Nuclear Magnetic Resonance (NMR) Spectroscopy

The spectroscopy of nuclear magnetic resonance often referred to NMR spectroscopy, also known as magnetic resonance spectroscopy (MRS) is a method of spectroscopy to detect local magnetic fields around the nuclei of atomic particles. It is a spectroscopy method that is based on an absorption process of electromagnetic radiation within the radio frequency range of from 4 to 900 MHz nuclei in atoms. In the last 50 decades, NMR has become the leading method to determine what the organic structure is. Out of all the methods for spectroscopy it is the sole one in that a full examination and understanding of the whole spectrum is typically required.

Principle of Nuclear Magnetic Resonance (NMR) Spectroscopy

The basic principle that drives NMR is that all nuclei are spin-driven and all nuclei are charged electrically. When the magnetic field of an external source is utilized, a energy transfer can occur between the energy of the base level to an energy level higher (generally one energy gap). The energy transfer occurs at a wavelength which corresponds to radio frequencies . And once the spin is back to original point, energy is released with the identical frequency. The signal that corresponds to the energy transfer is monitored in a variety of ways, and then processed to produce an NMR spectrum of the nucleus involved.

Working of Nuclear Magnetic Resonance (NMR) Spectroscopy

• The sample is put in an electric field. The NMR signal is generated by the excitation of nuclei of the samples by radio waves. The result is nuclear magnetic resonance. This can be detected using highly sensitive radio receivers.
• The magnetic field that surrounds the atoms in molecules alters the resonance frequency, thereby giving details of the structure and electronic makeup of the molecular structure and its functional groups.
• Because the fields are distinctive or highly distinctive to specific substances, NMR spectroscopy is the most reliable method for identifying the monomolecular organic compound.
• In addition to the identification of molecules, NMR spectroscopy provides detailed details about the structure and dynamics, the state of reaction and the chemical surroundings of molecules.
• The most popular kinds of NMR are carbon-13 and proton NMR spectrum, however it can be used with any type of material with nuclei that have spin.

Instrumentation of Nuclear Magnetic Resonance (NMR) Spectroscopy

1. Sample holder: glass tube that is 8.5 cm length, 0.3 centimeters in diameter.
2. Permanent magnet: They creates a an even magnetic field of 60 to 100 MHz.
3. Magnetic coils: Magnet coils are coils that create magnetic fields when they are surrounded by current.
4. Sweep generator: To generate the same quantity of magnetic fields that that passes through the sample.
5. Radio frequency transmitter: The radio frequency transmitter is a coil transmitter that emits an extremely short sound wave of radio.
6. Radio frequency receiver: A coil of a radio receiver that is able to detect radio frequencies released when nuclei relax to lower energy levels. 
7. Read out systems: Computers that analyze and records the data.

Background

In the last fifty years, the nuclear magnetic resonance technique also known as nmr has grown to become the most reliable method for determining the organic structure. Of all the methods for spectroscopy that exist, it is the only one that allows for a full study and analysis of the complete spectrum is usually required. While larger quantities of sample are required than mass spectroscopy however, nmr is not destructive and, with the latest instruments, good results can be obtained from specimens weighing less than 1 milligram. To make the most of nmr for analysis it is essential to know the physical concepts upon which the methods are founded.

The nuclei in many elemental isotopes exhibit a distinct spin (I). Certain nuclei have an integral spin (e.g. I = 1,2 3, ….), others contain fractional spins (e.g. I = 1/2 3/2 5/2, etc.) ….), and some do not have a spin I = 0. (e.g. 12C, 16O, 32S, ….). Isotopes that can be of great significance and importance for organic chemists include 1H 13C, 13C and 19F. 31P. All of them contain I = 1/2. Because the study of the spin state is quite easy, our discussion on the nmr is limited to these as well as other I = 1/2 nuclei.

The following features lead to the nmr phenomenon:

1. A spinning charge produces an electric field as illustrated by the animation to the right. The spin-magnet that results has magnetic moments (m) equal to its spin.
2. When there’s an external magnetic field (B0) There are two states of spin exist: with +1/2 as well as -1/2. In the magnetic field of lower energies, state +1/2 is in alignment with the field outside while more energetic -1/2 state is in opposition to the field externally. Notice that the arrow indicating the external field is North.

3. The energy difference between the two states is based on the strength of the magnetic field outside and is never tiny. The following diagram shows that both spin states share the same energy when the field is not present however they diverge when the field gets stronger. When the field is equal to Bx an equation for the energy gap is provided (remember the equation I = 1/2, and m is the magnetic time of the nucleus in this field).

4. The strong magnetic fields are required to perform NMR spectrum analysis. The most widely used unit for magnetic flux is called the Tesla (T). The magnetic field of the earth isn’t constant, but is around 10-4 T at the ground level. Modern nmr spectrometers employ powerful magnets that have fields of from 1 to 20 T. With these powerful fields, the energy gap between two states is only 0.1 Cal/mole. To understand this take note that infrared-related transitions are between 1 and 10 kcal/mole while electronic ones are 100 times more. To be used for nmr this tiny energetic difference (DE) is usually expressed as the frequency measured in MHz units (106 Hz) that ranges between 20 and 990 Mz, dependent on the strength of magnetic fields and the particular nucleus that is being examined. When a sample is exposed to radiation, the frequencies of radio (rf) energy that is identical to spin-state separation for a certain group of nuclei can cause the nuclei to undergo excitation from the +1/2 spin state to the spin state higher than -1/2. Be aware that this radiation falls within the radio and television broadcast spectrum. Nmr spectroscopy is the most energetically gentle probe to analyze how molecules form. The nucleus of the hydrogen atom (the proton) has the magnetic moment m = 2.7927 It has also been extensively studied than the rest of nuclei. The diagram previously shown can be modified to show energy changes for the proton’s spinning states (as frequencies) when you click anywhere within it.
5. for spin-1/2 nuclei, the energy differences between the two spin states with a certain magnetization strength is proportional to the magnetic moment. For the four nuclei common to all mentioned above that are magnetic, their magnetic moments are 1H m= 2.7927 19F M = 2.6273 31P m is 1.1305 and 13C = 0.7022. These moments occur in the nuclear magnetons, which contain 5.05078*10-27 JT-1. The following diagram shows the approximate frequencies that correspond to energies of spin separations of each of these nuclei under an outside magnetic field that is 2.35 T. The formula found in the colored box illustrates the direct relationship between the frequency (energy variance) in relation to magnetic moments (h = Planck’s constant 6.626069*10-34 Js).

Applications of Nuclear Magnetic Resonance (NMR) Spectroscopy

It is the science of studying the interactions between electromagnetic energy with material. NMR spectroscopy makes use of NMR phenomenon to investigate the physical, chemical and biological characteristics of matter.

• Analytical technique in chemistry that is used for quality control.
• It is utilized in research to determine the purity and content of a substance and also it’s molecular shape. For instance, NMR can quantitatively analyze mixtures of well-known compounds.
• NMR spectroscopy is commonly used by chemists to analyze chemical structure by using basic one-dimensional methods. Two-dimensional techniques can be used to analyze how molecules behave. complex molecules.
• These methods are replacing x ray crystallography for the purpose of determining the structure of proteins.
• Time domain NMR techniques for spectroscopy are employed to study molecular dynamics in solution.
• Solid-state NMR spectroscopy is employed to study the molecular structure of solids.
• Other scientists have also developed NMR methods of measuring the diffusion coefficient.