Nuclear Magnetic Resonance (NMR) Spectroscopy
Introduction
Nuclear magnetic resonance (NMR) spectroscopy uses radiofrequency radiation
to induce transitions between different nuclear spin states of samples
in a magnetic field. NMR spectroscopy can be used for quantitative measurements,
but it is most useful for determining the structure of molecules (along
with IR spectroscopy and mass
spectrometry). The utility of NMR spectroscopy for structural characterization
arises because different atoms in a molecule experience slightly different
magnetic fields and therefore transitions at slightly different resonance
frequencies in an NMR spectrum. Furthermore, splittings of the spectra
lines arise due to interactions between different nuclei, which provides
information about the proximity of different atoms in a molecule.
General Principles
Nuclei with an odd number of protons, neutrons, or both, will have an instrinsic
nuclear angular momentum or "nuclear spin".
Spin quantum number for various nuclei
Number of protons |
Number of Neutrons |
Spin Quantum Number (I) |
Examples |
Even |
Even |
0 |
12C, 16O, 32S |
Odd |
Even |
1/2 |
1H, 19F, 31P |
" |
" |
3/2 |
11B,35Cl, 79Br, 127I |
Even |
Odd |
1/2 |
13C |
" |
" |
5/2 |
17O |
Odd |
Odd |
1 |
2H, 14N |
When a nucleus with a non-zero spin is placed in a magnetic field, the
nuclear spin can align in either the same direction or in the opposite
direction as the external magnetic field. A nucleus that has its spin aligned
with the external field will have a lower energy than when it has its spin
aligned in the opposite direction to the field. Thus, these two nuclear
spin alignments have different energies and application of a magnetic field
results in an energy level diagram like:
Nuclear magnetic resonance (NMR) spectroscopy uses the transition between
these levels to detect and quantify nuclei. The magnitude of the energy
splitting between these levels for nuclei in a strong magnetic field is
in the range of radiofrequency (RF) radiation.
Absorption
of the RF radiation causes nuclear spins to realign or flip in the higher-energy
direction. After absorbing energy, nuclei will reemit
RF radiation and return to the lower-energy state.
The energy (and thus frequency) of an NMR transition depends on the
magnetic-field strength and a proportionality factor for each nucleus called
the magnetogyric ratio, γ. The frequency of
a transition is given by:
ν = γ/2π
H
where ν is the frequency of the resonant
radiation and H is the strength of the magnetic field.
Chemical Shift
The local environment around a given nucleus in a molecule perturbs the
local magnetic field that is exerted on that nucleus. Since the resonance
frequency of the transition, nuclei in different environments have slightly
different transition energies. This dependence of the transition energy
on the position of a particular atom in a molecule gives NMR spectroscopy
it's utility for structural characterization.
The resonance frequencies of different nuclei in an atom are described
by a relative shift compared to the frequency of a standard. This relative
shift is called the chemical shift, δ, and is
given by:
δ = [(νsample-
nref)/νref)]
1·106
where δ has units of ppm. For 1H
NMR spectroscopy the reference compound is tetramethylsilane, Si(CH3)4,
or TMS.
Instrumentation
There are two NMR spectrometer designs, continuous-wave (cw), and pulsed
or Fourier-transform
(FT-NMR). Pulsed FT-NMR instruments have largely replaced cw-NMR spectrometers.
Due to the lower maintenance and operating cost of cw instruments, they
are still used for routine 1H NMR spectroscopy at 60 MHz. (Low-resolution
cw instruments require only water-cooled electromagnets instead of the
liquid-He-cooled superconducting magnets found in higher-field FT-NMR spectrometers.)
These two spectrometer designs are described in separate cw-NMR
and FT-NMR documents.
For more extensive information and resources on NMR spectroscopy please
see Scott
van Bramer's NMR pages.