Abstract #2 on NMR Spectroscopy

There are 4 types of information obtainedfrom a 1H nmr spectrum which
can help determine the structure of an organic molecule.

1. The number of signalstells you how many different types of
Hydrogens(i.e. protons) there are in a molecule.

2. The integrationtells how many of each type of proton there are (i.e.
how many protons gave rise to the signal)

3. The chemical shift( how many ppm the signal is from TMS) tells you
what functional group is present. The chemical shift provides
information about the chemical environment of the proton.

4. The spin couplingobserved tells you how many equivalent protons
are on the adjacent carbon to the carbon bonded to the proton being
observed. (i.e. 3 sigma bonds away or protons one carbon away)

Some Important Notes:

Chemical shift equivalent Protons -
1. Protons which are related by symmetry, e.g. all six protons on
benzene are equivalent.
2. Protons which are related by rapid rotation, e.g. all three protons on
any methyl group.

Spin-Spin Coupling
1. For 1H nmr spectra, coupling is only observed between protons on
adjacent carbons i.e. to protons one carbon away(3 sigma bonds).
[There are exceptions to this rule but you need not worry about them
now.]

2. Chemically shift equivalent protons do not split each other,that is,
splitting is only observed between protons with different chemical
shifts.

3. In 13C spectra, only protons directly attached to the Carbon can split
it, therefore one can tell how many protons are attached to each
carbon. One can turn the coupling off (called decoupled spectra)
and get all singlets i.e. one line for each different carbon orone can
turn the coupling on (called coupled spectra) - this gives the number
of H’s connected to each carbon.