Empirical Correlations of Chemical Shifts

Conformational Analysis.— The major effort in this area has been directed towards empirical correlations of chemical shift and conformation. Tonelli and co-workers have examined the stereosequence dependence of C chemical shifts in polypropylene (PP) poly(vinyl chloride) (PVC), and polystyrene (PS) using the rotational isomeric-state scheme of polymer statistical mechanics. The structural variation of C shifts was shown to correlate well with the occurrence of three-bond gauche (y) interactions between carbon atoms together with y interactions between carbon and chlorine atoms in PVC and ring-current shifts in PS. It has been... 

The chemical shift is the cornerstone of the chemical applications of NMR. As we noted in Chapter 1, this accidental discovery converted a technique designed initially for probing the structure of the atomic nucleus into one that can provide detailed information about the structure and dynamics of molecules. In this chapter we examine the theoretical underpinning of the chemical shift, explore empirical correlations between chemical shifts and functional groups in organic molecules, and describe simple physical models that can help us to understand and predict chemical shifts. 

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]. 

It is not our concern here to delve into the theory of chemical shifts, since most studies mentioned are essentially empirical in their approach and have aimed to correlate the measured shifts with electronic factors. The main fact, however, is that unlike proton shifts, the shifts for other nuclei are governed by the Van Vleck second-order paramagnetic term, which is not easy to handle since its magnitude depends not only on the ground electronic state but also on the manifold of excited states. 

A semi-empirical molecular orbital method for the correlation of charge distributions with 13C shifts in amino acids was described [95]. Plotting of chemical shift parameters versus charge density changes of a-carbon atoms relative to the corresponding atoms in the parent hydrocarbons permits prediction of the chemical shifts of the a-carbons with an accuracy of about 10%. However, the slope (280 ppm per electron) in Fig. 5.12 is... 

The estimation of chemical shifts by examining the spectra of model compounds is not always feasible, and the prediction models fail to distinguish between two or more stereosequences as they cannot always be distinguished on the basis of intensity alone. To overcome these limitations, large numbers of organic compounds have been analyzed by NMR and their chemical shifts have been used to determine a set of empirical correlations that are used to determine the structure based on the polymer s NMR spectrum. The carbon chemical shifts of hydrocarbon-based polymers such as polyethylenes can be predicted by empirical additivity rules such as ... 

An interesting study of homolycorine type alkaloids with a saturated ring C has been made by Jeff and coworkers (208). They describe empirical correlations of A/-methyl chemical shifts with stereochemical assignments of the B/C and C/D ring junction. 

Because it is almost impossible to disentangle the various contributory effects, the theory of chemical shifts can be used only as a general guide. For the solution of problems, empirical correlations are almost invariably used. Some of the more fundamental of these are given in Table 12.1. 

By means of specifically deuteriated derivatives, the n.m.r. of all pentitols and hexitols in water and their acetates in chloroform have been assigned. Qualitative correlations between chemical shift, configuration, and preponderant conformation were made, leading to practical empirical rules. ... 

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