The total magnetic field experienced by a nucleus includes local magnetic fields induced by currents of electrons in the molecular orbital’s (note that electrons have a magnetic moment themselves). The electron distribution of the same type of nucleus (e.g. 1H, 13C, 15N) usually varies according to the local geometry (binding partners, bond lengths, angles between bonds, and with it the local magnetic field at each nucleus. This is reflected in the spin energy levels (and resonance frequencies).
The variations of nuclear
Magnetic resonance frequencies of the same kind of nucleus, due to variations in the electron distribution, is called the chemical shift. The size of the chemical shift is given with respect to a reference frequency or reference sample (see also chemical shift referencing), usually a molecule with a barely distorted electron distribution.The chemical shift is of great importance for NMR spectroscopy, a technique to explore molecular properties by looking at nuclear magnetic resonance phenomena.
Chemical shift δ is usually expressed in parts per million (ppm) by frequency, because it is calculated from:Since the numerator is usually in hertz, and the denominator in megahertz, delta is expressed in ppm.The detected frequencies (in Hz) for 1H, 13C, and 29Si nuclei are usually referenced against TMS (tetramethylsilane) or DSS, which is assigned the chemical shift of zero. Other standard materials are used for setting the chemical shift for other nuclei.
Thus, an NMR signal at 300 Hz from TMS at an applied frequency of 300MHz has a chemical shift of: Although the frequency depends on the applied field the chemical shift is independent of it. On the other hand the resolution of NMR will increase with applied magnetic field resulting in ever increasing chemical shift changes.