Nuclear Magnetic Resonance (NMR) spectroscopy is an analytical technique that is a result of the interaction between the radiofrequency waves and the nucleus of a molecule. This interaction induces a transition between magnetic energy levels of nuclei of the molecule.
The energy involved in radiofrequency radiation is very small which is too small to vibrate, rotate or excite an atom or molecule, but this energy is sufficient to affect the nuclear spin of the atoms of the molecule. Hence, the name of this technique is ‘Nuclear Magnetic Resonance’.
NMR was first accurately measured by Isidor Rabi of Columbia University, in 1938 and won Noble Prize in Physics for his work in 1944.
Main Character of Nuclear Magnetic Resonance- Proton
The particle involved in NMR is the proton, which is present in the nuclei of the molecule. This proton acts as a small magnet that shows magnetic moments. When radiofrequency waves are passed through these protons, they start resonating in the molecule.
The protons spin in two directions namely north and south with respect to the magnets. The position in the north direction is called the alpha-position and towards the south, it is called the beta-position. The frequency required for the movement of protons from alpha-position to beta-position is known as resonance frequency.
The change in the position of the proton causes a difference in the magnetic energy of the nuclei (𝚫𝗘), which is specific for each atom. This energy is affected by the shielding and deshielding effects of the electrons.
Shielding of electrons (electrons surrounding the proton) reduces the energy because the movement of protons is not at 180 degrees from alpha to beta-positions as the electrons hinder the movement of protons. While the deshielding of electrons provides more energy difference as the spin of protons from alpha to beta-position is at 180 degrees.
Working Principle of Nuclear Magnetic Resonance
The basic idea behind the working of Nuclear Magnetic Resonance is that the spinning of protons in a nucleus produces a magnetic field which results in the spinning motion of that proton.
According to quantum theory, a spinning nucleus can only have values for the spin angular momentum, which is given by:
[I(I+1)]¹⁄² h/2𝜋
where,
I = spin quantum number
h = Planck’s constant
𝜇 = 𝛾 * [I(I+1)]¹⁄² h/2𝜋
where,
𝜇 = magnetic moment of the nucleus
𝛾 = gyrometric ratio
When the protons interact with the gravitational force and the spin of the proton occurs, the motion produced is known as precessional motion or gyroscopic motion. And the frequency of the proton due to this motion is known as precessional frequency.
When the precessional frequency of a proton is the same as the radio frequency applied to the proton, then the absorption of radio waves by the proton occurs. The absorption of energy excites the proton from a lower energy level to a higher energy level.
𝜈 = 𝛾/2𝜋. B₀
Where,
𝜈 = precessional frequency
𝛾 = gyrometric ratio
B₀ = magnetic field
The above equation is called the Lamor equation which is the mathematical basis of NMR.
When a nucleus is placed in a system where it absorbs energy and becomes excited, then the nucleus loses its energy to return to its ground state. It then again absorbs the energy and becomes excited. This alternate process of the nucleus exciting and exciting causes its resonance.
Instrumentation of Nuclear Magnetic Resonance
The NMR instrument is a simple setup of the following components:
1. Sample Holder
Sample holders are tubes made up of glass, having a diameter of 0.3 cm and a length of 8.5 cm. These sample holders are inert, durable, and transparent to radiofrequency waves.
2. Magnets
Permanent magnets or electromagnets which provide strong and homogenous magnetic fields are suitable for the instrument. The usual strength of magnets is at least 20,000 gauss. The resolution is directly proportional to the strength of these magnets.
3. Sweep Coils and Sweep Generator
For the nucleus to resonate, the precession frequency of the nucleus should be equal to the radio wave frequency. This means if one of the frequencies is kept constant, then the other one must be changed to match the previous one. But this is not an easy process.
Therefore Helmholtz coils are employed in the pole faces of the permanent magnets. These coils are called sweep coils as they induce a magnetic field that can be varied by varying the current flowing through them. The phenomenon is called sweeping of the field.
A sweep generator is used to generate the varying magnetic field required for the resonance of the nucleus.
4. Radiofrequency Generator
A radio frequency oscillator is used to generate radio frequency waves. The oscillator is wound around the sample holder in the perpendicular direction of the applied magnetic field.
5. Radiofrequency Generator Receiver
A Radiofrequency receiver is a detector that detects radio waves. A receiver coil is attached to the sample holder and the detector receives the NMR signals. These signals are extremely weak, therefore they are first amplified and recorded by a readout system. The signals are recorded in the form of spectra.
What Do The NMR Spectra Interpret?
Nuclear Magnetic Resonance spectra elucidate a lot of information about the sample which can be interpreted in the following forms:
- The number of peaks in the spectra tells about different kinds of protons in different chemical environments present in the structure under examination.
- The position of signals elucidates the electronic environment of each proton.
- The relative number of protons of different kinds can be revealed from the intensities of different signals.
- The splitting of signals reveals information about the environment of absorbing protons with respect to the environment of neighboring protons.
Chemical Shift
Chemical shift is the shifting in the position of Nuclear Magnetic Resonance signals resulting from the shielding and deshielding of electrons which causes changes in the electronic environment of protons.
Since protons are surrounded by electrons, there is a production of a secondary magnetic field when it comes in contact with an external magnetic field, which is known as the electronic environment of the protons.
When the secondary magnetic field is in the direction of the applied magnetic field, then the proton is shielded and the signal is upfield. However, if the secondary magnetic field is in the opposite direction of the applied magnetic field, then the proton is deshielded and the signal is downfield.
The chemical shift is denoted by 𝛿, which is equated as:
𝛿 = 𝜈(sample) – 𝜈(reference) * 10⁴ ppm / 𝜈(reference)
where,
𝜈(sample)= resonating frequency of sample
𝜈(reference)= resonating frequency of reference
The reference sample used in NMR is Tetramethylsilane(TMS). It is highly shielded and has 𝛿 value of 0. It does not take part in chemical reactions.
The chemical shift is affected by certain factors such as- inductive effect, anisotropic effect, and hydrogen bonding.
Spin-Spin Coupling
The interaction between the spins of the neighboring nuclei in a molecule may cause the splitting of the lines in the NMR spectrum which is known as spin-spin coupling. The signal in the NMR spectrum is expected to be a single peak for an individual molecule/atom but actually, each peak observed in the spectrum is split into various peaks depending upon the structure of the molecule.
The different spin states and resultant magnetic moments of the neighboring protons lead to the modification in the actual magnetic field experienced by the given proton. The splitting of the signal leads to the splitting of a single peak into two equally spaced peaks(doublet) or three equally spaced peaks(triplet) or four equally spaced peaks(quartet) and so on.
The splitting of the signal arises due to the alignment of the spinning proton with respect to the applied magnetic field. The spin-spin interactions are independent of the strength of the applied field.
Equivalent nuclei do not interact with each other to cause spin-spin splitting.
The effectiveness of spin-spin coupling, which is responsible for the distance between the peaks in a multiplet, is called coupling constant, denoted by J and measured into Hertz or cycles per second.
Advantages of Nuclear Magnetic Resonance
- NMR is a qualitative as well as quantitative technique.
- It is a non-destructive analytical technique.
- Structural information of a compound is easily elucidated by this method.
- The analysis time is not slow.
- Sample preparation is easy and a small number of samples is required.
- Highly reproducible technique.
Disadvantages of Nuclear Magnetic Resonance
- The instrument is an expensive one.
- The resultant peaks can be mistaken for a single instead of two or more peaks.
- Quite an insensitive technique.
- Requires strong and high magnetic fields.
- It is motion sensitive, especially when spatial encoding using gradients is employed which leads to signal distortions that are visually most evident in artifacts on images or more subtly in quantitative measurements.
Applications of Nuclear Magnetic Resonance
- Forensic Toxicology- NMR is helpful in the examination of drugs, poisons, pesticides, etc.
- Questioned Documents- The instrument can analyze the inks.
- Forensic Biology- The biological fluids can be examined from this instrument.
- Forensic Chemistry- Fire accelerants, explosives, gunshot residues, and other chemical compounds can be examined by this technique.
- Fingerprint Examination- The fingerprint reagents are determined by this technique.
Conclusion
NMR is an analytical technique that uses radiofrequency waves to detect and identify a chemical compound in a given sample. It is one of the modern instrumental techniques that penetrate into the nucleus of an atom and elucidate the properties of the chemical compound.
NMR is a useful technique having applications in various scientific fields such as medicine, biotechnology, the chemical industry, etc. In forensics, it has applications in forensic toxicology, forensic biology, forensic chemistry, questioned documents, fingerprints, etc.
Also Have a Look at:
- X-Ray Diffraction Method
- How Does Atomic Absorption Spectroscopy Work?
- UV-Visible Spectroscopy
- Soil As A Forensic Evidence In An Investigation
Frequently Asked Questions
1. Difference Between NMR & MRI
Nuclear Magnetic resonance determines the chemical structure of matter whereas the goal of MRi is to generate detailed images of a body.
2. What Are The Main Components of NMR?
There are 4 main components of Nuclear Magnetic Resonance are Magnet, RF coil, Electronic Interface, Computer.
3. Who Invented Nuclear Magnetic Resonance?
NMR was invented by 2 American scientists in 1945 by Felix Bloch & Edward M. Purcell.
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