101 Spectroscopy
There are several spectroscopic techniques which can be used to identify organic molecules:
infrared (IR), mass spectroscopy (MS) UV/visible spectroscopy (UV/Vis) and nuclear mag-
netic resonance (NMR).
IR,NMRandUV/visspectroscopyarebasedonobservingthefrequenciesofelectromagnetic
radiation absorbed and emitted by molecules. MS is based on measuring the mass of the
molecule and any fragments of the molecule which may be produced in the MS instrument.
101.1 UV/Visible Spectroscopy
UV/Vis Spectroscopy uses ultraviolet and/or visible light to examine the electronic prop-
erties of molecules. Irradiating a molecule with UV or Visible light of a specific wavelength
can cause the electrons in a molecule to transition to an excited state. This technique is
most useful for analyzing molecules with conjugated systems or carbonyl bonds.
101.2 NMR Spectroscopy.
Nuclear Magnetic Resonance (NMR) Spectroscopy is one of the most useful analytical
techniques for determining the structure of an organic compound. There are two main
types of NMR,^1 H-NMR (Proton NMR) and^13 C-NMR (Carbon NMR). NMR is based on
the fact that the nuclei of atoms have a quantized property called spin. When a magnetic
field is applied to a^1 H or^13 C nucleus, the nucleus can align either with (spin +1/2) or
against (spin -1/2) the applied magnetic field.
These two states have different potential energies and the energy difference depends on the
strength of the magnetic field. The strength of the magnetic field about a nucleus, however,
depends on the chemical environment around the nucleus. For example, the negatively
charged electrons around and near the nucleus can shield the nucleus from the magnetic
field, lowering the strength of the effective magnetic field felt by the nucleus. This, in turn,
will lower the energy needed to transition between the +1/2 and -1/2 states. Therefore,
the transition energy will be lower for nuclei attached to electron donating groups (such
as alkyl groups) and higher for nuclei attached to electron withdrawing groups (such as a
hydroxyl group).
In an NMR machine, the compound being analyzed is placed in a strong magnetic field
and irradiated with radio waves to cause all the^1 H and^13 C nuclei to occupy the higher
energy -1/2 state. As the nuclei relax back to the +1/2 state, they release radio waves
corresponding to the energy of the difference between the two spin states. The radio waves