Nuclei with magnetic moments that are aligned with the field are said to be in the α-state (lower
energy). The nuclei can then be irradiated with radiofrequency pulses that match the energy gap
between the two states, which will excite some lower-energy nuclei into the β-state (higher energy).
The absorption of this radiation leads to excitation at different frequencies, depending on an atom’s
magnetic environment. In addition, the nuclear magnetic moments of atoms are affected by nearby
atoms that also possess magnetic moments.
Magnetic resonance imaging (MRI) is a noninvasive diagnostic tool that uses proton NMR, as shown
in Figure 11.3. Multiple cross-sectional scans of the patient’s body are taken, and the various
chemical shifts of absorbing protons are translated into specific shades of grey. This produces a
picture that shows the relative density of specific types of protons; for instance, a dark area on a T1-
weighted MRI tends to correspond to water, whereas a light area indicates fattier tissue. Comparison
with normal MRI then allows the diagnostician to detect abnormalities in the scanned region. We
mention this to explain the relevance of NMR spectroscopy to medicine; the MCAT will not test you
on the details of how MRI works.
Figure 11.3. Magnetic Resonance Imaging (MRI)A typical NMR spectrum is a plot of frequency vs. absorption of energy. Because different NMR
spectrometers operate at different magnetic field strengths, a standardized method of plotting the
NMR spectrum has been adopted. This standardized method, which is the only one seen on the
MCAT, uses an arbitrary variable called chemical shift (δ), with units of parts per million (ppm) of
spectrometer frequency. The chemical shift is plotted on the x-axis, and it increases toward the left
(referred to as downfield). To make sure that we know just how far downfield compounds are, we
use tetramethylsilane (TMS) as the calibration standard to mark 0 ppm; when counting peaks,
make sure to skip the TMS peak.