Page 104 - Tandem Techniques

Page 85

different values of the scanned magnetic field. This is because, due to their specific environment, they
experience shielding from the electron clouds from neighboring atoms and thus, although they all
absorb energy at the same frequency (because the frequency in this experiment is fixed) the applied
field must be different to compensate for their unique atomic environment. Now,

where (H) is the net magnetic field experienced by the proton,
(HF) is the high intensity applied magnetic field,
(Hs) is the small applied scanning magnetic field,
and (Hc) is the shielding effect provided by the atomic environment of the proton

and

Where (a) is the shielding effect of the electron environment of the proton.

In fact it is the chemical environment of the proton that will affect the diamagnetic shielding constant
(¬). Consequently, the relative positions of the absorption peaks, determined by the magnitude of (¬),
will disclose the nature of the chemical environment and contribute information with regard to the
overall structure of the molecule.

One further point. If the resolution of the NMR machine is increased (which, as in chromatography,
means that the widths of the peaks are reduced relative to their movement apart, viz. chemical shift)
then the proton peaks show a well-defined and predictable fine structure. An example of the spectrum
of ethyl alcohol that would be obtained on a NMR spectrometer having greater resolution is shown in
Figure 2.26. It is now seen that the magnetic field experienced by a proton is also influenced by protons
on the adjacent carbon atoms. For example, the methylene protons can contribute magnetic influence at
three different levels to the field experienced by the methyl protons. The magnetic fields due to the