Proton NMR, also notated as 1H NMR, is a spectroscopic technique that is commonly used for structure elucidation and purity assessments of organic compounds.

Principle of how proton NMR works.

In proton NMR, which stands for proton Nuclear Magnetic Resonance, a sample is placed in a strong magnetic field, and then hit with radiofrequency radiation. The protons will have aligned or anti-aligned their so-called spin state with the magnetic field, and the difference between these two states have a resonance energy. When hit by the appropriate frequency, the lower energy protons can flip their alignment, and it's this exact energy emitted from the protons that's detected in this technique, as they flip back to the lower energy state.