Solvent induced polarization

An illustrative example of a strongly solvent-dependent ESR speetrum is given in Fig. 6-10. It shows low-field portions of the ESR spectra of the stable, distillable ( ) 4-(methoxycarbonyl)-l-methylpyridinyl radieal, measured in three solvents of increasing polarity [381]. The different patterns of the ESR lines observed in different media arise from the solvent influence on the spin distribution, as measured by the HFS constants fl( H) and fl( " N). This change in spin distribution results from solvent-induced polarization of the earbonyl group, whieh is eoplanar and conjugated with the pyridinyl ring. 

The hcc s obtained at the B3LYP/EPR-II level are shown in Table 12. The calculated hcc s can be dissected into three terms a contribution due to the electronic and structural configurations assumed by the radicals in the gas phase (first column in Table 12) a contribution due to the solvent-induced polarization on the solute wave function without allowing any relaxation of the gas-phase geometry (direct solvent effect, second coliunn in Table 12), and a last contribution due to the solvent-induced geometry relaxation (indirect solvent effect, third column in Table 12). 

This partition of (7(R) is frequently used (see, for example, Section 3). The use of AMSOL has the merit of introducing, even if at a low computational level, solvent-induced polarization at an SCF level. 

As noted above, a dielectric enviroment reacts to the presence of the solute generating a reaction field, which in turn interacts with the solute charge distribution. Several studies [12,21-23,95-99] have shown that when a solute is immersed in water or other polar solvent, a rapid reorganization of the electron distribution occurs, leading to an increase in the molecular polarity. The magnitude of the solvent-induced polarization depends on the solvent and on the characteristics of the solute, especially its polarity and polarizability. 

The so-called self-assembly technique has its origin in 1946, when a paper was published by Bigelow et a] [116] and tluis is slightly younger tlian tlie LB teclmique. The autliors noted tliat a hydrophilic surface exposed to an amphiphilic compound dissolved in a non-polar solvent induces tlie amphiphilic material to fonn a monolayer on it. 

Continuum models of solvation treat the solute microscopically, and the surrounding solvent macroscopically, according to the above principles. The simplest treatment is the Onsager (1936) model, where aspirin in solution would be modelled according to Figure 15.4. The solute is embedded in a spherical cavity, whose radius can be estimated by calculating the molecular volume. A dipole in the solute molecule induces polarization in the solvent continuum, which in turn interacts with the solute dipole, leading to stabilization. 

Because of these observations, comparative experiments with peptides of different proline content in a solvent less polar than water, are recommended. (Pro-Pro-Gly)n and (Pro-Ala-Gly)n, in methanol/acetic add (volume ratio 9 1) show a temperature-induced triple helix-coil transition which is characterized by the following parameters92,150) (Pro-Pro-Gly)n AH°s = -1.9 kJ/rnol tripeptide AS° = -5.4 J tnor1 K (Pro-Ala-Gly) A HI = -0.9 kJ/mol tripeptide A 5° = -3.8 J mol-1 K ... 

A detailed examination of the kinetics of dimethylaminolysis of N3P3C16 by Krishnamurthy and co-workers has revealed that there is a gradual and subtle mechanistic change that occurs as the degree of replacement of chlorines increases (92). While the first chlorine replacement follows an Sn2 pathway involving the formation of a neutral five-coordinate intermediate [Fig. 8(A)], at the second stage the mechanism can be induced to follow a concerted path [Fig. 8(B)] by using acetonitrile as the solvent. The polar transition state of the concerted path reaction pathway is stabilized in acetonitrile. This postulate has sup-... 

Duckett381 reported on the use of parahydrogen-induced polarization (PHIP) to delineate the pathways involved in the catalytic hydrogenation of alkenes and alkynes by [Ru3(CO)12 x(PPh3)x] (x = 1 or 2) and showed that the mechanism is highly dependent on the solvent. Bassett and... 

Recently, Hajek and Radoiu observed that MW not only increase the rate of heterogeneous catalytic reactions, but also affect the product selectivity [85], The results were explained in terms of MW-induced polarization, involving the absorption of MW by highly polarized reagent molecules on the active site of the catalyst. On the other hand there is little, if any, activation of homogeneous catalytic reactions in polar solvents [86], presumably due to the high absorbent power of MW irradiation by the solvent. 

It is important to keep in mind that the ability of dipoles to associate is influenced by their environment. Many studies on association of dipoles have been carried out in solutions containing the polar molecules. If the molecules of the solvent are polar or can have polarity induced in them (see Section 6.3), association of the solute molecules will be hindered. The solvent molecules will surround the polar solute molecules, which will inhibit their interaction with other solute molecules. The solute... 

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). 

In the case of charged molecules, an additional friction force should be introduced as a result of the induced polarization of the surrounding solvent molecules. 

No particular relationship between solvent polarity and the observed change is apparent. Rader notes that the experimental conditions are such that no appreciable intramolecular hydrogen-bonding occurs between methanol molecules, presumably ruling out the possibility that the change arises from solvent induced variation of the self-association equilibrium. Differences in solvent-solute association are still a possible interaction mechanism. 

Good selectivity was always obtained when the esterification reactions were done in a more polar solvent (Scheme 10). The reason is likely due to decreased reactivity of the esterification reagent from solvent-induced destabilization of the stannylene intermediates. If the experiments were performed in polar solvents, higher yields of 15 and 38 would be acquired. 

Fruitful interplay between experiment and theory has led to an increasingly detailed understanding of equilibrium and dynamic solvation properties in bulk solution. However, applying these ideas to solvent-solute and surface-solute interactions at interfaces is not straightforward due to the inherent anisotropic, short-range forces found in these environments. Our research will examine how different solvents and substrates conspire to alter solution-phase surface chemistry from the bulk solution limit. In particular, we intend to determine systematically and quantitatively the origins of interfacial polarity at solid-liquid interfaces as well as identify how surface-induced polar ordering... 

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