Chemical Shift
What is meant by a chemical shift?
|
|
Chemical shift refers to the slight difference in resonance frequencies of two otherwise identical nuclei residing in different molecular environments. This frequency difference arises because the resonance frequency of a particular nucleus is not determined by the strength of the externally applied magnetic field (Bo), but by the local field (Bloc) experienced by the nucleus at the atomic level.
All nuclei therefore do not resonate at precisely the same frequency. The exact frequency of each nucleus depends both on the particular molecule where it resides as well as its specific position on that molecule. For ¹H nuclei chemical shifts are relatively small (less than a few hundred Hz at 1.5T), but are nevertheless detectable. Chemical shifts can provide important information about the molecular composition of the tissue imaged and form the basis of MR Spectroscopy (MRS).
All nuclei therefore do not resonate at precisely the same frequency. The exact frequency of each nucleus depends both on the particular molecule where it resides as well as its specific position on that molecule. For ¹H nuclei chemical shifts are relatively small (less than a few hundred Hz at 1.5T), but are nevertheless detectable. Chemical shifts can provide important information about the molecular composition of the tissue imaged and form the basis of MR Spectroscopy (MRS).
When placed in an external magnetic field (Bo) a reactive response of the electron clouds around a nucleus produces an induced field (Bind) that typically opposes the applied field. This is called "shielding". The resonant frequency of the nucleus depends on Bloc = Bo − Bind.
|
Chemical shifts result from diamagnetic susceptibility effects at the atomic/ molecular level. Susceptibility is a response of matter to an external magnetic field that may arise from several different mechanisms (described in a prior Q&A). The easiest way to think about the phenomenon is to imagine that the electrons circulating around the nucleus produce an "induced" magnetic field (Bind) that opposes Bo. The electron cloud thus acts as a "shield" to protect the nucleus from the full force of Bo. The shielding factor is commonly given the symbol σ, where Bind = σBo. The local effective field experienced by the nucleus is therefore Bloc, where
Bloc = Bo − Bind = Bo (1 − σ)
|
Older literature sometimes also refered to σ as the "chemical shift". This terminology has created some confusion in that Bind and Bloc are not measured directly. In practice, chemists measure differences in NMR frequencies between a sample under investigation and an external reference standard. In 2001 the International Union of Physicists and Chemists (IUPAC) recommended abandoning the use of σ in favor of the δ-scale, with chemical shift (δ) defined as
δ = (fsamp − fref ) / fref
where fsamp is the NMR frequency of the nuclear species under investigation and fref is the NMR frequency of a reference compound.
The frequency difference (fsamp − fref ) is usually on the order of a few hundred to a few thousand Hz, while the reference frequency (fref ) is measured in MHz. The chemical shift (δ) is therefore a small number, expressed in units of parts per million (ppm). The suffix ppm is interchangeable with x10 −6, just as the symbol % is interchangeable with x0.01 or x10−2.
The chemical shift (δ) is a dimensionless constant specific for each type of nucleus (¹H, ³¹P, etc) and chemical configuration (—CH3, —OH, --PO4, etc). Tables of chemical shift values are readily available (now embedded into computer/smart phone applications) allowing chemists rapid identification of detected nuclei in MRS experiments.
The use of ppm allows description of chemical shifts independent of field strength. Consider two nuclei with chemical shifts differing by 1.0 ppm. At 1.5T (where the Larmor reference frequency is about 64 MHz), these two nuclei would therefore differ in absolute frequencies by 64 MHz x 1 ppm = 64 Hz. At 3.0T (where the Larmor frequency is 128 MHz) the two would differ by 128 MHz x 1 ppm = 128 Hz.
References
Harris RK, Becker ED, Cabral de Menezes SM, et al. NMR Nomenclature. Nuclear Spin Properties and Conventions for Chemical Shifts (IUPAC Recommendations 2001). Pure Appl Chem 2001; 73:1795-1818.
Harris RK, Becker ED, Cabral de Menezes SM, et al. Further conventions for NMR shielding and chemical shifts (IUPAC recommendations 2008). Pure Appl Chem 2008; 80:59-84.
Proctor WG, Yu FC. The dependence of a nuclear magnetic resonance frequency upon chemical compound. Phys Rev 1950; 77:717. (This is the famous paper where changes in resonance frequency — later to be known as "chemical shifts" — were first reported among several nitrogen compounds, the basis for NMR spectroscopy).
Harris RK, Becker ED, Cabral de Menezes SM, et al. NMR Nomenclature. Nuclear Spin Properties and Conventions for Chemical Shifts (IUPAC Recommendations 2001). Pure Appl Chem 2001; 73:1795-1818.
Harris RK, Becker ED, Cabral de Menezes SM, et al. Further conventions for NMR shielding and chemical shifts (IUPAC recommendations 2008). Pure Appl Chem 2008; 80:59-84.
Proctor WG, Yu FC. The dependence of a nuclear magnetic resonance frequency upon chemical compound. Phys Rev 1950; 77:717. (This is the famous paper where changes in resonance frequency — later to be known as "chemical shifts" — were first reported among several nitrogen compounds, the basis for NMR spectroscopy).
Related Questions
How are chemical shifts measured?
How are chemical shifts measured?