Of chemical shift

The variation of chemical shifts as a function of dilution could be accounted for only qualitatively (235) because of the large diversity of solute-solvent interactions resulting from the nature and the shape of the solvent molecule (Table 1-34). 

Enonnous numbers of chemical shifts have been recorded, particularly for FI and Many algoritlnns for the prediction of shifts have been extracted from these, so that the spectra of most organic componnds can be predicted at a useful level of accuracy, usmg data tables available in several convenient texts [12, F3,14 and 15]. Alternatively, computer programs are available that store data from 10 -10 spectra and then use direct 

Table 7.43 Estimation of Chemical Shift for Protons of —CHj— and Methine

Table 7.44 Estimation of Chemical Shift of Proton Attached to a Double Bond 7.95

Table 7.50 Estimation of Chemical Shifts of Alkane Carbons 7.102

TABLE 7.52 Estimation of Chemical Shifts of Carbon Attached to a Double Bond The olefinic carbon chemical shift is calculated from the equation 

The most obvious feature of these C chemical shifts is that the closer the carbon is to the electronegative chlorine the more deshielded it is Peak assignments will not always be this easy but the correspondence with electronegativity is so pronounced that spec trum simulators are available that allow reliable prediction of chemical shifts from structural formulas These simulators are based on arithmetic formulas that combine experimentally derived chemical shift increments for the various structural units within a molecule 

Figure 10.2.5. Example of a local RDF descriptoT for proton 6 used in the prediction of chemical shifts (e = 20 A

Ab-initio calculations are particularly usefiil for the prediction of chemical shifts of unusual species". In this context unusual species" means chemical entities that are not frequently found in the available large databases of chemical shifts, e.g., charged intermediates of reactions, radicals, and structures containing elements other than H, C, O, N, S, P, halogens, and a few common metals. 

The similarity of the retrieved protons to those of the query structure, and the distribution of chemical shifts among protons with the same HOSE codes, can be used as measures of prediction reliability. When common substructures cannot be found for a given proton (within a predefined number of bond spheres) interpolations are applied to obtain a prediction proprietary methods are often used in commercial programs. 

The first step for any structure elucidation is the assignment of the frequencies (chemical shifts) of the protons and other NMR-active nuclei ( C, N). Although the frequencies of the nuclei in the magnetic field depend on the local electronic environment produced by the three-dimensional structure, a direct correlation to structure is very complicated. The application of chemical shift in structure calculation has been limited to final structure refinements, using empirical relations [14,15] for proton and chemical shifts and ab initio calculation for chemical shifts of certain residues [16]. 

First-order spectra (mulliplels) are observed when the eoupling constant is small compared with the frequency difference of chemical shifts between the coupling nuclei This is referred to as an A n spin system, where nucleus A has the smaller and nucleus X has the considerably larger chemical shift. An AX system (Fig. 1.4) consists of an T doublet and an X doublet with the common coupling constant J x The chemical shifts are measured from the centres of eaeh doublet to the reference resonance. 

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

Pyrazine and its derivatives have been extensively studied by proton and NMR spectroscopy and conflicting reports on the reliability of additivity rules and/or correlation of chemical shifts with calculated electron densities have appeared. 

Structure calculation algorithms in general assume that the experimental list of restraints is completely free of errors. This is usually true only in the final stages of a structure calculation, when all errors (e.g., in the assignment of chemical shifts or NOEs) have been identified, often in a laborious iterative process. Many effects can produce inconsistent or incorrect restraints, e.g., artifact peaks, imprecise peak positions, and insufficient error bounds to correct for spin diffusion. 

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