NMR spectroscopy has become one of the most frequently used analytical tools to elucidate
chemical structure. It is particularly useful for carbon-based chemical compounds, also
known as organic compounds, the molecules that make up all living things and many substances
that we interact with on a daily basis. In this video we’ll take a look at how NMR
spectroscopy works and provide a visual metaphor for what is happening at the nuclear level.
If we consider a chemical compound, we are particularly interested in what atoms are
present and what the arrangement of those atoms are. If we were able to look inside
an atom, we’d see a dense and positively charged NUCLEUS. It is the nucleus that NMR
spectroscopy detects and gives us a lot of information about the overall structure of
the molecule. Here’s how it works. Ordinarily, nuclei
are oriented randomly, but If we place a sample of a chemical compound in a strong magnetic
field at the heart of the NMR instrument, the nuclei line up (like a compass needle
aligning in a magnetic field) and start to precess like a spinning top as it spins on
a table. By convention, the magnetic field (designated
B0) is oriented along the Z-axis. Different nuclei have different spins, and the ones
with spin one-half can exist in one of two possible spin states. These include the following
atomic isotopes The hydrogen nucleus (usually referred to
as simply the “proton”) is present in almost all organic compounds and is the one
most frequently analyzed by NMR. The precession frequency or Larmor frequency
is given by the following equation and is dependent on the strength of the magnetic
field and the gyromagnetic ratio which is a constant for a given isotope.
If we apply a short radiofrequency pulse (or Rf pulse) at right angles to B0, the nuclei
will flip down to the X-Y plane in unison and continue to precess.
Let’s represent the average magnetic moment of a set of nuclei by this sphere. At equilibrium
the magnetization vector will be aligned along B0 as depicted here.
As we’ve seen the Rf pulse will flip the magnetization vector to the XY plane. The
rotating magnetization vector induces a current in a receiver coil. This signal is detected
and amplified. We can see that the oscillating signal sweeps out a sine wave with decreasing
magnitude as the nucleus re-aligns with the magnetic field B0. This wave is called the
free-induction decay or FID. If each proton sweeps out an FID at the same
frequency, then each proton would look identical and we wouldn’t be able to learn anything
about the overall structure of the molecule. But here’s the key: Each proton is in a
unique chemical environment. Let’s consider two protons in different
parts of the molecule, each will be exposed to a different magnetic field depending on
the electron density distribution. The electron creates its own magnetic field which opposes
the applied magnetic field B0. A proton with a high electron density around it experiences
a lower magnetic field and therefore a lower Larmor frequency, and a proton in an electron-
poor environment has a higher frequency. Typically, protons in an electron-rich environment are
referred to as “shielded” and those in an electron poor environment are referred
to as “de-shielded.” Aromatic rings, such as a benzene ring, can induce their own magnetic
fields, which can further modify the magnetic field around the proton.
Now if we pulse these with an Rf pulse, we will get two signals in our FID, one oscillating
at a high frequency and one oscillating at a low frequency.
The FID is a representation of the wave in the time domain. The Fourier transformation
mathematically converts this to the frequency domain, which allows for a visual representation
in an NMR spectrum, with the shielded protons giving peaks at the right side and the de-shielded
protons giving peaks on the left side. Let’s look at an example. Here’s a simple
molecule with hydrogens in 5 different electronic environments. Each type of proton will experience
a slightly different magnetic field strength and will precess at its own frequency giving
5 overlapping signals in the FID. After Fourier transformation, we will get
a spectrum with 5 peaks, each peak representing a proton (or set of protons) in a particular
chemical environment. With this spectrum in hand, we can start to deduce the chemical
space around each proton. We have seen that an isolated proton (or group
of identical protons) will give an NMR spectrum with a single peak (known as a singlet). Sometimes
you will have protons that are close enough that their magnetic spin states interact (what’s
known as spin-spin coupling). In this case the peaks do not appear as a singlet, but
rather appear as a split peak, known as a doublet. For example, if proton A has a neighbor,
proton B, in a different electronic environment, proton A will be affected by its neighbor.
Proton B will either be aligned up or down, thus proton A will be a doublet. Likewise,
proton B will be split by proton A, and it will be a doublet also.
Two neighboring protons will give a triplet and three neighboring protons will give a
quartet. You can even see quintets and septets in some cases. Spin-spin coupling can give
us valuable information about the connectivity of the atoms in a molecule because it shows
us which protons are close neighbors to which other protons.
I hope this video has provided some useful images to provide a visual metaphor of how
NMR spectroscopy works. As a next step you may want to focus on the equations that are
used to describe the behavior of nuclei in a magnetic field. You may also want to get
a deeper understanding of how the NMR spectrum is used to interpret chemical structure, and
a lot of that skill will come with practical experience and seeing first-hand the relationship
between structures and their resulting spectra. Thanks for watching. See you later!