Instrumental Methods
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When the reaction product(s) is obtained in pure form, modern instrumental methods of structure determination, rather than traditional wet methods, provide the fastest way to determine the functionality and connectivity present.
INSTRUMENTAL METHODS
When
the reaction product(s) is obtained in pure form, modern instrumental methods
of structure determination, rather than traditional wet methods, provide the
fastest way to determine the functionality and connectivity present. Today’s
chemist has a large number of tools available with which to probe the structure
of molecules, but for determining the structures of organic molecules the “big
three” are nuclear magnetic resonance (NMR) spectroscopy, infrared (IR)
spectroscopy, and mass spectrometry (MS). The frequency of use of these
techniques generally falls in the same order (NMR > IR > MS).
The
first two of these methods (NMR and IR) are spectroscopic techniques in which
the molecule is interrogated with electromagnetic radiation in a particular
range of frequencies (NMR uses radio frequencies and IR spectroscopy uses
infrared radiation). Only certain frequencies will be absorbed by a particular
compound, and those frequencies that are absorbed can be used to infer
structural details about the compound.
Mass
spectrometry is not a spectroscopic technique because absorption of
electromagnetic energy is not involved in any way. In MS the molecule is
frag-mented into ions and these charged pieces are separated on the basis of
their mass-to-charge ratio. Since the usual charge is +1, the masses of the pieces are
determined. Knowledge of the masses of the pieces allows the structure of the
compound to be reconstructed.
Another
distinction between these techniques is the structural information they are
capable of revealing. Both NMR spectroscopy and MS establish connectivity
between atoms and groups in a molecule (albeit slightly differently). In
addition the functional groups present are suggested. Infrared spectroscopy
does not generally establish connectivity but is unmatched for identifying
functional groups present in the molecule. By combining these three major
methods, the functional groups, the molecular weight, and the connectivity of
the atoms and groups can be established rapidly and efficiently. It is normally
a trivial step then to write the structure of the molecule.
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