Chemical shift solvent effects

Several other nmr procedures have been used for the determination of fractionation factors. These have advantages in some systems. Instead of determining the effect of the concentration of an exchanging site on the averaged chemical shift, the effect on the averaged relaxation rate of water protons can be used in a very similar way (Silverman, 1981 Kassebaum and Silverman, 1989), For example, addition of the enzyme Co(ii)-carbonic anhydrase to an aqueous solution increases the observed value of XjT because the proton-relaxation rate is the average of that for the bulk solvent (cfl. 0.3 s ) and that for water bound to the cobalt ca. 6x 10 s ). The average is different in an H2O/D2O mixture if the bulk solvent and the Cobound solvent have different deuterium contents, and it has been used to determine a value for the fractionation factor of Co-bound water molecules in the enzyme. 

Donor strengths, taken from ref. 207b, based upon the solvent effect on the symmetric stretching frequency of the soft Lewis acid HgBr2. Gutmann s donor number taken from ref 207b, based upon AHr for the process of coordination of an isolated solvent molecule to the moderately hard SbCL molecule in dichioroethane. ° Bulk donor number calculated as described in ref 209 from the solvent effect on the adsorption spectrum of VO(acac)2. Taken from ref 58, based on the NMR chemical shift of triethylphosphine oxide in the respective pure solvent. Taken from ref 61, based on the solvatochromic shift of a pyridinium-A-phenoxide betaine dye. 

Using a multiple linear regression computer program, a set of substituent parameters was calculated for a number of the most commonly occurring groups. The calculated substituent effects allow a prediction of the chemical shifts of the exterior and central carbon atoms of the allene with standard deviations of l.Sand 2.3 ppm, respectively Although most compounds were measured as neat liquids, for a number of compounds duplicatel measurements were obtained in various solvents. 

Representative chemical shifts from the large amount of available data on isothiazoles are included in Table 4. The chemical shifts of the ring hydrogens depend on electron density, ring currents and substituent anisotropies, and substituent effects can usually be predicted, at least qualitatively, by comparison with other aromatic systems. The resonance of H(5) is usually at a lower field than that of H(3) but in some cases this order is reversed. As is discussed later (Section 4.17.3.4) the chemical shift of H(5) is more sensitive to substitution in the 4-position than is that of H(3), and it is also worth noting that the resonance of H(5) is shifted downfield (typically 0.5 p.p.m.) when DMSO is used as solvent, a reflection of the ability of this hydrogen atom to interact with proton acceptors. This matter is discussed again in Section 4.17.3.7. 

Figure 2.11. Proton-Proton shift correlations of a-pinene (1) [purity 99 %, CDCls, 5 % v/v, 25 °C, 500 MHz, 8 scans, 256 experiments], (a) HH COSY (b) HH TOCSY (c) selective one-dimensional HH TOCSY, soft pulse irradiation at Sh = 5.20 (signal not shown), compared with the NMR spectrum on top deviations of chemical shifts from those in other experiments (Fig. 2.14, 2.16) arise from solvent effects...

The natural product is the asperuloside described in the literature The assignments for the carbon pairs C-1/C-1, C-6 /C-1 and C-11 / CO (acetyl) have been interchanged. Deviations of C chemical shifts (CDCI3-D2O from the values tabulated here [(CD3)2SO] are due mainly to solvent effects. Here the difference between the measurements a and d shows that the use of D2O exchange to locate the OH protons where the CH COSY plot is available is unnecessary since OH... 

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. 

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. 

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