Nuclear magnetic resonance solvent effects

The use of computer simulations to study internal motions and thermodynamic properties is receiving increased attention. One important use of the method is to provide a more fundamental understanding of the molecular information contained in various kinds of experiments on these complex systems. In the first part of this paper we review recent work in our laboratory concerned with the use of computer simulations for the interpretation of experimental probes of molecular structure and dynamics of proteins and nucleic acids. The interplay between computer simulations and three experimental techniques is emphasized (1) nuclear magnetic resonance relaxation spectroscopy, (2) refinement of macro-molecular x-ray structures, and (3) vibrational spectroscopy. The treatment of solvent effects in biopolymer simulations is a difficult problem. It is not possible to study systematically the effect of solvent conditions, e.g. added salt concentration, on biopolymer properties by means of simulations alone. In the last part of the paper we review a more analytical approach we have developed to study polyelectrolyte properties of solvated biopolymers. The results are compared with computer simulations. 

Ronayne, J., Williams, D. H. Solvent Effects in Proton Magnetic Resonance Spectroscopy. Annual Review of NMR Spectroscopy, Vol. 2,pp. 83-124, New York 1969. Laszlo, P. Solvent Effects and Nuclear Magnetic Resonance. Progr. N.M.R. Spectroscopy 3, 231-403(1968). 

Buckingham, A. D., Schaefer, T., Schneider, W. G. Solvent Effects in Nuclear Magnetic Resonance Spectra. J. Chem. Phys. 32, 1227 (1960). 

Fields, R., Green, M., Jones, A. Bis(trifluoromethyl)phosphine Solvent Effects on Nuclear Magnetic Resonance Parameters. J. Chem. Soc. A, 1969, 2740. 

Nuclear magnetic resonance studies on meso-ionic l,2,4-triazol-3-ones (200) were used to examine their relationship to the alternative l,3,4-oxadiazol-2-imine structure (153). The effect of solvent polarity upon the ultraviolet spectrum of anhydro-3-hydroxy-1,4-diphenyl-1,2,4-triazolium hydroxide (200, R = = Ph, R = H) has been discussed... 

There are many more solvent effects on spectroscopic quantities, that cannot be even briefly discussed here, and more specialized works on solvent effects should be consulted. These solvent effects include effects on the line shape and particularly line width of the nuclear magnetic resonance signals and their spin-spin coupling constants, solvent effects on electron spin resonance (ESR) spectra, on circular dichroism (CD) and optical rotatory dispersion (ORD), on vibrational line shapes in both the infrared and the UV/visible spectral ranges, among others. 

Solvent effects on nuclear magnetic resonance (NMR) spectra have been studied extensively, and they are described mainly in terms of the observed chemical shifts, 8, corrected for the solvent bulk magnetic susceptibility (Table 3.5). The shifts depend on the nucleus studied and the compound of which it is a constituent, and some nuclei/compounds show particularly large shifts. These can then be employed as probes for certain properties of the solvents. Examples are the chemical shifts of 31P in triethylphosphine oxide, the 13C shifts in the 2-or 3-positions, relative to the 4-position in pyridine N-oxide, and the 13C shifts in N-dimethyl or N-diethyl-benzamide, for the carbonyl carbon relative to those in positions 2 (or 6), 3 (or 5) and 4 in the aromatic ring (Chapter 4) (Marcus 1993). These shifts are particularly sensitive to the hydrogen bond donation abilities a (Lewis acidity) of the solvents. In all cases there is, again, a trade off between non-specific dipole-dipole and dipole-induced dipole effects and those ascribable to specific electron pair donation of the solvent to the solute or vice versa to form solvates. 

Duthaler, R.O. and Roberts, J.D., Nitrogen-15 nuclear magnetic resonance spectroscopy solvent effects on the 15N chemical shifts of saturated amines and their hydrochlorides, J. Magn. Reson., 34, 129, 1979. 

The papers of Reichardt on solvents, solvent effects, and solvatochromic dyes often have useful historical introductions.435 436 Hargittai has interviewed the various discoverers of buckminster fullerene.437 Marsden and Rae have traced the history of nuclear magnetic resonance in Australia, 1952-1986.438... 

With the exception of l,6-naphthyridin-8-ols, nuclear hdyroxy-1,6-naphthyridines would be expected to exist as the corresponding naphthyridinones. This has been confirmed by infrared,1035 ultraviolet,1026 and nuclear magnetic resonance spectral means.639 Other physicochemical studies of these oxy-1,6-naphthyridines include IR stretching frequencies1019 and ionization constants1040 of 1,6-naphthyr-idin-8-ols the mass spectra of 1,6-naphthyridinones the effect of solvents on the UV spectrum of an 8-hydroxy-1,6-naphthyridine derivative 773 and X-ray analysis of the dilactam, 4-hexyl-3,4,4a,5-tetrahydro-l,6-naphthyridine-2,7(l//,6Z/)-dione (1), in connection with its crystallization as self-assembled hydrogen-bonded polymers.549... 

Even with these limitations, nuclear magnetic resonance has made significant contributions to four areas of the chemistry of the platinum group metals bonding problems, molecular stereochemistry, solvation and solvent effects, and dynamic systems—reaction rates. Selected examples in each of these areas are discussed in turn. Because of space limitations, this review is not meant to be comprehensive. 

Spectroscopic investigations, such as infrared and nuclear magnetic resonance, should provide more information at the molecular level of the effect of solute species on the properties of a solvent. Cox (1973) has outlined the results of several such investigations which provide qualitative confirmation of the interpretations of solute-solvent interactions given above. 

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