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3.8.2 Nmr chemical shifts in diamagnetic molecules

Chemical shift values (δ) are defined to be δ = 10⁶ (v_R − v)/v_R where v and v_R 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.

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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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