NMR SPECTROSCOPY
NMR spectroscopy is used to come up with the carbon hydrogen framework in a
compound. It does this by identifying environments in a compound (proton NMR)
or by determining the carbon environments (carbon 13 NMR).
You need to understand a bit of physics behind NMR spectroscopy.
1. Bonds can stretch, bend or vibrate when they absorb a certain amount of
energy.
2. Absorption of energy by the bond occurs when the incidence energy (energy
subjected to the molecules) matches the energy between the ground state and
the excited state of that particular atom.
3. The source of energy used in NMR is radio waves. Radio frequency waves are
very long waves with low energy, when these low energy waves interact with
a molecule the atoms undergo a nuclear spin.
The particles inside the nucleus are always in opposite spin. This spinning effect
results into a current loop where there is a magnetic field, therefore each particle
behaves like a tiny magnet with the north and the south.
In carbon 13 the magnetic fields of 6 protons cancel out with the magnetic fields of 6
neutrons. 1 neutron remains and its magnetic field is not cancelled out. This is what
makes the nucleus behave like a tiny magnet. This is also the reason why any
particle (atom) that has an odd number of particles in its nucleus can be used in
NMR.
What happens when these atoms with odd number of particles in the nucleus are
exposed within a magnetic field?
When nuclei of hydrogen atoms are not subjected to magnetic field, there are in
random spin.
When these hydrogen atoms are placed in magnetic fields since themselves are tiny
magnets they are oriented within the magnetic field.
Once these hydrogen atoms in the magnetic fields are exposed to radio waves, their
environment can be determined depending on their neighbors.
Proton NMR
This is the study of hydrocarbon frame work using different environments of
hydrogens within the molecule.
How to determine hydrogen environments within a molecule.
An environment of any given hydrogen atoms shall always be determined by the
immediate neighboring hydrogen atoms.
The environments you have correspond to number of peaks on the NMR spectrum
results. The hydrogens in the same environment shall always absorb the same
amount of energy while getting promoted from low energy level to high energy
level.
The total amount of energy that they will absorb shall be incorporated in one peak.
NMR spectrum
Once a substance is analyzed in an NMR spectronometry machine and different
hydrogen environment are determined, the results obtained possesses intensity of
the peaks on the y axis against chemical shifts in ppm (parts per million).
Factors effecting the position of the peaks on the delta scale

When hydrogen atoms in a methyl group or free hydrogen atoms are next to a
high electronegative atoms (O,Cl,N…)these hydrogens tend to absorb more
radio wave energy and their peaks are found on to the left hand side of the
graph(down field).

When hydrogen atoms in a methyl group or free hydrogen atoms are nearby
are less electronegative atoms they tend to absorb less energy, so their peaks
are fund to the right hand side of the graph.
High resolution NMR
High resolution NMR, solves a challenge of singlet chemical shifts in low resolution
that tend to overlap even when hydrogens are in different environments.
e.g
The overlap of the peaks is due to magnetic fields generated by the spinning nuclei
are slightly interfered with the neighboring nuclei and this is called spin spin
coupling. To solve this challenge a high resolution proton NMR is used.
Peak split = n+1
Where n is the number of hydrogen atoms on the adjacent carbon atom.
Example
Reminder

Every hydrogen next to an electronegative atom is in its own environment.

Study spin spin coupling effect due to the central carbon atom.

Look at effect due to the next hydrogen atom.
Identification of hydrogen environments bonded to high electronegative atoms(N
O).
To determine whether the functional group is NH2 or OH the substance is dissolved
in heavy water (D2O).
D is an isotope of hydrogen with mass number of 2.
Once an alcohol or amine is dissolved in D2O the peak that was falling out the range
will disappear.
Solvents used in proton NMR spectroscopy
Trimethyl silicate

It is inert

Most organic compounds are soluble in it.

All the hydrogens are in the same environment.
Tetrachloromethane.

When used in hydrogen NMR there is no peak
that is formed due to the solvent since there are no
hydrogens.

It is inert
CDCL3

The hydrogen is hydrogen 2 isotope hence no peak produced due to the
solvent.
Carbon 13 NMR spectroscopy
Analysis of carbon 13 spectra is very similar to that of proton NMR spectra, just look
for different environments of a carbon in a molecule and match them to their
corresponding chemical shift.
NB: Take care when interpreting carbon 13 NMR spectra because the heights of the
lines are not always proportional to the number of carbons in that environment.
Determining carbon environments in a given molecule.
When determining a carbon environment the following must be considered.

The hybridization a carbon in the following must be considered

The other atoms it is bonded to directly
Examples
1.
2.