Nuclear Magnetic Resonance Spectroscopy: Principles, Instruments, and Spectra Interpretation

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’. 

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 concerning 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. 

Principle of Nuclear Magnetic Resonance Spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful analytical technique used to study the structure and dynamics of molecules, particularly in organic chemistry and biochemistry. The principle of NMR is based on the magnetic properties of certain atomic nuclei, such as hydrogen (protons) and carbon-13.

The fundamental principle behind NMR is the interaction of nuclei with an external magnetic field. When placed in a strong magnetic field, certain nuclei with a non-zero nuclear spin (such as ^1H or ^13C) can absorb radiofrequency (RF) energy and undergo a transition from a lower energy state to a higher energy state. The energy required for this transition is proportional to the strength of the external magnetic field.

The magnetic properties of the nuclei are influenced by the local environment, including the presence of neighboring nuclei and their magnetic moments. This interaction leads to the splitting of NMR signals, known as chemical shifts and coupling constants, providing valuable information about the molecular structure.

Chemical Shift

Chemical shift is the shifting in 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. But 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

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), 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 concerning 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.   

Instrumentation of Nuclear Magnetic Resonance

The NMR instrument is a simple setup of the following components:

I. 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.

II. Magnets

Permanent magnets or electromagnets that 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.

III. Sweep Coils & 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.

Since this is not an easy process 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.

IV. Radio Frequency 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.

V. Radio Frequency 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 Does the NMR Spectra Interpret?

An NMR spectra elucidates 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 concerning the environment of neighboring protons.
Conclusion

Nuclear Magnetic Resonance (NMR) spectroscopy is a vital analytical technique that provides detailed information about the structure and dynamics of molecules. Its core principle involves the interaction of atomic nuclei with magnetic fields, yielding chemical shifts and coupling constants that serve as unique molecular fingerprints.

Widely applied in organic chemistry, biochemistry, and medicine, NMR has become an indispensable tool for researchers, offering insights into molecular structures and behaviors.

With its non-invasive nature and evolving technology, NMR continues to drive groundbreaking discoveries and advancements across various scientific disciplines.

Suksham Gupta

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