infrared, using the appropriate detectors) and can be displayed in the form of a spectrum
of absorption versus the irradiating frequency. The great information content of this
spectrum derives from the fact that each nucleus of a molecule (e.g.,each proton) will
have a slightly different resonance frequency, depending on its “environment” (the
atoms and electrons that surround it). In other words, its magnetic momentum will be
“shielded” differently in different functional groups. This makes it easy to distinguish,
for example, the protons on a C-CH group from an O-CH 3 group or an N-CH 3 group,
aliphatic or aromatic protons, carboxylic acid or aldehyde protons, and so on, because
they absorb at different frequencies. In the same fashion, every carbon atom in a mole-
cule can be distinguished by^13 C magnetic resonance spectroscopy.
The only drawback to NMR is its low sensitivity. Concentrations in the millimolar
range are sometimes required, although with computer enhancement techniques (such
as Fourier transform) signals at 10–6–10–5M concentrations can be detected. This is
especially important for nuclei that have a low natural abundance, such as^13 C (1.1%)
or deuterium,^2 H (0.015%).
Fourier-transform (pulsed) proton NMR techniques allow an even more sophisticated
assignment of resonances to specific protons. If the single high-frequency pulse is
replaced by two pulses of variable pulse separation, the introduction of a second time
parameter yields a two-dimensional NMR spectrum, with two frequency axes.
Resonances on the diagonal are the normal, one-dimensional spectrum, but off-diagonal
resonances show the mutual interaction of protons through several bonds. This allows the
assignment of all protons even in very large molecules; recently, the three-dimensional
spectrum of a small protein has been deduced by use of a three-pulse method.
Nuclear magnetic resonance permits counting of the protons in a molecule. The area
under each NMR resonance peak is proportional to the protons contained in that func-
tional group. One of the easily identifiable groups in the spectrum is used as a relative
standard; electronic integration of the peak areas will give the number of protons in each
group of signals, clarifying the assignment of resonances to specific structural features.
The detection of relaxation rates is a further application of NMR spectroscopy. When
a particular nucleus, such as a methyl proton, is irradiated by a strong radiofrequency
and absorbs it, the populations of protons in the high- and low-spin states are equalized
and the signal disappears after a while. It will be recalled that the NMR signal is based
on energy absorption; if all of the nuclei of a given type are in the high-spin state,
absorption is not possible and “saturation” occurs. Upon removal of the strong irradiat-
ing frequency, the high- and low-spin populations will once again become unequal by
transferring energy either to the solvent (spin–lattice relaxation,T 1 ) or to another spin
in the molecule (spin–spin relaxation,T 2 ), and the appropriate spectrum line will
assume its original amplitude. The time necessary for this recovery is called the relax-
ation time,whereas its reciprocal is the relaxation rate.We shall see in some later
examples how relaxation rates can be used in elucidating molecular interactions.
Another tool in NMR spectral analysis is the observation of slight shifts of the various
peaks. Hydrogen bonding and charge-transfer complex formation will shift resonances
downfield (to lower frequencies) and upfield, respectively. On the other hand, the coupling
constant, or separation distance between the sublines of doublets or triplets, is a result
of line splitting by neighboring protons. Thus, line multiplicity (in addition to line posi-
tion) is used in determining the nature of a proton and its neighbor. An ethyl group, for
DRUG MOLECULES: STRUCTURE AND PROPERTIES 59