Effect of solvent on chemical shift

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]. 

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). 

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 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. 

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. ... 

Approximate chemical-shift ranges for protons of various structural types in parts per million (8) from tetramethylsilane. Protons in specific compounds may appear outside of the cited range depending on the shielding or deshielding effect of substituents. The chemical shifts of 0—H and N—H protons depend on the conditions (solvent, temperature, concentration) under which the spectrum is recorded. 

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. 

Fig. 8. Effect of solvent on P chemical shift of 3, 5 -cAMP from Lemer and Kearns (1980). Copyright 1980 American Chemical Society.

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. 

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