NMR Spectroscopy – A Level Chemistry B OCR Revision

Basics of NMR Spectroscopy

Nuclear Magnetic Resonance (NMR) Spectroscopy is a powerful technique used for determining the structure of organic compounds.
It works by exploiting the magnetic properties of certain nuclei such as proton (1H) and carbon (13C).
When a sample is placed in a magnetic field, these nuclei absorb and re-emit electromagnetic radiation in the radio frequency range.
The frequency at which a particular nucleus re-emits the radiation depends on its chemical environment, enabling identification of the structure.

An NMR spectra is a plot of intensity against chemical shift.
Chemical shift, measured in parts per million (ppm), shows the frequencies at which different nuclei absorbed and re-emitted the radio waves.
Each group of equivalent protons or carbons gives rise to a signal (peak) in the NMR spectrum.
The number of peaks shows the number of different environments of the particular nucleus in a molecule.

Chemical shift depends on the molecular environment around the nucleus.
More shielded nuclei resonate at lower frequencies (upfield, smaller chemical shift values), whereas less shielded nuclei resonate at higher frequencies (downfield, larger chemical shift values).

Spin-spin splitting or coupling occurs when the spin of one nucleus influences others with which it is bonded, leading to splitting of peaks in the spectrum.
The number of sub-peaks or splitting can provide information on the number of adjacent protons or carbons, guided by the N + 1 rule, where N is the number of identical nuclei adjacent to the one responsible for the original peak.

Integration of NMR signals reveals the relative numbers of protons or carbons causing each signal.
The area under each peak is proportional to the number of equivalent protons or carbons.

NMR Spectroscopy is widely used in organic chemistry for structural determination of molecules.
It is also used in medicine for MRI scans, helping to visualise tissues and organs in the body without the need for invasive surgery.

Mastering NMR principles and being able to interpret NMR spectra is crucial for organic chemists and researchers, enabling the 3D structural elucidation of complex organic molecules.