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?
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.
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.
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.
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.
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.
What we can learn from NMR spectra
- Chemical shift: Information about the local chemical environment of atoms within the molecule.
- Spin-Spin coupling constant: Information about adjacent atoms by looking at the "splitting" of the peaks.
- Relaxation time: Information on molecular dynamics.
- Area under the peak: Using integration you can determine the number of atoms for a given peak.
- 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.
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