1. Nuclei like ¹H and ¹³C have an odd number of protons and/or neutrons, giving them nuclear spin.
2. When exposed to an external magnetic field, the spins of these nuclei align either with or against the field.
3. These nuclei absorb energy and rapidly flip (i.e. ressonate) between two spin states when exposed to radio frequency radiation.
4. The energy required for resonance to occur is measured and recorded as an NMR spectrum, providing information about molecular structure.

Chemical shift refers to the difference in resonant frequency between a nucleus and a reference standard, caused by differences in shielding.

Nuclei in different chemical environments will have slightly different resonant frequencies.

δ

Parts per million (ppm)

The electrons surrounding a nucleus act to shield it from the external magnetic field. Nuclei that are more shielded by surrounding electrons will experience a weaker magnetic field. This results in a lower resonance frequency and a smaller chemical shift value for more shielded nuclei.

For example, electronegative atoms like oxygen draw electron density away from directly bonded carbon atoms, decreasing their shielding and increasing their chemical shift.

TMS gives a single peak with all 12 hydrogen atoms resonating at the same frequency. This peak is defined as 0 ppm, so TMS provides a reference point to measure other chemical shifts.

The x-axis represents the chemical shift (δ), measured in ppm.

The y-axis represents the energy absorbed from the radio signal.

Chemical shift increases in value from right to left. 

This means the TMS peak at 0 ppm is always found at the far right of the chemical shift scale.

The chemical environment of an atom refers to its position within a molecule, encompassing all atoms, functional groups, and bonds directly connected to the nucleus.