Chemical shift

In nuclear magnetic resonance (NMR) spectroscopy, the chemical shift is the resonant frequency of a nucleus relative to a standard in a magnetic field. Often the position and number of chemical shifts are diagnostic of the structure of a molecule. Chemical shifts are also used to describe signals in other forms of spectroscopy such as photoemission spectroscopy. In nuclear magnetic resonance (NMR) spectroscopy, the chemical shift is the resonant frequency of a nucleus relative to a standard in a magnetic field. Often the position and number of chemical shifts are diagnostic of the structure of a molecule. Chemical shifts are also used to describe signals in other forms of spectroscopy such as photoemission spectroscopy. Some atomic nuclei possess a magnetic moment (nuclear spin), which gives rise to different energy levels and resonance frequencies in a magnetic field. The total magnetic field experienced by a nucleus includes local magnetic fields induced by currents of electrons in the molecular orbitals (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 so on), 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 operating (or Larmor) frequency ω0 of a magnet is calculated from the Larmor equation where B0 is the actual strength of the magnet in units like Teslas or Gauss, and γ is the gyromagnetic ratio of the nucleus being tested which is in turn calculated from its magnetic moment μ and spin number I with the nuclear magneton μN and the Planck constant h: Thus for example, the proton operating frequency for a 1 T magnet is calculated as: MRI scanners are often referred to by their field strengths B0 (eg 'a 7T scanner'), whereas NMR spectrometers are commonly referred to by the corresponding proton Larmor frequency (eg 'a 300 MHz spectrometer; which has a 7T B0). While chemical shift is referenced in order that the units are equivalent across different field strengths, the actual frequency separation in Hertz scales with field strength (B0), meaning that larger B0 machines give spectra with peaks that are less likely to be overlapping, a significant advantage for analysis. (Larger field machines are also favoured on account of having intrinsically higher signal arising from the Boltzmann distribution of magnetic spin states.) Chemical shift δ is usually expressed in parts per million (ppm) by frequency, because it is calculated from: where νsample is the absolute resonance frequency of the sample and νref is the absolute resonance frequency of a standard reference compound, measured in the same applied magnetic field B0. Since the numerator is usually expressed in hertz, and the denominator in megahertz, δ is expressed in ppm. The detected frequencies (in Hz) for 1H, 13C, and 29Si nuclei are usually referenced against TMS (tetramethylsilane), TSP (Trimethylsilylpropanoic acid), or DSS, which by the definition above have a chemical shift of zero if chosen as the reference. Other standard materials are used for setting the chemical shift for other nuclei.