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Here B
reference
is the magnetic field from the reference nuclei and B
sample
is the
field at the sample nuclei,
ν
0
is the instrument frequency (300 – 800 MHz). A
common reference substance is tetramethylsilane (Si(CH
3
)
4
).
For protons the chemical shift is related to the electron density around the
nucleus. Electronegative substituents reduce the density, leading to ‘downfield
shift’, or larger
δ
values.
The figure below illustrates the chemical shift concept.
Figure 9
The proton NMR spectrum of glyceraldehyde in an organic solvent is useful to
illustrate the main principles relevant for our use on carbohydrates: