Chemical Shift in ¹H-NMR Spectroscopy

Abstract

The phenomenon of nuclear magnetic resonance is based on the fact that nuclei of certain elements possess a spin angular momentum and an associated magnetic moment. When such nuclei are placed in a magnetic field, they can adopt one of a number of quantized orientations, each orientation corresponding to a particular energy level. The orientation with the lowest energy is the one in which the nuclear magnetic moment is most closely aligned with the external magnetic field while the orientation with the highest energy is the one in which the nuclear magnetic moment is least closely aligned with the magnetic field. Nuclear magnetic resonance involves transitions between these energy levels (or changes in the orientation of the nuclei) with respect to the external magnetic field. These transitions may be induced by the absorption of radio frequency radiation of the correct frequency, which is measurable on a recorder in the form of an NMR signal of the nucleus. Pauli first postulated such magnetic properties of nuclei in 1924* but it was not until 1946 that the first NMR experiments were independently carried out by two groups, Bloch^† at Stanford University and Pureed^‡ at Harvard University. These experiments have ushered in a new era in which nuclear magnetic resonance spectroscopy has become well established as a powerful tool for structure elucidation, and more recently has also been increasingly employed to obtain sectional pictures of human organs.