Electron shielding, solvent effects

Keywords NMR Chemical shielding Solvent effects QM/MM Electronic polarization effects... 

Other measures of nucleophilicity have been proposed. Brauman et al. studied Sn2 reactions in the gas phase and applied Marcus theory to obtain the intrinsic barriers of identity reactions. These quantities were interpreted as intrinsic nucleo-philicities. Streitwieser has shown that the reactivity of anionic nucleophiles toward methyl iodide in dimethylformamide (DMF) is correlated with the overall heat of reaction in the gas phase he concludes that bond strength and electron affinity are the important factors controlling nucleophilicity. The dominant role of the solvent in controlling nucleophilicity was shown by Parker, who found solvent effects on nucleophilic reactivity of many orders of magnitude. For example, most anions are more nucleophilic in DMF than in methanol by factors as large as 10, because they are less effectively shielded by solvation in the aprotic solvent. Liotta et al. have measured rates of substitution by anionic nucleophiles in acetonitrile solution containing a crown ether, which forms an inclusion complex with the cation (K ) of the nucleophile. These rates correlate with gas phase rates of the same nucleophiles, which, in this crown ether-acetonitrile system, are considered to be naked anions. The solvation of anionic nucleophiles is treated in Section 8.3. 

These solvent effects change the electron shielding around the nucleus in question, hence its response to the applied magnetic and electrical fields. The... 

Solvent effects on nuclear magnetic properties are well known, and have been studied for a long time. Both the NMR shielding constant and the nuclear spin-spin coupling constant depend on the electronic structure of the whole system. This means that both are sensitive to the weak intermolecular interactions between solute and solvent molecules. 

On the basis of such a classification an empirical approach based on the so-called solvent empirical parameters was formulated to evaluate solvent effects on nuclear shieldings. In brief, this approach, originally proposed by Kamlet, Taft and co-workers [20] for electronic excitations, does not involve QM or other types of calculations but introduces a numerical treatment of experimental data obtained for a given reference system to obtain an estimate of solvent effects on various properties. 

In the last years, these semiclassical analyses have been substituted by (or supported with) explicit descriptions of the electronic aspects of the solvent effects on NMR properties and in particular on the nuclear shielding. This change of perspective has been made possible by the large development of QM solvation models which have been coupled to QM methodologies initially formulated for isolated systems. 

We have presented and compared different solvation models (continuum, discrete, continuum + discrete) to study solvent effects on molecular properties. In particular, the nitrogen nuclear shielding, which is known to be very sensitive to even small modifications of electronic and/or nuclear charge distributions, has been analyzed. Such alternation/combination of different models has been required to study the complex nature of solute-solvent interactions when both long-range polar and shorter-range specific H-bond effects are active. 

Semipermeable membrane a membrane that allows solvent but not solute molecules to pass through. (17.6) Shielding the effect by which the other electrons screen, or shield, a given electron from some of the nuclear charge. (12.14)... 

The relationship between ring proton chemical shifts and 7r-electron densities of aromatic molecules and ions is important. S.C.F. molecular orbital calculations have been used to evaluate ring currents and associated chemical shifts for pyridine and other heterocycles 276a. When corrections are made for polar solvent effects and ring current effects, a roughly linear relationship between chemical shift and electron density is obtained for pyridine and other heterocycles 276 With 4-substituted pyridines, a simple additivity relationship of substituent contribution to the proton shieldings in these compounds can be applied, as for substituted benzenesi277 Correlations of proton chemical shifts and 7r-electron densities have been made for a... 

Solvent effects can be produced by specific interactions, such as protonation or hydrogen bonding and nonspecific interactions which may arise from solvent polarity effects. Both of these types of interaction are found in studies of solvent effects on nitrogen nuclear shieldings. The extent of substituent effects depends upon the position of substitution and the electronic nature of the substituent. 

Kohn—Sham stabilities MNR shielding tensors electron affinities cavity solvent effects analytic excited states... 

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