NMR Basics for the absolute novice

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: computer, spectrometer, and super-conducting magnet

5 components of every NMR system

  • A stable magnet that produces a homogeneous magnetic field
  • A Radio Frequency (RF) transmitter that produces the necessary electromagnetic radiation
  • A highly sensitive RF receiver that can detect the weak signals produced by the resonating nuclei
  • A console to control the RF pulses and convert the signals detected by the receiver to a digital format
  • 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?

spinning Hydrogen nucleus
NMR spectrum of the hydrogen nuclei of ethanol
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

Alignment to the magnetic field
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

2. Excitation

After 90-degree pulsing by broad-band RF
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.

3. Measure

Relaxation after Free Induction Decay
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.

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

There are three main things that we can learn from the NMR spectra.
Horizontal axis (chemical shift): Information on the type of functional group and molecule conformation
Integration ratio (signal area ratio): Information on quantities such as composition ratios, mixing ratios, etc.
Splitting pattern (coupling): Information on neighboring atoms
The first is information about the horizontal axis, called the chemical shift. The horizontal axis contains information about the type of functional group and molecule conformation . From the position where the spectrum appears (numerical value on the horizontal axis), it is possible to predict what kind of functional group and molecule conformation are contained in the molecule to be measured.
The second is the integration ratio (signal area ratio). By comparing the integral values of each signal, it is possible to compare the number of functional groups contained in a molecule and to obtain information on the mixing ratio of a mixed sample consisting of multiple molecules.
The third is a signal splitting called coupling. The signal is split due to the influence of another nuclear spin existing near the nuclear spin of interest. Figure 1 shows the 1H NMR spectrum of ethanol. The methyl and methylene group signals show that the signal is not a single signal, but is split into multiple signals. Since the splitting pattern of the signal depends on the number and type of other nuclear spins existing nearby, it is possible to predict the substituents contained in the system from the splitting pattern.
Fig.1 1H NMR spectrum of ethanol (CH3CH2OH)

Fig.1 1H NMR spectrum of ethanol (CH3CH2OH)

Reasons for causing differences in horizontal axis (chemical shift)

The difference of chemical shift is due to the strength of the magnetic field received (felt) by the nuclear spin we are focusing on.
As shown in Fig 2. depending on the height of the electron density existing near the nuclear spin, the strength of shielding of the magnetic field (the strength of the magnetic field that the nuclear spin receives) varies.
The electron density existing near the nuclear spin depends on the magnitude of the electronegativity of the atoms existing near the nuclear spin of interest.
When an O (oxygen) atom with high electronegativity exists nearby, electrons are attracted by the O atom, the electron density near the nuclear spin of interest decreases, and the magnitude of the magnetic field that the nuclear spin receives becomes greater.
As the electron density near the nuclear spin decreases(shielding becomes lower), the corresponding signal shifts to the left.
Fig. 2 Difference of strength of shielding the magnetic field
Fig. 2 Difference of strength of shielding the magnetic field

Example of chemical shift table of 1H

Fig. 3 Correlation diagram of typical functional groups and 1HNMR signal positions

Fig. 3 Correlation diagram of typical functional groups and 1HNMR signal positions

Figure 3 shows a correlation diagram of typical functional groups and 1HNMR signal positions. In the NMR spectra, the right side is generally called as the high-field side and the left side as the low-field side. The signal appearing at 0 ppm is the signal of the reference material TMS (tetramethylsilane). The chemical shift value is a numerical value that represents the shift from other signals) So, it is necessary to calibrate the reference point with a reference material such as TMS etc. 1H signals from alkyl chains, such as methyl, methylene, and methine, often appear near 1 ppm. And as mentioned above, 1H signals near alcohol and ether groups with neighboring oxygen atoms and 1H signals derived from amino groups with neighboring nitrogen atoms are detected near 3ppm to 4ppm.

The signal appearing near 5 ppm is an alkene-derived 1H signal. Furthermore, 1H signals derived from aromatic rings are observed around 7 ppm, and a signal derived from formyl groups such as aldehydes appears around 9 ppm. Signals derived from carboxyl and phenol groups appear around 11 ppm. The position at which the signal appears allows a rough prediction of the type of functional group.

When performing a structural analysis using NMR, please be careful of heavy water exchange in the case where OH or COH groups are included.

In solution NMR, the sample is dissolved in a heavy solvent for measurement. If the solvent to be used is heavy water or heavy methanol, heavy water exchange occurs between the D(2H) in the solvent molecule and the 1H in the OH or COH groups, and the 1H signal from the OH or COH groups may not be observed.

Integration ratio

Fig.4 1HNMR spectrum of benzyl acetate

The following is a brief introduction to the use of integration ratios. Figure 4 shows the structural formula of benzyl acetate and the 1H spectrum. Looking at the molecular structure of benzyl acetate, we can guess that 1H signals would be observed in three areas related with the CH3 group, the CH2 group, and the aromatic group.

Furthermore, a closer look reveals that benzyl acetate has three 1Hs derived from CH3, two 1Hs derived from CH2, and five 1Hs derived from one substituted aromatic CH. The integration ratio of each signal is calculated to be CH3:CH2:CH = 3:2:5, which indicates that the values predicted from the structure and the actual measured values coincide.

It can also be seen that CH3 is shifted to the left from the area where 1H signal derived from CH3 is often observed (around 1 ppm) due to the influence of the neighboring O atoms.

Examples of the use of integration ratios in mixed samples include the following
  • Relative quantitative evaluation by comparison of integration values of each component
  • Absolute quantitative evaluation using a standard substance of known purity (q-NMR)
  • Calculation of reaction rate by comparison of integration values before and after the reaction
In both examples, it is important to find the signal that is specific to each component and that can be integrated correctly (i.e., not overlapping with other signals).

Coupling and Spin-Spin Coupling Constant "J"

Fig.5 1H NMR spectrum of 2,4 dimethyl pyrimidine

Fig.5 1H NMR spectrum of 2,4 dimethyl pyrimidine

Finally, we introduce couplings. Coupling refers to the interaction between the nuclear spin of interest and another neighboring nuclear spin. In 1D measurements of 1H NMR, the interaction, "coupling" occurs when nuclear spins are in proximity to each other and induces the NMR signal splits. The unit of the splitting width of spin coupling is expressed in Hz. This number is called the spin coupling constant or J-coupling constant (j-value).

Formula for calculating spin coupling constant or J-coupling constant (j-value)
It is also known that the splitting widths have the same j-value when coupled to each other. In the compound in Fig. 5, Ha and Hx are coupled, so the splitting widths of both Ha and Hx have the same value, 6hz.Thus, when a split peak is observed, the j-value information can be used to determine which signals are coupled.

Splitting pattern due to coupling

Let us explain a little more about the splitting pattern caused by coupling. An unsplit signal is called a singlet, denoted by the symbol "s"; a two-divided signal is a doublet, denoted by the symbol "d"; a three-divided signal is a triplet, denoted by the symbol "t"; a triplet has a signal strength ratio of 1:2:1, : A signal that splits into four is a quartet, denoted by the symbol "q." The signal strength ratio for a quartet is 1:3:3:1. Signals with five or more segments are multiplets, indicated by the symbol "m".

Using the 1H NMR spectrum of ethanol as an example, we will explain the splitting of the 1H signal. Focusing on the signal derived from the CH3 group around 1ppm, the number of 1H near by CH3 group is 2 (coupled with CH2 group), so it splits into 2+1=3.

Looking at the signal derived from the CH2 group around 3.5 ppm, the number of 1H near by CH2 group is 3 (coupled with CH3 group), which splits into 3+1=4.

Because the OH signal around 5ppm is not coupled to the near by 1H, it does not split and is in the singlet state. Basically, we can see that the signal splits into the "n+1", "n" means the number of nuclei spins positioning around the nuclear spin of interest.

  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.
NMR spectrum of the hydrogen nuclei of ethanol

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

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)

Relaxation Time (molecular mobility, interatomic distance)
Diffusion Coefficient (molecular weight, conformation of polymer)

Diffusion Coefficient (molecular weight, conformation of polymer)

Organic Chemistry, Polymer Chemistry

JEOL NMR Spectrometers

The ECZ Luminous (JNM-ECZL series) is an FT NMR spectrometer equipped with state-of-the-art digital and high frequency technology. The Smart Transceiver System, a high-speed, high-precision digital high-frequency control circuit, enables further miniaturization and high reliability of the spectrometer. It is capable of high-field and solid-state NMR measurements while maintaining the size of a conventional low-field solution NMR system.
With the new Multi Frequency Drive System one can run multi-nuclei pulse trains on a single physical RF channel. This allows to run modern complex NMR experiments on a spectrometer with very simple configuration.
This series is regularly offered for spectrometers of all NMR frequencies that are manufactured by JEOL.
To Learn More please visit our ECZL NMR Product Page.
ECZ Luminous (JNM-ECZL series) FT NMR spectrometer
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