NMR Basics for the absolute novice

Nuclear Magnetic Resonance (often abbreviated as NMR) is a phenomenon observed when an appropriate nucleus is placed in a magnetic field. An NMR instrument allows for the molecular structure of a material to be determined by observing and measuring the interaction of Radio Frequency (RF) energy with the nuclei in a given sample in order to generate a spectrum . The most commonly obtained spectra are those for hydrogen (often called protons or abbreviated as 1H) and carbon (13C).

For the analysis of molecular structure at the atomic level, electron microscopes and X-ray diffraction instruments can also be used, but the advantages of NMR are that sample measurements are non-destructive and there is less sample preparation required.

Fields of application include bio, foods, and chemistry, as well as new fields such as battery films and organic electronic materials, which are improving and developing at remarkable speed. NMR has become an indispensable analytical tool in cutting-edge science and technology fields.

NMR instrument composition

NMR instrument composition

 

5 components of every NMR system

  1. A stable magnet that produces a homogeneous magnetic field
  2. A Radio Frequency (RF) transmitter that produces the necessary electromagnetic radiation
  3. A highly sensitive RF receiver that can detect the weak signals produced by the resonating nuclei
  4. A console to control the RF pulses and convert the signals detected by the receiver to a digital format
  5. Software which can be used to help the user interpret the data produced by the instrument

Principles of nuclear magnetic resonance (NMR)

When a nucleus that possesses a magnetic moment (such as a hydrogen nucleus 1H, or carbon nucleus 13C) is placed in a strong magnetic field, it will begin to precess at a particular frequency like a spinning top. This precession is the fundamental attribute of nuclei that allows us to to use NMR. So, how do we get from spinning protons to an NMR spectrum which we can use to interpret the structure of a molecule, such as ethanol shown here?

Precession
Arrow
Spectra

The steps below detail the process of obtaining an NMR spectrum of a molecule of interest. This overview is intended as a simplified look at NMR technology and theory in helping to answer the question: "What is NMR?"

1. Sample and Magnet

Samples are placed in a "probe" which positions the sample precisely in a strong magnetic field that is generated by the superconducting magnet. Both solid and liquid samples can be analyzed using NMR. However, most samples are those dissolved in a solvent; sometimes a deuterated solvent is used. (A sample holder depicted here shows the holder with an NMR tube holding a liquid solution of the dissolved sample.) Each NMR active nucleus in the sample has its own tiny magnetic moment. The microscopic magnetic moments of the nuclei in the sample align and form a net macroscopic magnetization vector that is aligned with the static magnetic field generated by the magnet.

Sample Holder for NMR

Alignment to the magnetic field

2. Excitation

Strong electric currents are generated in the probe coils (a "pulse" of broad-band RF) in order to form a secondary oscillating magnetic field. This causes the macroscopic magnetization to rotate to some extent (often 90°) into to the horizontal or xy plane. These electrical currents are generated by the spectrometer.

Magnetic moment rotates to a different orientation

3. Measure

After excitation, the net macroscopic magnetization precesses around the primary static magnetic field and returns to the z plane (going vertical), inducing weak currents (decay) in the probe coils. This resonance signal, also known as a Free Induction Decay (FID), is recorded by the spectrometer as a function of time.

Free Induction Decay

4. Process

The FID is usually a complex exponential decay pattern. It is converted from the time domain into the frequency domain by performing a Fourier Transformation (FT). Multiple scans are usually necessary to increase the signal-to-noise ratio, or S/N, so that the peaks being used for elucidating the structure can be discerned from the background noise. Sometimes thousands of scans are required.

Fourier Transformation

5. Interpret

As successive scans are added to the database and a spectrum is obtained, the final step is interpretation. Information such as chemical shift, peak shape, linewidth, and intensity can help determine structural information as well as chemical processes that may be occurring in the sample.

NMR System

What we can learn from NMR spectra

  1. Chemical shift: Information about the local chemical environment of atoms within the molecule.
  2. Spin-Spin coupling constant: Information about adjacent atoms by looking at the "splitting" of the peaks.
  3. Relaxation time: Information on molecular dynamics.
  4. Area under the peak: Using integration you can determine the number of atoms for a given peak.
  5. Signal intensity: Quantitative information, e.g. atomic ratios within a molecule that can be helpful in determining the molecular structure, and proportions of different compounds in a mixture.
What we can learn from NMR spectra

Interested in learning about the history of NMR and the innovations since the technique's discovery?

Read our article: NMR Innovation: A Manufacturer’s Perspective on Analytical Advances and New Applications

You can also read an in-depth published article on the history of JEOL NMR and ESR


NMR application fields

Analysis of Molecular Structure and Identification of Unknown Chemical Substance

Analysis of Molecular Structure and Identification of Unknown Chemical Substance

A very wide range of applications including Organic Chemistry, Inorganic Chemistry, Biochemistry, Pharmaceutical Analysis, New Materials, Petrochemistry, etc.

Quantitative Analysis

Quantitative Analysis

Polymer Chemistry, Quality Control of Synthetic Chemicals, Food Chemistry

Relaxation Time (molecular mobility, interatomic distance)

Relaxation Time (molecular mobility, interatomic distance)

Organic Chemistry, Polymer Chemistry

Diffusion Coefficient (molecular weight, conformation of polymer)

Organic Chemistry, Polymer Chemistry