Nuclear Magnetic Resonance Spectroscopy (NMR) :
Principle : In NMR substances absorb energy in the radio frequency region of the electro-
magnetic spectrum under influence of a strong magnetic field. It is a well known fact that the
nuclei of the atoms bonded to each other in molecules spin on an axis like a top. Since nuclei
are positively charged, this spin will create a small magnetic field. If an external magnetic
field is applied to these nuclei this magnetic field will split into two energy levels. The
energy difference is very small and corresponds to radiofrequency energy which is unique for
every molecule and will give the information regarding the nature of the compounds and the
presence of various functional groups and their environment.
Since this technique is mostly measures the spinning of the hydrogen nuclei (almost all the
organic compounds contain hydrogen atoms!), it is sometimes referred as Proton Magnetic
Resonance (PMR) spectroscopy.
Instrumentation : The instrumentation for this technique includes powerful magnet, radio-
frequency signal generator, amplifier, detector, etc. The following is the outline of the
instrument:
Applications : The application lies mostly in the identification and structural analysis of
organic compounds and thus, it is mostly a tool for qualitative analysis. It gives valuable
information regarding the position of the functional groups in a molecule and provides
distinguished spectra for the isomer. Much precise information on the structure of the
compounds can be obtained using the same technique with other magnetic nuclei like C 13
,
O17
, the instrumentation being the same except that the sweep of the magnetic field is varied.
Disadvantages: Very expensive and the instrumentation is complex and needs exceptional
skills to operate. Its sensitivity ranges from moderate to poor, however, can get clear
information using C13
or O17
NMR. The usage of the solvents is limited and in most of the
situations deuterated solvents are required.
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