How To Interpret 1H
NMR spectra can be a very challenging task; you must admit that an NMR
spectrum looks nothing
like the molecule that generates it. Nonetheless,
of NMR spectra doesn't have to be terribly difficult. You
just have to make sure you use all the information available, and be
pointers that will be helpful as you solve any 1H
NMR spectrum. While
not necessary, the NMR
a tool that can really
help you solve these problems by letting you visualize how everything
fits together. It can also help you recognize any mistakes
first thing to remember is that NMR interpretation is like solving
a puzzle. Just like you can't put every puzzle piece you pick
its correct place, you cannot expect to look at an NMR spectrum and
immediately write down the structure. You have to work at it,
trying first one idea, then another. And just like you can
when the picture puzzle is finished because all the pieces match with
their neighbors, similarly the NMR spectrum is solved when all the
molecular pieces from the NMR match their neighbors.
Guidelines for solving
- Each peak in the spectrum corresponds to a
fragment of the molecule.
- Each peak contains THREE different kind of
the molecular fragment: Integration, Chemical
Multiplicity (or splitting). You
CANNOT solve the spectrum without
using all three pieces of information.
- Once you have connected all the fragments, it
does NOT mean
you have successfully solved the structure. You must make
that each connection is consistent with the information in the
spectrum. CHECK IT! (The NMR Mosaic is excellent at helping
you make sure the pieces truly match.)
Roadmap for solving the structure.
- Use the
molecular formula (if given) to figure
out the degree
of unsaturation (the number of rings and/or multiple bonds).
- Use the Infrared spectrum
(if given) to
out the functional groups present.
- Look for any recognizable
groups in the NMR
as aromatic rings, aldehydes, carboxyllic acids, or alcohols.
- Use the integration
to determine the
number of hydrogens contributing to each peak.
- OFTEN (but not always) the smallest
integration is 1 hydrogen.
- OFTEN (but not always) peaks around 0.7-1.1d are methyl
groups (3 hydrogens).
- If you know the molecular formula, the
hydrogen equals the total integration of all peaks divided by the total
number of hydrogens.
- Then make
fragment each peak corresponds to
(remember, each peak corresponds to one fragment of the molecule, often
a but now always a CHx) :
- Use the integration to calculate the number of
hydrogens contributing to each peak (a CH, CH2,
- Use the chemical shift to determine which
any) functional group(s) is attached to that fragment.
- Use the multiplicity to determine how many
hydrogens are adjacent to the fragment.
- Put the
fragments together. You have
two kinds of information telling you how to connect the
If the pieces
don't all match,
something is not right. Find the piece(s) that fit the worst
check them first; they are frequently the ones interpreted incorrectly.
Is the splitting correct? Can the chemical shifts
another functional group? Can a CH really be an OH?
- The chemical shifts tells you which
are connected to which functional groups.
- The splitting tells you how many hydrogens
on immediately adjacent carbons.
- Check the
interpretation of the
splitting. For instance a quartet almost always means the
next to a CH3 group. But once in a while it may be next to a
and a CH. Either can be right, and often it is impossible to
which is correct until you start putting the fragments together.
It just depends on the specifi spectrum you are solving.
The NMR Mosaic can help you avoid problems that have plagued students