Scaling Chemical Shift Values

Rablen examined the proton NMR shifts of 80 organic molecules using three different DFT functionals (B3LYP, B3P86, and B3PW91) and three different 

6-31+G(d). The root mean square error (RMSE) is less than 0.15 ppm for both models. 

04 ppm for the aromatic test suite. This same computational level did well for the amides (RMSE = 0.10 ppm), but the best agreement with the experimental values in D2O was with the HF/6-311G(d,p) values (RMSE = 0.08 ppm). 

Based on these studies, two different approaches are routinely taken toward computing NMR chemical shifts. The first is to determine a scaling factor for the particular problem at hand. This will mean choosing a particular computational method, that is, a particular density functional and a basis set. Next, a set of molecules related to the molecule of interest are selected. The computed chemical shifts of these reference compounds are compared to their experimental values to determine the linear scaling factors. This linear scaling is then applied to the computed chemical shifts of the molecule of interest. 

The downside to this approach is that a new scaling factor must be determined for every new study. The second approach is to take a more general view, that a single scaling factor might be appropriate for a broad set of compounds. One will still need a unique scaling factor for every computational method, but perhaps it is possible to also settle on a generic computational method. 

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TABLE 7.69 Fluorine-19 Chemical Shifts Values given in ppm on the 5 scale, relative to CCI3F. 

In the following 55 problems, the chemical shift value (ppm) is given in the scale below the spectra and the coupling constant (Hz) is written immediately above or below the appropriate multi-plet. Proton NMR data are italicised throughout in order to distinguish them from the parameters of other nuclei ( C, N). 

The behaviour of the mutant enzymes where, for example, histidine-152 has been changed to alanine is compared with that of wild type enzymes.60 The 31P NMR chemical shift values and signal width for H152A mutant enzyme have shown the presence of two conformers open and closed forms of the enzyme that interconvert slowly on the NMR time scale. The tightness of the binding of the cofactor to the protein surface and its protonation state have been also discussed for intermediate Schiff bases in different steps of the catalytic cycle (Table 1). 

Also because of the Larmor equation (1.8), the frequency or field differences /1vs or ABS are proportional to the swept radio frequency Vj (in MHz) or the field strength of B0 (in T). Therefore, chemical shifts dvs (or ABS) obtained at different radio frequencies v, (or field strengths B0) have to be adjusted to the same radio frequency (or field) before comparison. In order to get chemical shift values which are independent of the frequency or field strength used, the d scale of chemical shifts is introduced. <5 values are obtained by dividing the frequency differences Avs (in Hz) by the frequency iq used (in MHz = 106 Hz). 

TABLE 6.34 Proton Chemical Shifts Values are given on the 8 scale r = 10.00—5. 

Fig. 3.64 Proton magnetic resonance of m-dinitrobenzene in CDC13 solution sweep offset 460 Hz, sweep width 100 Hz, The <5 scale is not applicable in this case actual S values may be obtained by dividing the chemical shift values in Hz by 60 (cf. Fig. 3.47) and relevant discussion p. 325).

A typical demonstration of the utility of theoretical calculations in the assignment of uncertain signals is reported by Quin et al.30 31 As already found by Bagno,116 the experimental values reported in the literature for S-methyltetrahydrothiophenium salt 1, 750 ppm (referred to CS2), did not fit well with the scaled values, calculated by the B3LYP DFT approach and the EMPI method, which were 87.4 and 121 ppm respectively. With the aid of AIM calculations, it was verified that this discrepancy cannot be ascribed to intermolecular interactions in solution, neither salt formation nor interaction with counterions. The experimental redetermination of chemical shift values has given a value of 95 ppm (ext. ref. CS2), in agreement with calculated values. For S-methylthianium ion 16, a value of 68 ppm has been calculated, compared to an experimental value reported in the literature of 670 ppm.29... 

In the following tables 33S NMR parameters of organic and inorganic compounds have been reported. Line width values, LW, are in hertz. Chemical shift values, 8, have been referred to SO4. Data in the literature referred to CS2 and sulpholane have been converted in the SO4- scale by the following relationships,10 obtained from the chemical shift values reported by Belton et al/ (referred to Cs2S04 2M in water) ... 

This is a simple procedure whereby a reference peak (e.g., TMS in organic solvents) is selected with a cursor (Bruker uses a triangle or vertical arrow, Varian a vertical red line) and given a specific chemical-shift value. Without this reference, the chemical-shift scale... 

An electronic measure, the NMR chemical shift values of the amide proton in coil conformations (Table 3) also show a high degree of correlation (r = 0.70-0.89) with hydrophihcity scales and with strand versus coil conformations (Table 4). NMR studies reveal that the amide proton is shielded to a greater extent in coil conformations as compared with extended ( 3) structures (37) increased electron density exists at this atom in the coil conformation. Taken together, the data suggest strong interactions between hydrophihcity and electronic parameters in folding and provide support for additional refinement of the Hp index. 

Figure 11 shows plots of the H chemical shift error (terror) against the measured value ( measure) of the peak shift of silicon rubber, where measure is determined from the offset frequency dependency of the peak shift, (terror is the difference between the calculated chemical shift value (Scaled) using 0.40 and the... 

The location of the peak(s) on the frequency scale (its chemical shift ) depends on the magnetic environment of the absorbing proton. Extensive tabulations of empirically determined chemical shifts for various types of protons are available, but it is important for the organic chemist to learn some of the commoner chemical shift values given in Tables 3-1, 3-2, and... 

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