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An Introduction to NMR Spectroscopy
Shelby Feinberg and Steven Zumdahl
Nuclear magnetic resonance spectroscopy (NMR) has risen to the same level of
importance as electronic and vibrational spectroscopy as a tool for studying molecular properties,
particularly structural properties. Although the following discussion of NMR specifically deals
with hydrogen atom nuclei in organic molecules, the principles described here apply to other
types of molecules as well. Many other types of nuclei (1!, 1"#, 1$, etc.) have nuclear spins and
thus can be studied using NMR techni%ues.
!ertain nuclei, such as the hydrogen nucleus (but not carbon&1' or oygen&1), have a
nuclear spin. *he spinning nucleus generates a small magnetic field termed . +hen placed in a
strong eternal magnetic field, called o, the nucleus can eist in two distinct spin states- a low
energy state A, in which is aligned with the eternal magnetic field, o, and a high energy state
, in which is opposed to the eternal magnetic field, o(figure 1). Alignment of with ois
the more stable and lower energy state.
Figure 1
/n NMR, transitions from the more stable alignment, A, (with the field) to the less stable
alignment, , (against the field) occur when the nucleus absorbs electromagnetic energy that is
Ho
Ho
Spin + 1/2Parallel
ALow Energy
Spin 1/2Anti-parallel
BHigh Energy
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eactly e%ual to the energy separation between the states (0). *his amount of energy is usually
found in the radiofre%uency range. *he condition for absorption of energy is called the condition
of resonance. /t can be calculated as the following-
h,2'h304 ==
h 5 $lanc67s constant
5 the strength of the applied magnetic field, o, at the nucleus
5 the gyromagnetic ratio (a constant that is characteristic of a particular nucleus)
5 the fre%uency of the electromagnetic energy absorbed that causes the change in
spin states
*here are three features of NMR spectra that we will focus on- the number and si8e of
signals, the chemical shift, and spin&spin coupling.
Number and Size of Signals
9et7s consider how the NMR spectrometer can distinguish between hydrogen nuclei and
produce multiple signals. Magnetically e%uivalent hydrogen nuclei produce one signal. *hese
hydrogen nuclei eperience the same local environment. #or eample, in a molecule such as
diethyl ether (#igure '), there are two sets of magnetically e%uivalent hydrogens. *he hydrogens
labeled aare si magnetically e%uivalent methyl hydrogens, while the hydrogens labeled bare
four magnetically e%uivalent methylene hydrogens. Notice that the methyl ( a) hydrogens are all
located ad:acent to a carbon containing two hydrogen atoms. Additionally, the methylene (b)
hydrogens are all located ad:acent to an oygen atom and a carbon atom containing three
hydrogen atoms.
Figure 2
CH3 CH2 O CH2 CH3
a b b a
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ecause of rapid rotations about sigma bonds and molecular symmetry, the si methyl hydrogens
(a) and the four methylene hydrogens (b) comprise two individual magnetically e%uivalent
groups of hydrogens. *he methyl hydrogens (a) eperience a different total magnetic field than
the methylene hydrogens (b), because of different local magnetic fields. As a result, the
resonance energy, 0, corresponding to the fre%uency of absorption, , will be different for these
two groups of hydrogen nuclei. *hus, the NMR spectrometer can distinguish between groups of
hydrogen nuclei which eperience different local magnetic fields. ;ifferent fre%uencies are
re%uired for the two different groups of hydrogens, thereby producing two distinct signals.
*he areas of the signals are directly proportional to the number of hydrogens.
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eternal magnetic field and the magnetic field produced by neighboring electrons. A mathe&
matical representation of the value of this resonance energy is the following-
1h),,(2'
h304 eo =+=
/f we vary the fre%uency () of the electromagnetic energy until h5 0, absorption will cause a
transition in spin states, and a signal will be recorded by the NMR spectrometer.
*he position of an NMR signal is recorded relative to the position of the signal of an
internal standard. *his standard is commonly tetramethyl silane (*M
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ydrogen nuclei which absorb at large are said to be deshielded from the eternal
magnetic field. *hese signals appear downfield (towards 1 ppm) from *M< because the
fre%uency at which they absorb differs greatly from the fre%uency of the *M< hydrogens. As a
result, the value of sis large. ;eshielding is caused by ad:acent atoms which are strongly
electronegative (e.g. oygen, nitrogen, halogen) or groups of atoms which possess &electron
clouds (e.g. !5B, !5!, aromatics).
9et7s now loo6 at the NMR spectrum of ben8yl acetate (#igure =).
Figure 4
AAA AA
c
b
a
*M