NUCLEAR MAGNETIC RESONANCE 101
To produce an observable magnetic effect, the NMR technique resorts to
an additional perturbation for detection of a resonance. The additional per-
turbation results from the application of a sinusoidally oscillating magnetic
fi eld with frequency = γB 0 /2 π along the y axis of Figure 3.10. The perturbing
fi eld is generated by passing a radio - frequency (RF) alternating current
through a coil wrapped around the sample space. The fi eld generates a vector
(calledB 1 ) around which the nuclei precess. The precession yields a resultant
magnetization vectorMxy and, consequently, a net absorption of energy by
the nuclei. When B 1 is cut off, the precession stops; however, the signal output
due to the loss of the energy previously absorbed by the nuclei is observable
for between 10 ms and 10 s. The point at which maximum current has been
induced in the coil duringB 1 ’ s application is known as a 90 ° pulse. If one
observes only the resultant nuclear magnetization, this laboratory frame of
reference is known as the rotating frame. A simplifi ed version of this concept
is shown in Figure 3.11. In this scheme the resultant nuclear magnetization
Mxy is illustrated with B 1 ’ s position frozen. The fact that all low - energy nuclei
act in concert and at a single frequency leads to the name “ resonance
spectroscopy. ”
A nuclear magnetic resonance spectrometer measures the frequency of
“ nuclear resonance ” with suffi cient accuracy to provide useful information.
The spectrometer features a strong stable magnet containing a gap within
which the sample is placed. The sample is surrounded by a transmitter – receiver
coil. The usual modern system contains a superconducting cryomagnet for
generation of magnetic fi eld strengths between 200 and 750 to 1000 MHz.
3.4.2 Nuclear Screening and the Chemical Shift,
If all the nuclei being detected in an NMR experiment (all protons in an
organic ligand molecule, for instance) resonated at the same frequency, chem-
ists would not be very interested because little information about structure,
Figure 3.10 Freely precessing nuclei in a magnetic fi eld. The fi gure represents low -
energy nuclei in a sample arising from different atoms (drawn with the same origin).
(Adapted with permission of Nelson Thornes Ltd. from Figure 1.2b of reference 21 .)
Bo
Mz
My= Mx= 0
y
z
x
