The Effect of Solvent on Chemical Shift

FIGURE 6.17 The appearance of the methyl resonances of iV,iV-dimethylformamide with increasing temperature. 

Depending on the rate of rotation, an averaging of the two NH absorptions could lead to peak broadening (see Sections 6.1, 6.2C, and 6.4). Thus, in amides, three different peak-broadening factors must always be considered  

The last two effects should disappear at higher temperatures, which increase either the rate of rotation or the rate of proton exchange. 

Chemists generally obtain the NMR spectrum of a substance by following a typical routine. The substance must be dissolved in a solvent, and the solvent that is selected should have certain desirable properties. It should be inexpensive, it should dissolve a wide range of substances, and it should contain deuterium for locking and shimming purposes on Fourier transform (FT) 

348 Nuclear M netic Resonance Spectroscopy Part Four 

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Barlin and Batterham223 have studied the effects of solvent on chemical shifts of the anionic and cationic species of imidazoles. Protonation shifts, obtained by direct comparison of spectra in deuteriochloroform and trifluoroacetic acid, observed for 1-methyl-imidazoles are consistent with stabilization of the resulting cations by an amidinium-type resonance (47). Thus, for 1-methylimidazole, which... 

Both non-empirical <85JMR262> and semiempirical methods <83OMR(2i)50l> have been applied successfully to the interpretation of NMR spectra, particularly to the effects of solvent on chemical shifts. 

A more detailed study (50) of the 119Sn chemical shifts of trimethyl-and triethyl-tin chloride as a function of concentration and temperature in various polar solvents has revealed the effect of complexing on chemical shift. The formation of a 1 1 complex of trialkyltin chloride in a polar donor solvent, L, may be written as ... 

The intermolecular solute/solvent interactions may arise from nonspecific interaction forces such as dispersion, dipole-dipole, dipole-induced dipole, etc., as well as from specific interactions found in protic and aromatic solvents. Solvent effects on NMR spectra were first observed by Bothner-By and Glick [226] and independently by Reeves and Schneider [227] in 1957. Since then, the influence of solvent on chemical shifts (and coupling constants) has been extensively studied by scores of workers and has been thoroughly reviewed by several specialists [1-4, 288-237]. 

A phenomenological study was performed to determine the effect of solvent on Sn NMR spectra of these organoraetallic polymers. Samples were dissolved in chloroform, benzene, n-hexane, acetone, tetrahydrofuran, methanol, and pyridine. The Sn NMR spectra in these solvents are given in Figure 1. The appearance and location of the H Sn resonance changes drastically over the range of selected solvents. The chemical shift moves upfield in the order chloroform, benzene, n-hexane, acetone, tetrahydrofuran, pyridine, and methanol. The amount of structural information and, conversely, the broadening of the resonance increases in the same order with methanol and pyridine reversed. 

The sequence analysis, tacticity, primary and secondary structure, influences of (a) cis-trans isomerization, (b) neighboring residue effects, and (c) type and concentration of solvent on chemical shifts for a number of synthetic polypeptides have been reported by Kricheldorf and coworkers. The investigated polypeptides (excluding homopolyglycine which is usually considered as a polyamide) include various poly-, oligo-, and copolypeptides (10, 13, 18-19, 20-23), and particularly polymers based on glycyl-glycine units, (15, 16, 21, 24) D- and L-amino acids (17, 25-28), and L-lysine and iso-L-lysine (29). 

The PMR spectra obtained showed the effect of the lithium on the chemical shift of the protons on the terminal monomer unit in a series of polydienes, including butadiene, isoprene eind 2,3-dimethylbutadiene, and the effect of solvents on these spectra. 

The effect of a change in temperature or solvent on chemical shift is a parameter which must be carefully studied. On the basis of several examples it was proposed that evidence for hydrogen bonding (36) could be derived from the failure of an amide-proton chemical shift to be sensitive to a change in temperature. Later observations from this laboratory have shown that the chemical shift of certain amide protons not hydrogen bonded also may not be sensitive to a change in temperature. 

The reader may gain better appreciation of the many basic differences responsible for the division into different classes of heteronin by comparing certain representative members, directly or through appropriate models, in terms of the information presented in Table II. First, one notes that the classification of oxonin (24a) as atropic, V-methylazonine (27a) as nondescript, and 1//-azonine or its anion as diatropic, originally proposed on the basis of 1H NMR chemical shifts (data shown in first three rows), was confirmed by the determination of solvent shift character (S values)38,39 that revealed 1//-azonine to possess significant diatropic influence (comparable to that of naphthalene + 1.3538), the V-methyl counterpart to exhibit a far weaker effect in the same direction, and oxonin to be atropic or mildly paratropic under this criterion, its S value being closely similar to that of the family s 8rr-electron polyenic model, all-m-cyclononatetraene (24 X = CH2). Major differences between oxonin and parent azonine are also seen to exist in terms of thermal stability and 13C NMR and UV spectroscopy, all of which serve further to emphasize the close structural similarity of oxonin with n-... 

Different methods for the study of selective solvation have been developed [118, 120] conductance and Hittorf transference measurements [119], NMR measurements (especially the effect of solvent composition on the chemical shift of a nucleus in the solute) [106-109], and optical spectra measurements like IR absorption shifts [111] or UV/Vis absorption shifts of solvatochromic dyes in binary solvent mixtures [124, 249, 371]. Recently, the preferential solvation of ionic (tetralkylammonium salts) and neutral solutes (phenol, nitroanilines) has been studied particularly successfully by H NMR spectroscopy through the analysis of the relative intensities of intermolecular H NOESY cross-peaks [372]. 

Solvent effects on the fluorescence spectrum and the effect of protonation on electron density and chemical shift in NMR spectrum of pyridazine were investigated. 

Solvent isotope effects (H2O D2O) on chemical shifts are much larger in o-and p-fluorophenolates than in the corresponding phenols and much larger than that in the m-fluorophenol, thereby relating the strength of the solvation of the fluorine to its electronegativity. ... 

Theoretically, any external standard may be used, provided that it is clearly defined, and that precise data are available which make possible the accurate recalculation of nitrogen chemical shifts referred to other standards. Practically, there are limitations concerning the concentration of nitrogen nuclei in the reference compound, the sensitivity of its nitrogen chemical shift to concentration and solvent effects in the case of solutions, effects due to possible impurities, nuclear Overhauser effects on the resonance upon proton decoupling etc. 

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