Nuclear Magnetic Resonance (NMR) Spectroscopy

Introduction

Nuclei with an odd number of protons, neutrons, or both, will have an instrinsic nuclear spin.

Spin quantum number for various nuclei

Number of protons	Number of Neutrons	Spin Quantum Number	Examples
Even	Even	0	12C, 16O, 32S
Odd	Even	1/2	1H, 19F, 31P
"	"	3/2	11B,35Cl, 79Br
Even	Odd	1/2	13C
"	"	3/2	127I
"	"	5/2	17O
Odd	Odd	1	2H, 14N

When a nucleus with a non-zero spin is placed in a magnetic field, the nuclear spin can align in either the same direction or in the opposite direction as the field. These two nuclear spin alignments have different energies and application of a magnetic field lifts the degeneracy of the nuclear spins. A nucleus that has its spin aligned with the field will have a lower energy than when it has its spin aligned in the opposite direction to the field.

Nuclear magnetic resonance (NMR) spectroscopy is the absorption of radiofrequency radiation by a nucleus in a strong magnetic field. Absorption of the radiation causes the nuclear spin to realign or flip in the higher-energy direction. After absorbing energy the nuclei will reemit RF radiation and return to the lower-energy state.

The energy of a NMR transition depends on the magnetic-field strength and a proportionality factor for each nucleus called the magnetogyric ratio. The local environment around a given nucleus in a molecule will slightly perturb the local magnetic field exerted on that nucleus and affect its exact transition energy. This dependence of the transition energy on the position of a particular atom in a molecule makes NMR spectroscopy extremely useful for determining the structure of molecules.

Instrumentation

There are two NMR spectrometer designs, continuous-wave (cw), and pulsed or Fourier-transform (FT-NMR). CW-NMR spectrometers have largely been replaced with pulsed FT-NMR instruments. However due to the lower maintenance and operating cost of cw instruments, they are still commonly used for routine ¹H NMR spectroscopy at 60 MHz. (Low-resolution cw instruments require only water-cooled electromagnets instead of the liquid-He-cooled superconducting magnets found in higher-field FT-NMR spectrometers.) These two spectrometer designs are described in separate CW-NMR and FT-NMR documents.