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Nuclear Magnetic Resonance Spectroscopy

Key Information & Summary

  • Nuclear Magnetic Resonance (NMR) Chromatography is a technique that provides information about the chemical structure of a compound.
  • NMR spectroscopy operates by applying magnetic field to nuclei that behave as magnets because of their charge and spin.
  • Nuclei are present in a microenvironment where they may or may not be surrounded by electrons. Nuclei surrounded by electrons are called shielded while those that are not surrounded by electrons are called deshielded.
  • When nuclei in different microenvironment absorb energy and go to high energy state, they are said be in resonance.
  • An NMR spectrum provides a signal or peak representing the energy necessary to bring each nucleus into resonance.
  • NMR is used to determine structures of molecules and can analyse various samples including biological, petroleum and environmental samples.

Introduction

Nuclear Magnetic Resonance (NMR) spectroscopy is an analytical technique used to determine chemical structure of a compound. It provides qualitative and quantitative information about the sample. It provides a complete analysis and interpretation of a sample. It is non-destructive and modern instrumentation allows quick and precise analysis with small amounts of samples.

α and β-spin states

A nucleus behaves like a magnet because of its charge and spin (I). Atoms mostly used in organic chemistry like 1H, 13C, 19F and 31P, all have I = 1/2 spin. Atoms have odd number of protons or odd number of neutrons or both behave like a magnet. The spin of a nucleus generates a magnetic field (B). When a magnet is brought in the vicinity of another magnet, it can effect of orientation of other magnet. For example, in a simple magnet, North Pole of a magnet is oriented toward the south pole of another magnet.

NMR spectroscopy operates by applying a magnetic field to nuclei that behave as magnets. When a nucleus is placed in an NMR chamber and external magnetic field (applied magnetic field, B) is applied to the nucleus, the nucleus may orient itself with the magnetic field. This is called α-spin state or low energy state. The magnet may also orient itself opposite direction of magnetic field. This is called β-spin state or high energy state. Although very few magnets go to high energy state.

Shielded versus deshielded nuclei

In real circumstances, the nuclei may be surrounded by a particular number of electrons. These electrons shield the nuclei from the applied magnetic fields. These kinds of nuclei that are surrounded by electrons are called shielded electrons and this phenomenon is called diamagnetic shielding. Shielded nuclei may not change their orientation on application of external magnetic field due to the diamagnetic shielding effect.  On the other hand, the nuclei that are not shielded by electrons are called deshielded electrons. Deshielded nuclei change their orientation on application of an external magnetic field.

(Nuclei behaving as magnet are represented by an arrow)

Resonance  

When the shielded and deshielded nuclei are subjected to radiofrequency radiations they may absorb the energy from radiations and go to β-spin state or high energy state. Since both kinds of nuclei are in same energy state, they are said to be in resonance. The deshielded nucleus will need more energy (E1) to reach β-spin state or resonance while the shielded nucleus will need less energy (E2) to reach β-spin state or resonance. In other words, E1 will be more than E2. This means that nuclei in different electronic environments (shielded or deshielded) require different amounts of energy to bring them into resonance.

Different nuclei require different amount of energy to go to β-spin state. The amount of energy required to bring a nucleus from low energy state to higher energy state is ΔE and can be calculated with the following formula:

ΔE = hv

h is Planck’s constant and v is frequency of radiofrequency radiation)

v = (γ / 2ℼ) B0

γ is gyromagnetic ratio of nuclei under study. Different nuclei have different gyromagnetic ratios. B0 is applied magnetic field.

The NMR spectrum

An NMR spectrum provides a signal or peak representing the energy necessary to bring each nucleus into resonance. For example, if the nuclei discussed above are put into the NMR spectrum, it would show two signals or peaks. Each signal would represent the amount of energy required to put a nucleus to resonance. Two peaks mean that there are two nuclei in the sample and each nucleus is in a different electronic environment. The peak “a” shows a deshielded nucleus that needs more energy to bring it to resonance while the peak “b” shows a shielded nucleus that needs less energy to bring it to resonance. The peaks for deshielded and shielded microenvironment are also called downfield and upfield peaks, respectively.

Examples

For example, in CH3CH2Cl, H atoms are in two electronic microenvironments. Cl is electronegative and attracts electrons. Cl will pull the electrons from the nearby hydrogen nuclei (CH2) and make them deshielded. On the other hand, the hydrogen nuclei far from Cl (CH3) will have less pulling effect or they will be more shielded. If CH3CH2Cl is put into an NMR machine, the shielded (lower energy) and deshielded (higher energy) hydrogen nuclei will appear on NMR spectrum as two peaks.

Applications

NMR has revolutionized the analysis of chemistry of various samples including biological, petroleum and environmental samples. NMR is used for determining the structures of molecules and exact position of functional groups in certain compounds. For example, if an NMR sample contains a chemical compound whose possible molecular formula is C3H7Br and has no double or triple bonds. The possible structures could be 1-bromopropane or 2-bromopropane as shown below. For 1-bromopropane, hydrogen nucleus near Br will be most deshielded while hydrogen nucleus farthest from Br will be most shielded. Hydrogen in the middle will be intermediate shielded due to small pulling effect from Br. Therefore, if Br is attached at carbon number 1, the NMR spectrum is expected to show three peaks or signals. In case of 2-bromopropane, hydrogen nuclei will be in two microenvironments: shielded or deshielded. Therefore, two peaks or signals are expected for this structure. In this way, NMR helps to solve the structures of various chemical compounds.

NMR is used as a diagnostic tool in medical examinations of human body. NMR also known as magnetic resonance imaging (MRI) is used to provide detailed images of all body parts, even the soft tissues like brain, heart etc. It is especially useful because it does not use harmful radiations as used in computed tomography (CT) scans.

Graphical Summary

References for further reading

https://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/nmr1.htm

https://www.ias.ac.in/article/fulltext/reso/009/01/0034-0049

https://www.chemguide.co.uk/analysis/nmr/background.html#top

https://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/nmr1.htm