Chemical shift in organic compounds

Witanowski M, Stefaniak L, Januszewski H. In Nitrogen Chemical Shifts in Organic Compounds in Nitrogen NMR, Witanowski M, Webb GA (Eds.), Plenum Press, London, New York, 164, 1973. 

Fig. 3.45 l3C chemical shifts in organic compounds. Data reproduced from E. Breitmaier, G. Jung and W. Voelter (1971). Angew. Chem. Int. Edit. Engl., 10,679. 

Raymond AJ, Mehdi M (2004) The prediction of IH NMR chemical shifts in organic compounds. Spectrosc Eur 16(4) 20-22... 

Fast and accurate predictions of H NMR chemical shifts of organic compounds arc of great intcrc.st for automatic stnicturc elucidation, for the analysi.s of combinatorial libraries, and, of course, for assisting experimental chemists in the structural characterization of small data sets of compounds. 

There remains one topic to be discussed in our survey of chemical bonding in organic compounds. For most compounds, all the molecules have the same structure, whether or not this structure can be satisfactorily represented by a Lewis formula. But for many other compounds there is a mixture of two or more structurally distinct compounds that are in rapid equilibrium. When this phenomenon, called tauto-merism, exists, there is a rapid shift back and forth among the molecules. In most cases, it is a proton that shifts from one atom of a molecule to another. 

Nitrogen chemical shifts of organic compounds are reviewed in Chapter 5 of Reference 4. The range of N chemical shifts is about 600 ppm wide. If extreme values for metal complexes are included, it extends to over 1400 ppm. Nitromethane is the recommended reference (it can be added in a sealed-off capillary). Values are also frequently quoted with respect to a saturated aqueous solution of ammonium chloride or ammonium nitrate. The following N shift values can be used to convert the 5-values ... 

Fig. 83 Proton chemical shifts for organic compounds in dilute solutions of CC14 or CDC13. R and Ar stand for alkyl and aiyl groups.

Another approach has been developed for the prediction chemical shifts in H-NMR spectroscopy. In this case, special proton descriptors were applied to characterize the chemical environment of protons. It can be shown that 3D proton descriptors in combination with geometric descriptors can successfully be used for the fast and accurate prediction of H-NMR chemical shifts of organic compounds. The results indicate that a neural network can make predictions of at least the same quality as those of commercial packages, especially with rigid structures where 3D effects are strong. The performance of the method is remarkable considering a relatively small data set that is required for training. A particularly useful feature of the neural network approach is that the system can be easily dynamically trained for specific types of compounds. 

Instead of measuring chemical shifts in absolute terms, we measure them with respect to a standard—tetramethylsilane (CFl3)4Si, abbreviated TMS. The protons of TMS are more shielded than those of most organic compounds, so all of the signals in... 

The net result of such effects is the quite amazing difference of more than 850ppm between the chemical shift of F2 (+423 ppm) and C1F (-437 ppm), all of which is not of great consequence to organic chemists, who would rarely if ever find themselves in a situation requiring knowledge of the chemical shifts of the compounds in the two lists above. 

Table 2 gives the approximate range of nitrogen chemical shifts in the organic compounds with which this book deals. The 14N and 15N NMR spectra of amino- and nitro-aliphatic compounds have been reviewed1,2,15 and the effects of the alkyl group on the 15N chemical shifts have been investigated. 

Table 5.3 Typical Chemical Shift Ranges in Organic Compounds...

Typical C Chemical Shift Ranges in Organic Compounds C Chemical Shifts (5) for sp Carbons in Alkyl Derivatives C Chemical Shifts (5) for sp Carbons in Vinyl Derivatives... 

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