Dipolar solvation mechanism

This appears to be a better scheme than distinguishing between truly nonpolar and nondipolar solvents. A polar molecule can be defined as having a strongly polar bond, bnt need not necessarily be a dipole. In this framework, the solvating power of the nondipolar solvents need no longer be viewed as anomalous or as essentially dependent on specific solvation effects. Beyond this it shonld be emphasized that matty liquids have both a dipole moment and a quadmpole moment, water for example. However, for dipolar solvents snch as acetonitrile, acetone, and dimethyl snlfoxide, the dipolar solvation mechanism will prevail. For less dipolar solvents like tetrahydrofurane, quadrapoles and dipoles might equally contribute to the solvation energetics. ... 

Another class of systems for which the use of the continuum dielectric theory would be unable to capture an essential solvation mechanism are supercritical fluids. In these systems, an essential component of solvation is the local density enhancement [26,33,72], A change in the solute dipole on electronic excitation triggers a change in the extent of solvent clustering around the solute. The dynamics of the resulting density fluctuations is unlikely to be adequately modeled by using the dielectric permittivity as input in the case of dipolar supercritical fluids. 

Going back to the interpretation of correlated rotations, the issues resumed above give us a picture of short-range order where a central molecule is surrounded by those of its first coordination shell, reminiscent of molecular aggregates that are known to occur about electrons, radicals or ions, and thus refe-red by analogy as a "dipolar solvation" process. At present, the way the dynamics would be affected by dipolar self-solvation deserves further research concerning mechanisms that entail rotational coherence. 

The dependence of the SD mechanism on the form of AE can be investigated via the comparison of the corresponding influence spectra. - This is illustrated in Fig. 4 where Psolv ( ) for electrostatic dipolar-symmetry and Lennard-Jones (LJ) AE s are shown. In addition of the total solvation spectra, the subspectra corresponding to the solute and the solvent molecule that has the strongest influence on the solute as measured by the square of its influence coefficient Cj... 

Short-lived organic radicals, electron spin resonance studies of, 5, 53 Small-ring hydrocarbons, gas-phase pyrolysis of, 4, 147 Solid state, tautomerism in the, 32, 129 Solid-state chemistry, topochemical phenomena in, 15, 63 Solids, organic, electrical conduction in, 16, 159 Solutions, reactions in, entropies of activation and mechanisms, 1, 1 Solvation and protonation in strong aqueous acids, 13, 83 Solvent effects, reaction coordinates, and reorganization energies on nucleophilic substitution reactions in aqueous solution, 38, 161 Solvent, protic and dipolar aprotic, rates of bimolecular substitution-reactions in,... 

Solvation of thiolates is similarly low in both protic and dipolar aprotic solvents because of the size and polarisability of the large weakly basic sulfur atom, so is unlikely to contribute appreciably to the observed solvent effect. The intermediate nitro radical anion is stabilised by H-bonding in a manner which retards its dissociation in the SrnI mechanism (upper equation in Scheme 10.35). In contrast, the electron flow in the direct substitution at X (lower equation in Scheme 10.35) is such that solvation by methanol promotes the departure of the nucleofuge. In summary, protic solvation lowers the rate of the radical/radical anion reactions, but increases the rate of the polar abstraction yielding disulfide. 

The photostimulated reactions of thiolate anions with 2-halo-2 -nitropropane derivatives yield both oc-nitrosulphides via an S l pathway and disulphides (equation 71a)282 284. In contrast with the case of the oxidative dimerisation products of the mono-enolates, the disulphides are formed via an ionic mechanism nucleophilic attack by the thiolate anion on the a-halogen and subsequent reaction of a second thiolate with the sulphenyl halide. As expected for such a process, disulphide formation is favoured (and thus a-nitrosulphide formation is disfavoured) the more nucleophilic the thiolate (i.e. derived from a less acidic thiol) and the easier the abstraction of the halo-substituent (i.e. I > Br > Cl). Use of the protic solvent methanol instead of the usual dipolar aprotic solvents for the reaction of equation 71a is detrimental to the yield of the S l substitution products exclusively disulphides are formed285 (equation 71b). Methanol solvation probably retards the dissociation of the radical anion intermediate in the SRN reaction, into radical and anion, and hence retards the chain reaction relative to the ionic reaction. The non-nucleophilic methylsulphinate ion gives only an S l reaction product with 2-bromo-2-nitropropane286. 

Factors such as solvation and, in the case of ion-radicals, the counterion, may influence the properties of radicals. It is beyond the scope of this chapter to describe ion aggregation mechanisms and the factors which govern the hyperfine splittings manifested by counterions. The subject has been reviewed, however, by a prime mover in the field.12 Suffice it to say that the association of an ion-radical with a counterion may lead to a considerable redistribution of spin within the radical with consequences for the chemistry. For example, disproportionation equilibria and persistence may be influenced by the nature of the association.68 Closely allied to the phenomenon of ion association is, of course, solvation. Whether or not an ion-pair or other ionic assemblage exists in preference to free ions depends on the extent of the solvation of the ions. Nonionic radicals are also subject to variation in properties with change in solvent principally owing to interaction of the solvent with dipolar charges within the radical. 

The proposed mechanism is in accordance with the observed solvent dependence of the reaction. Whereas the dipolar sulfoxide is expected to be more strongly solvated with an increase in solvent polarity, the less dipolar sulfenate should be relatively insensitive to such a solvent change. Stabilization of the sulfoxide, relative to the less dipolar activated complex (which should be similar to the sulfenate intermediate), increases the enthalpy of activation, AH. This is refiected in the necessity of breaking increasingly strong solute-solvent interactions. On the other hand, because desolvation on activation is expected to increase the degrees of freedom in the system, a more positive AS is expected to work in the opposite direction and effect a compensating increase in k with... 

In water, ionization of the C-Br bond occurs first (Ei mechanism) to give the intermediate resonance-stabilized benzylic zwitterion C. After fast rotation about the C-C bond, carbon dioxide leaves conformer D perpendicularly to the plane of the car-benium ion, to give mainly the most stable ( )-isomer of / -bromostyrene. In butanone, after fast rotation about the C-C bond, elimination of CO2 and Br occurs in a concerted single-step (E2 mechanism) for stereoelectronic reasons (Br and C02 must be anti to one another) to give conformer B, which decomposes exclusively to the thermodynamically less stable (Z)-isomer. In more polar solvents, the partly zwitterionic activated complex, leading to zwitterion C in the rate-determining step, will clearly be more stabilized by solvation than the less dipolar activated complex leading directly to the (Z)-isomer of / -bromostyrene from conformer B [851]. 

Electric polarization, dipole moments and other related physical quantities, such as multipole moments and polarizabilities, constitute another group of both local and molecular descriptors, which can be defined either in terms of classical physics or quantum mechanics. They encode information about the charge distribution in molecules [Bbttcher et al, 1973]. They are particularly important in modelling solvation properties of compounds which depend on solute/solvent interactions and in fact are frequently used to represent the -> dipolarity/polarizability term in - linear solvation energy relationships. Moreover, they can be used to model the polar interactions which contribute to the determination of the -> lipophilicity of compounds. 

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