What is NMR Spectroscopy?

NMR stands for Nuclear Magnetic Resonance in which the study of different molecules is done based on the interaction of radiofrequency electromagnetic radiations with the nuclei of molecules placed in a strong magnetic field. A precise study of spectra of visible light and all bands of electromagnetic radiation is called spectroscopy. 

General Introduction about NMR Spectroscopy

• This spectroscopy is considered to be one of the prominent techniques for determining the structure of organic compounds. Among all types of spectroscopy, NMR is considered to be the only one for which a complete analysis and interpretation of the entire spectrum is normally expected. NMR spectroscopy is non-destructive and is done with modern instruments because of which good data may be obtained from samples weighing less than a milligram. 
• NMR spectroscopy is based on the absorption process of electromagnetic radiations in the radiofrequency region of 4 to 900 MHz. This absorption process is done by the nuclei of atoms. 

Physical principle of NMR spectroscopy

To successfully use NMR analytical tools, it is necessary to understand the physical principles on which the methods are based. 

• As there are so many elemental isotopes that show characteristic features of spin, i.e., some nuclei show integral spin I =1,2,3,...., some show integral spin in fractional form I = ½ whereas some show no spin, i.e., I =0. From the analysis, it is shown that NMR spectroscopy is limited to those having spin (I) = 1 or ½. 
• A strong magnetic field is necessary for NMR spectroscopy. Tesla is the international unit that is used for measuring magnetic flux. In modern NMR spectroscopy, a high magnetic field between 1 to 20 Tesla is used.
• Energy transfer is possible between ground state to an excited state and radiofrequency is applied only when radiofrequency energy matches with the energy difference. 

Working principle of nuclear magnetic resonance

• Nuclear magnetic resonance is based on the principle of the spins of atomic nuclei. A nucleus which has either an odd atomic mass or atomic number shows nuclear spin. A nucleus is a charged particle that is under continuous motion and because of this it develops a magnetic field. 
• When the nucleus which has non-zero spins is placed under a strong magnetic field with respect to the applied magnetic field, with the supply of appropriate energy, these nuclei move from higher energy states to lower energy states.
• The difference in energy between a higher energy state and a lower energy state depends on the applied field. Thus the amount of energy absorbed during this transition is a function of the nucleus type and its chemical environment in the molecule.

Applications of NMR

• In spectroscopy, study of the interaction of electromagnetic radiation with matter is done. NMR spectroscopy is used to study the physical, chemical, and biological properties of various molecules.
• It is used in analytical chemistry for quality control checking. 
• It is used in various chemical research for determining the content and purity of a sample along with its molecular structure. For example, NMR can be used for the quantitative analysis of mixtures containing known compounds.
• This study is used by various chemists for studying the chemical structure by using simple one-dimensional techniques; whereas two-dimensional techniques are used in determining the complicated structure of the molecules. 
• This technique is also used in determining the structure of a protein by replacing x-ray crystallography. 
• This technique is also used to probe molecular dynamics in solution.
• Solid-state NMR spectroscopy is used in determining the molecular structure of solids. 
• It is also used in measuring the diffusion coefficients of the gases. 

Chemical shift in NMR Spectroscopy

• When the charge undergoes a spinning movement, it generates a magnetic field which results in a magnetic moment proportional to the spin. As in the presence of an external magnetic field, there exist two kinds of spin: one spin up and one spin down. Here one spin shows alignment with the magnetic field and the other shows alignment opposite to the magnetic field.  
• When there is a difference in the resonant frequency of the spinning proton and signal of the reference molecules then it is called resonating frequency. The chemical change in nuclear magnetic resonance is one of the most important properties usable for molecular structure determination. 
• Several nuclei are also detected from NMR spectroscopy like ¹H (proton), ¹³C (carbon 13), ¹⁵N (nitrogen 15), ¹⁹F (fluorine 19) and many more. Among all these atoms ¹H and ¹³C are the most widely used. The similarity which is present in all atoms is an odd atomic number or mass number. 
• In NMR chemical shifts are used to explain signals in other forms of spectroscopy like photoemission spectroscopy.

Influencing factors on chemical shifts

Factors that affect the chemical shift are as follows:

• Electronegativity : With the increase in electronegativity of the surrounding groups, a decrease in electron density is observed. This decrease in electron density further causes an increase in chemical shift value due to the shielding of the nucleus.
• Anisotropy : Chemical shifts are also dependent on the orientation of neighbouring bonds, mainly the π bonds.
• H-bond : Hydrogen bonding occurs because of electronegative atoms in the neighbourhood of protons. The resulting deshielding leads to higher values of chemical shifts.

Shielding in NMR

An external magnetic field has an easily influencing ability to the electron density found around the nucleus of the hydrogen atom. This phenomenon is called the shielding effect; electron density shields the proton in the nucleus from the external magnetic field. This shielding is done by decreasing the net magnetic field that is experienced.

Major features which affect the shielding of the nucleus are electronegativity, the magnetic anisotropy of π systems, and hydrogen bonding as discussed above.

NMR instrumentation

NMR instrumentation has nine major parts; they are listed below:

1. Sample holder – A glass tube having a diameter of 8.5 cm long and 0.3 cm. 
2. Magnetic coils –This coil helps in generating magnetic fields whenever current flows through it. 
3. Permanent magnet – Its major function is to provide a homogenous magnetic field at 60 – 100 MHz. 
4. Sweep generator – It helps in modifying magnetic field strength which is already applied.
5. Radiofrequency transmitter – This transmitter produces a powerful but short pulse of the radio waves.
6. Radiofrequency – It is useful in detecting receiver radio frequencies.
7. RF detector – It helps in detecting unabsorbed radio frequencies.
8. Recorder – It records those NMR signals which are captured by the RF detector.
9. Readout system – It is a cryptographic system that records the data.

Context and Applications

This topic is important for both undergraduate exams like NEET, JEE, PAT and other competitive exams and graduate exams mainly for Bachelors and Masters in Chemistry.

Related concepts

• Spectroscopy
• Types of spectroscopy
• Acquisition of spectra
• Sample handling

Practice Problems

1. Explain the basic principle behind NMR?

Ans. The basic principle of NMR spectroscopy is that many nuclei have a spin and all nuclei are electrically charged. So, after applying an external magnetic field, an energy transfer is possible between the base energy to a higher energy level mainly causing a single energy gap. 

2. Explain which elements are used in NMR spectroscopy?

Ans. NMR is considered a physical phenomenon in which nuclei in a magnetic field absorb and re-emit electromagnetic radiation.

Deuterium is an isotope of hydrogen having a neutron in the nucleus and does not show a spin. Deuterated chloroform is the most common solvent used in NMR spectroscopy.