A Level Chemistry Revision: Organic Chemistry – Nuclear Magnetic Resonance Spectroscopy

In our latest A level chemistry revision guide, we take a look at Nuclear Magnetic Resonance (NMR) spectroscopy. Read on to find out more about this important analytical technique so you can enter the exam room better prepared.

Introduction to NMR spectroscopy

NMR spectroscopy is an analytical technique used to determine the chemical structure of a compound. It provides both qualitative and quantitative information and enables you to quickly and precisely analyse small amounts of samples.

How does NMR spectroscopy work?

NMR spectroscopy uses magnetic fields to stimulate the signature resonance of nuclei, which can be detected as radio wave signals. The method is only applicable to nuclei with odd numbers of weight units. It can’t be used to identify carbon-12, for example, but it can identify carbon-13. This is because the spin and charge of odd atomic nuclei mean they act like magnets.

Autosampler of NMR spectrometer loaded with samples for analysis
Autosampler of NMR spectrometer loaded with samples for analysis

Nuclei can either be shielded meaning they’re surrounded by electrons in a microenvironment or deshielded (not surrounded by electrons). This is crucial when it comes to absorbing energy and resonating at a particular predicted frequency. The resonating frequency corresponds to the energy state and species of nuclei.

In a graph, the NMR spectrum is represented by signal peaks that denote the known chemical environment and the sample being analysed. These peaks show the energy levels required to make each nucleus resonate. See the example below for the analysis of tetramethylsilane (TMS).

A graph showing analysis of tetramethylsilane (TMS)

The α and β-spin states

When a sample substance is subjected to a very strong magnetic field, its atomic nuclei will either orient with the magnetic field (α-spin) or go against it (β-spin). This behaviour is comparable to swimming in a raging river. In the α-spin state, the nuclei would be swimming with the current, while in the β-spin state they’d be swimming against the current. Unsurprisingly, the former requires less energy than the latter. 

The charge and spin of atomic nuclei mean they act like small magnets. However, not all atoms can be practically used in NMR spectroscopy. Only those with an odd mass nuclei (i.e. an odd number of protons or neutrons) are suitable. This is a necessary condition if you want to obtain a proper reading of the resonant radiofrequency.

The atoms commonly used in NMR spectroscopy include the isotopes 1H, 13C, 19F, and 31P. All of these atoms have a spin (I) = ½ and their nuclei collectively behave like magnets when subjected to a very strong magnetic field.

The spin of the nucleus is responsible for generating a magnetic field. However, the magnetic orientations are random; they only become coherent when subjected to a strong magnetic field. This results in resonance, which can be detected as radio wave frequency.

A graphic showing NMR spectroscopy instrumentation

Shielded vs deshielded nuclei

Ideally, the atomic nuclei of a substance should coherently respond to an external magnetic field by either orienting themselves in the direction of the field or against it.

In reality, however, the nuclei are surrounded by electrons, which can shield them against external magnetic fields. This is known as the diamagnetic shielding effect. When this occurs, the shielded nuclei may only exhibit partial orientation in response to an external magnetic field. 

Some atoms have nuclei that are not shielded by electrons. These deshielded nuclei can easily change their orientation when an external magnetic field is applied, making them ideal atoms for analyses.

Resonance of nuclei

Atomic nuclei can resonate just like an echo or two guitar strings producing the same note. The resonance of a nucleus corresponds with the amount of energy that’s input. Whether shielded or unshielded, nuclei absorb energy when subjected to radiofrequency radiations. When nuclei reach the same energy state, they are in resonance. 

To reach a β-spin state, a deshielded nucleus needs a higher energy input. A shielded nucleus, meanwhile, requires less energy to reach the same state. Nuclei with different electronic environments therefore need different energy intensities to resonate with each other.

The NMR spectrum

Whether an element or a compound, substances have a unique NMR spectrum that you can analyse and compare with known spectrums. This serves as the basis for identifying a particular substance. The spectrums are represented by peaks in graphs produced by the NMR spectroscopy instrument.

Peaks correspond to the resonance energy of the nuclei in a substance and can be compared to a database or other known substances. This allows you to determine the composition (e.g. functional group) and structure of an organic compound.

It’s important to note that no two magnets will have the exact same fields. The resonant radio frequencies will also vary, meaning you can’t assign an exact number to the peaks. This problem is solved by simply including known substances with the test sample. The peaks can then be compared based on the chemical shift, which serves as the calibration. See the table below to compare the chemical shift range of some common examples.

The chemical shift range of protons

Applications of NMR spectroscopy

NMR spectroscopy is mainly used as an analytical tool in organic chemistry and biochemistry. It can determine the composition and structure of any organic molecule that has a spin value. The technique can also help to identify chemical and physical properties such as conformational change, phase changes and solubility.

This makes NMR spectroscopy a powerful tool in environmental studies, where it’s used to identify and quantify environmental pollutants. A graphic showing how NMR identifies environmental pollutants

It also has applications in forensic science, especially in identifying possible poisons or traces of organic materials at a crime scene. In the pharmaceutical industry, NMR spectroscopy is often used to design new drugs.

For more guides to help you prepare for your A level chemistry exam, visit our A level resources hub.

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