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Chapter: 3 Chemistry
    Section: 3.8 Molecular spectroscopy
        SubSection: 3.8.2 Nmr chemical shifts in diamagnetic molecules

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3.8.2 Nmr chemical shifts in diamagnetic molecules

Chemical shift values (δ) are defined to be δ = 106 (vRv)/vR where v and vR are the resonance frequencies of the nuclei in a sample and of the same nuclei in a reference standard. The chemical shift value is thus dimensionless and is expressed in parts per million (ppm). A material is chosen as the most convenient reference standard and its resonance frequency is assigned a chemical shift value of zero. The sign convention recommended by IUPAC for all nuclei is that the signals of frequencies greater than that of the reference standard are given positive chemical shift values. A contrary sign convention was used in early nmr studies of certain nuclei, for example 19F and 31P, and care needs to be exercised, even today, to ascertain the sign convention authors are using, especially for nuclei other than 1H and 13C. A most convenient presentation of the relationship between molecular structure and chemical shift is the bar chart. Such charts exist for many active nmr elements and bar charts for 1H, 13C, 15N, and 19F are given here. The charts for 1H and 13C are widely used because of their importance in organic, organo-metallic and metal complex chemistry. The nmr spectroscopy of 15N has not hitherto been widely studied because of the experimental difficulties associated with the small magnetic moment and low natural abundance of that nuclide, but recent improvements in nmr spectrometers have largely overcome the problem of low sensitivity. Consequently the nmr spectroscopy of 15N is becoming increasingly important because of the widespread occurrence and range of bond types of nitrogen.

19F nmr spectroscopy, with a nuclear spin of , 100% natural abundance and high sensitivity, has been studied for many years on continuous wave and Fourier transform instruments, and in view of the increasing rôle of organofluorine chemistry is becoming even more important.

N.B. The multiplicity of the 13C nmr signal recorded under conditions of full proton-coupling or proton off-resonance partial decoupling is as follows: Methyl carbon → quartet; methylene carbon → triplet; methine carbon → doublet; quaternary carbon → singlet.

Such internuclear spin–spin couplings are of great importance in molecular structure analyses.



(Click the ThumbNail Images Below to view Larger Images)

Ranges of 1H
chemical shifts in functional groups


 

Ranges of 1H
chemical shifts in functional groups
(contd)

     

Ranges of 15N
chemical shifts in functional groups


 

Ranges of 15N
chemical shifts in ligands attached to metals

     

Ranges of 19F
chemical shifts in functional groups


 

Ranges of 19F
chemical shifts in functional groups
(contd)

     

Ranges of 13C
chemical shifts in functional groups


   


References

N. F. Chamberlain (1974) The Practice of NMR Spectroscopy—with Spectra-Structure Correlations for
  Hydrogen-1, Plenum Press.
F. W. Wehrli and T. Wirthlin (1978) Interpretation of Carbon-13 NMR Spectra, Heyden.
T. Clerc, E. Pretsch, J. Seibl, and W. Simon (1989) Tables of Spectral Data for Structure Determination of Organic Compounds, 2nd English edn, Springer-Verlag.
G. C. Levy and R. L. Lichter (1979) 15N NMR Spectroscopy, Wiley.
J. Mason (1983) Chemistry in Britain, 19, 654.
J. W. Emsley, J. Feeney and L. H. Sutcliffe (eds) (1971) Progress in Nuclear Magnetic Resonance Spectroscopy
  7, Pergamon Press.
E. G. Brame (1962) Anal. Chem., 34, 591–2.



J.G.E.Phillips

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