Albery, 1975a) the viscosity of D20 is also 20% greater than H20. Hence any reaction which is diffusion controlled will be 20% slower in D20 than in H20. This effect would certainly apply to transition state D in Fig. 3 where in the transition state the leaving group is diffusing away. A similar effect may also apply to the classical SN1 and SN2 transition states, if the rotational diffusion of water molecules to form the solvation shell is part of the motion along the reaction co-ordinate in the transition state. Robertson (Laughton and Robertson, 1959 Heppolette and Robertson, 1961) has indeed correlated solvent isotope effects for both SN1 and SN2 reactions with the relative fluidities of H20 and D20. However, while the correlation shows that this is a possible explanation, it may also be that the temperature variation of the solvent isotope effect and of the relative fluidities just happen to be very similar (see below). 

Extensions of EET theories to account for coherence in the rate expression [28-30] and complicated donor-acceptor aggregates [23-27] have been described in the literature. To understand the effect of the medium on the EET rate we need to focus on the electronic coupling component of the theory [11]. A primary assumption of Forster theory, relevant to the medium effect, is that the electronic coupling can be approximated as a dipole-dipole interaction between transition dipole moments of the donor and acceptor molecules,... 

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