Tables of Chemical Shifts

Structural analysis of an unknown organic material normally begins with an examination of the and spectra. Resonance positions are analyzed, if possible, with the benefit of knowledge of the molecular formula and structural information based on synthetic precursors. Representative chemical shifts are given in Tables 3-8 through 3-12, drawn from references at the end of this chapter. 

1 From Shoolery s rule, calculate the expected resonance position for the CH2 resonance in (a) CH3CH2I. (b) NC—CH2CH = CH2, (c) CH3OCH2C6H5, and (d) CH3C=CCH2Br. 

2 The proton resonance positions of the cis- and trcms-, 2-dibromoethenes are 8 6.65 and 7.03. Which comes from the cis isomer and which from the trans isomer  

3 A trisubstituted benzene possessing one bromine and two methoxy substituents exhibits three aromatic resonances, at 8 6.40, 6.46, and 7.41. What is the substitution pattern  

4 Octahedral cobalt complexes, CoL, have three filled t2 and two empty e molecular orbitals. The Co chemical shifts of several such complexes are linear with the wavelength of the first absorption (longest wavelength) in the UV/visible spectrum. Explain the linearity in terms of Ramsey s equation for shielding. 

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The inclusion of the number of protons at each chemical shift proved to be key the number of false positives increased 5-7 fold (depending on how diverse the set of test compounds were) if this parameter was excluded. Various refinements were tested, but the most successful was the J Filter which disallowed a combination in which the number of couplings measured at a given chemical shift in the experimental spectrum was greater than in the predicted. This proved to be particularly valuable in the differentiation of isomers, where changes in the overall table of chemical shifts alone, could be quite subtle. The inconsistent appearance of labile protons in the experimental spectrum reduced the accuracy and it was asserted that where possible, they should be removed from both the experimental spectrum and the predicted spectrum. The mismatch level not only encoded the divergence of the experimental spectrum and the postulated structure, but also encoded... 

Tables of chemical shifts can be used lo determine the molecular environment of all nuclei, but are especially useful in H and C NMR spectroscopy.

Since the chemical shift of a proton is determined by its environment, we can construct a table of approximate chemical shifts for many types of compounds. Let s begin with a short table of representative chemical shifts (Table 13-3) and consider the reasons for some of the more interesting and unusual values. A comprehensive table of chemical shifts appears in Appendix 1. 

Note These values are approximate, as all chemical shifts are affected by neighboring substituents. The numbers given here assume that alkyl groups are the only other substituents present. A more complete table of chemical shifts appears in Appendix 1. 

These hints are neither exact nor complete. They are simple methods for making educated guesses about the major features of a compound from its NMR spectrum. The hints can be used to draw partial structures to examine all the possible ways they might be combined to give a molecule that corresponds with the spectrum. Figure 13-38 gives a graphic presentation of some of the most common chemical shifts. A more complete table of chemical shifts appears in Appendix 1. 

Silicon only has one naturally occurring isotope ( Si) with a nonzero nuclear spin and Si NMR spectroscopy has become one of the most widely used techniques for the identification of silicon compounds. Unfortunately, the natural abundance of Si is only 4.7%, which combined with its long spin-lattice relaxation times, means that relatively long acquisition times may be needed to obtain high quality spectra. Si NMR spectroscopy has been the subject of a number of reviews and tables of chemical shift data and coupling constants are available. " ... 

Extensive tables of chemical shifts and coupling constants can be found in B. E. Mann, C NMR Chemical Shifts and Coupling Constants of Organometallic Compounds, in Adv. Organomet. Chem., 1974,... 

For extensive tables of chemical shifts for organic ligands, see Footnote 23a. 

Phenol and the simple aliphatic substituted phenols are soluble in CCI4 and CDCI3. As indicated above in the table of chemical shifts, the presence of other functional groups may require the use of D20, Poiysol, Acetone or DMSO-d6. 

The two exchangeable Primary Amide protons resonate at low field as either one or two very broad bands. The table of chemical shifts provided below indicates that the aliphatic Primary Amides resonate at slightly higher field than the aromatic compounds. The chemical shift(s) of these protons vary over a relatively wide range of values due to their sensitivity to the concentration of the sample solution, the solvent employed and the temperature at which the solution was examined, in addition to any possible hydrogen bonding effects and other structural considerations. 

We have included a discussion of some new techniques including NOE difference spectroscopy. We have also expanded our discussion of the information available from a DEPT experiment. We have added more tables of spectral data, and we have expanded the appendices to include more tables of chemical shift and coupling constant values. 

More extensive tables of chemical shifts can be found in the references by Silverstein and Webster, Pavia et al and Lambert et al. listed in the bibliography. 

As with NMR spectroscopy, tables of chemical shifts have been developed for carbon atoms in different environments one example is provided by Table 8.5. Examination of the data in the table shows that some of the structural features in a molecule that produce downfield chemical shifts in NMR spectra do the same in... 

It is needless to say how useful have been NMR spectra in chemistry. A simple proof of this is tables and tables of chemical shifts for various types of atomic nuclei, particularly for proton chemical shifts and chemical shifts. Clearly, chemical shifts are atomic properties, but if one would add all carbon chemical shifts in a hydrocarbon, one would obtain a numerical quantity that can be viewed as a molecular entity. Let us elaborate on this. Paraphrasing what one finds in Wikipedia and the Merriam-Webster Dictionary, one can write the following explanations for entity ... 

Charts and tables of chemical shifts are used to aid the interpretation of proton and carbon-13 spectra, and examples of these are given in Topic E13. 

Lithium covalently bound to carbon may be observed by Li NMR in lithium alkyls. In such molecules Li has a small chemical shift range (- ll ppm). Tables of chemical shifts and coupling constants are to be found in the review by Gunther. 

NMR spectroscopy is very useful for identifying organic compounds, provided that they can be obtained in a reasonably pure state. Kdnig has published a table of chemical shifts of functional groups found in common surfactants (39). This allows use of proton magnetic resonance to identify components of commercial products, where the range of possible structures is limited. Carminati and coworkers recommend the use of C NMR for the identification of unknown surfactants, both alone and in formulated products. With experience, not only the surfactants, but other components of products can be identified (40). 

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