Solvent effects on chemical shift

The solvent induced shifts are very high and have been shown in a great number of compounds. Only studies specifically devoted to solvent effects will be discussed here  

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Benzene solutions of polar molecules Solvent effects on chemical shifts 121... 

Give a clear indicaUon of solvent, concentration, and temperature. These parameters have a much greater effect on chemical shifts and coupling constants for fluorine than for protons. 

Pi-complexing is most commonly used to rationalize effects observed in aromatic solvents. The most frequent evidence cited is magnetic anisotropy effects on chemical shifts in the solute molecule. As was the case for hydrogen bonding no quantitative correlations with substantive parameters such as ultraviolet spectral shifts have been attempted. 

Although observations are scattered throughout the literature, solvent effects on 29Si NMR parameters have been studied systematically only to a slight extent. Attention has been focussed mainly on effects on chemical shifts despite the fact that all parameters are affected by solvent, concentration and temperature. 

As has been shown, several solvents produce specific effects on chemical shifts, and care should be taken when assigning lines or structures on the basis of comparison of chemical shifts obtained in different solvents or when using literature data with no experimental details given. 

The concentration of the sample in a particular solvent has little effect on chemical-shift values and, because of the inherently low sensitivity of 13C-n.m.r. spectroscopy, it is advantageous to use as concentrated solutions as possible when measuring these spectra. However, increased concentration, and consequently increased viscosity, causes line broadening due to decreased, spin-lattice relaxation-times (Tj values),18 and thus, poorer resolution. Certain solvents that tend to give viscous solutions (for example, Me2SO-d6) may also give decreased resolution. 

Deuterium isotope effects on chemical shifts of phenols of which the OH proton has been exchanged by deuterium can be measured in two different ways. If the OH(D) proton is exchanging slowly (see Section II.B) two different resonances are observed, one due to the protio and one due to the deuterio species (see Figure 1). The relative intensities will depend on the H D ratio, perhaps not in a quantitative way due to fractionation (see Section II.O). If exchange is fast on the NMR time scale only one resonance for the X-nuclei (e.g. C) is observed, the position of which depends on the H D ratio. In order to determine the isotope effects properly, a series of experiments must be conducted varying the H D ratios of the exchanging species, typically 1 5, 1 2, 1 1, 2 1 and pure solvent . The exchanging species is typically H2O D2O but could equally well be deuteriated alcohols, ROD. 

Some precise data (61) are reported in Table XVI on the nitrogen chemical shifts of CH3CN, CN0, NCO0 and NCSe. Attention is drawn to the appreciable solvent effects on the shift of CH3CN, giving a range of 20 ppm. 

A review with 116 references was given. Isotope effects on chemical shifts, nA C(D), nA H(D), 1A N(D) and 1A C( 0), and solvent isotope effects in proteins are reviewed and references are provided to related cases. 

The term has previously been applied to the effect on chemical shift of specific solute—solvent interactions in solution, and it has been pointed out that these should cause a small paramagnetic solvent shift in this usage, shifts due to anisotropy effects associated with specific interactions are included in the term. In this discussion, however, we shall take to represent the effect of solvent anisotropy in a geometrically specific solute—solvent orientation which has been brought... 

The chemical shifts of polar molecules are frequently found to be solvent dependent. Becconsall and Hampson have studied the solvent effects on the shifts of methyl iodide and acetonitrile. The results obtained from dilution studies in various solvents may be explained as arising from a reaction field around the solute molecules. The spherical cavity model due to Onsager was used to describe this effect, and this model was completely consistent with the experimental data when a modified value for the dielectric constant, s, of the particular solvent was used. 

However, even with this technique we must take into account the possibility of medium effects on chemical shifts. So far two techniques have been used to compensate for this effect one procedure uses the intramolecular shift between two groups (e.g. CH2 and CH3 in an ethyl derivative), and the other the intermolecular shift relative to a suitable chosen reference (in most cases the trimethylammonium ion) as an internal reference. The first method has the advantage that it does not require the introduction of any other substances into the solutions, but it suffers from the disadvantage that the relative intramolecular shifts are small. The use of an internal standard, whose chemical shift is also affected by the solvent, may alter the acidity of the solution, although the use of small amounts (0-05 m) of trimethylammonium sulfate should give negligible changes. 

Since then, the generality and importance of solvent effects on chemical reactivity and physical properties of species in dilute solutions has been widely acknowledged. Solvent-solute interactions for reactants and for products account for observed shifts in chemical equilibria those involving reactants and transition states determine changes in the rates of elementary processes. Shifts of the absorption and/or fluorescence maxima originate in differential solvent-solute interactions of the ground and electronically excited states of a dissolved species. The perturbations induced by the solvents are reflected by concurrent variations of such physical properties of the solute as ir, nmr, and epr spectra and partial molar properties. 

Dr. B. Chawla (68a) in this laboratory has recently carried out a study of solvent effects on the shifts of several aromatic compounds. A series of typical results is given in Table 16. It shows the influence of the environment on the chemical shifts of the para and meta carbon atoms of a,a,a-trifluoromethyl-benzene relative to the signal of benzene. 

SR = calculated from measured spin rotation constants in molecular beam experiments. Aik.Hal. = from theoretical calculations of shieldings in alkali halide crystals. Aik.Hal. (P) = from pressure dependence of alkali halide shifts in combination with theoretical models for shielding values. T2 = calculated from experimental value of line width at infinite dilution. 6VS.T2 = calculated from concentration dependence of chemical shifts and line widths. H2O/D2O = calculated from solvent isotope effects on chemical shifts. NaVwa" = estimated from difference in chemical shift between Na and Na . Atomic beam = calculated from magnetic moment of free atom determined in atomic beam experiments. 

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