Download Nuclear Magnetic Resonance Spectroscopy

Nuclear Magnetic Resonance Spectroscopy
Topics
1. Theory behind NMR
2. Chemical shifts
3. Integration of signals
4. Splitting of signals
One of the most powerful tools available to chemists for determining molecular structure.
The test sample can be recovered since the measurement does not destroy the molecule.
Theory behind NMR
The proton in the nucleus has a positive charge. The protons act as if they were spinning and
they generate a magnetic field. Nuclei which have an odd number of protons can behave as if
they were tiny bar magnets. Of importance to organic chemists is that the hydrogen nucleus, one
proton, and the carbon-13 nucleus, 13 protons, both have this property.
We will discuss proton magnetic resonance spectroscopy, i.e. that arising from the hydrogen
nucleus.
When a molecule containing hydrogen atoms is placed in a strong magnetic field, the spinning
proton of the nucleus of each hydrogen atom aligns either with the magnetic field or against the
magnetic field. The energy difference between the two states is very very small, but those
protons aligned with the field are of slightly lower energy than those aligned against the field.
Therefore, there are a few more protons aligned with the field. When a beam of electromagnetic
energy of the proper energy is applied to the sample, some of the protons which are aligned with
the field absorb energy and flip to become aligned against the field. This is the state calle
‘resonance’. As the magnetic field strength is increased, the energy needed to flip the proton
becomes higher so that the frequency of the electromagnetic energy which causes resonance is
also higher.
Chemical Shift
If all of the protons in a molecule resonated at exactly the same energy, then there would be only
one signal and the spectrum would not be useful. However, protons in a molecule are
surrounded by electrons and these electrons also set up tiny electric fields around the proton.
These fields make small changes in the energy needed to cause the proton to resonate. Therefore
the environment of the proton in the molecule affects the energy at which is resonates.
The energy at which the proton in an organic molecule resonates is measured relative to the
energy at which the protons in tetramethylsilane, (CH3)4Si resonate. All of the protons in
tetramethylsilane (TMS) are identical and so only one signal results. This molecule was chosen
as the reference since the protons in it resonate at a slightly higher frequency than the protons in
organic molecules so there is no interference. The position of the TMS signal is set to zero and
the signals of the other protons are expressed relative to this signal on the so-called delta(δ)-scale
where
δ = peak distance in Hertz (Hz) from the TMS peak
spectrometric frequency in MHz
Thus 1δ = 1 part per million of the spectrometer operating frequency and 1δ = 60 Hz at a
spectrometer frequency of 60MHz, 100 Hz at a frequency of 100 MHz and 600 Hz at a
frequency of 600 MHz.
The reason for measuring the chemical shift in this way is that the δ value of the signal is
constant regardless of the operating frequency of the spectrometer. A proton that gives a signal
at 2.5 δ on a 60 MHz instrument will also give a signal at 2.5 δ on a 600 MHz instrument.
Protons with a high electron density around them resonate at high field – closer to TMS. These
are called ‘shielded protons’. They require a higher energy to resonate. Protons that are less
shielded resonate at lower energy and so their signals are further downfield from TMS. Each
chemically non-equivalent set of protons gives a signal at a different value of δ .
Integration
The area under each peak is proportional to the number of protons giving rise to the peak.
Spin-Spin Splitting
This is a very useful feature of the signals in an NMR spectrum.
The number of peaks in a signal from a proton or set of equivalent protons on a given carbon
atom is equal to n + 1, where n is the number of protons on adjacent carbons.