Nuclear Magnetic Resonance or NMR is a technique that makes use of magnetic fields and electromagnetic frequencies to study the molecular structure of a sample. It is an analytical technique used to determine the purity and the composition of an element or a compound. Once the basic structure of the sample is known, NMR can be used to determine molecular conformation in solution, and to study its physical properties, such as phase changes, solubility, diffusion, etc., at the molecular level. The phenomenon of NMR was first observed in 1946 by the Swiss-American physicist Felix Bloch and American physicist Edward M. Purcell independently of each other.
Principle of Working of NMR
The basic principle behind Nuclear Magnetic Resonance is that the nuclei of the sample substance having at least one unpaired neutron or proton act as tiny magnets when exposed to the magnetic field. When such a sample substance is placed in a strong magnetic field, the force exerted by the magnetic field causes the nuclei to spin with respect to their own axis in a similar way as a spinning top. This means that all nuclei are electrically charged, and many of them spin. When an external magnetic field is applied across the spinning nuclei, an energy transfer takes place between the base energy and a higher energy level. This energy transfer takes place at a wavelength that corresponds to a particular radiofrequency. This causes a part of the energy possessed by the radio wave striking the sample to get absorbed. This selective absorption is known as resonance. In other words, when the natural frequency of the spinning nuclei matches the frequency of an external radio wave striking the sample, a selective absorption or resonance takes place. When the spin returns to its base level, energy is emitted at the same frequency. The resonance may also be produced by manually tuning the natural frequency of the nuclear magnets to the frequency of a weak radio wave.
Working of NMR
The sample to be studied at a molecular level is placed in the NMR probe, which is inserted into the centre of the magnet. High magnetic fields used in NMR spectroscopy are generated by the superconducting magnets, while the low magnetic fields can be generated with the help of permanent magnets or electromagnets. The sample is placed in such a way that it experiences the strongest magnetic field and highest homogeneity of the magnet. The coils present in the NMR probe tend to excite the sample, and the detector records the responses at radiofrequency. The nuclei of the sample start to behave like microscopic magnets and tend to spin around the main magnetic field. This spinning motion exhibited by the nuclei is known as the precession. The frequency with which the nuclei spins is proportional to the strength of the magnetic field. When a radio frequency wave sweeps over the spinning nuclei, the direction of the spin tends to flip to the opposite plane. The magnetic field then induces an electric current into the receiver coil. The received signal is recorded and amplified. It can be identified easily that the resultant signal is a sine wave with decreasing magnitude. This is because the nuclei soon get realigned along the external magnetic field. The output wave here is known as the free induction decay. Each of the protons is present in a different environment, which is why each of them produces a unique free induction decay, thereby allowing us to learn about the internal structure of the sample with clarity. On plotting the Fourier transform of the obtained output signal, one can easily observe that the upfield or shielded protons correspond to the high peaks present on the right of the frequency spectrum, while the downfield or de-shielded protons represent the peaks present on the left of the spectrum. Local field differences can also be created externally by changing the electron densities in a molecule. A higher electron density corresponds to more shielding, which means a lower rate of spinning and a lower local field. The excited nuclei tend to spin like a magnet and create a magnetic field around them. The magnitude of this electric current is proportional to the intensity of the magnetic field present around the nuclei, which can be used to measure the frequency of spin with the help of NMR. Obtaining the Fourier transformation of the recorded time-domain signals gives a frequency-domain spectrum. Different frequencies of the plotted frequency spectrum correspond to different nuclei. Since all of the resultant frequencies depend on the chemical structure of the nuclei, each of the frequencies is known as a chemical shift. The spectra for all hydrogen nuclei is measured at once, known as the ethanol spectrum. It consists of three different peaks at different chemical shifts. The spin of the nuclei also affects the neighbouring elements, i.e., the shared electrons present in the chemical bond. Each spin can have two states, i.e., lower energy +1/2 state, and higher energy -1/2 spin state. The magnetic moment of the lower energy state is aligned with respect to the external field, while the magnetic moment of the higher energy spin state is opposite to the magnetic field. There exists a small difference in the energy between the two spin states, which depends on the intensity of the external magnetic field. This small energy difference is denoted by ΔE. For NMR purposes, ΔE is given as frequency in units of MHz, and the range lies from 20 MHz to 900 MHz. Irradiating a sample with a radio frequency wave having energy proportional to the energy difference of the spin states of a specific set of nuclei is responsible for excitation of the nuclei present in the +1/2 state to the higher -1/2 spin state.
Uses of NMR
1. The medical application of NMR is Magnetic Resonance Imaging, i.e., a non-invasive and painless method of getting an image of the internal organs and structures of a living being.
2. Nuclear magnetic resonance spectroscopy is mainly used to determine the purity of various solids and liquids; therefore, it is mostly applicable in the quality control departments.
3. NMR is used to study the molecular structure of a variety of solids and liquids.
4. Nuclear Magnetic Resonance is also used to measure the nuclear magnetic moments, i.e., the characteristic magnetic moment of specific nuclei.
5. NMR spectroscopy is best suited to study the structure of carbon-based compounds or organic compounds.