NationMaster - Encyclopedia: Electron spin resonance

Electron Paramagnetic Resonance (EPR) or Electron Spin Resonance (ESR) is a spectroscopic technique which detects species that have unpaired electrons, generally meaning that the molecule in question is a free radical if it is an organic molecule, or that it has transition metal ions if it is an inorganic complex. Because most stable molecules have a closed-shell configuration without a suitable unpaired spin, the technique is less widely used than nuclear magnetic resonance (NMR). Extremely high resolution spectrum of the Sun showing thousands of elemental absorption lines (fraunhofer lines) Spectroscopy is the study of matter and its properties by investigating light, sound, or particles that are emitted, absorbed or scattered by the matter under investigation. ... The Electron is a fundamental subatomic particle that carries an electric charge. ... In chemistry free radicals are uncharged atomic or molecular species with unpaired electrons or an otherwise open shell configuration. ... Organic chemistry is a specific discipline within the subject of chemistry. ... In chemistry, a molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical bonds [1] [2] [3] [4] [5]. Chemical substances are not infinitely divisible into smaller fractions of the same substance: a molecule is generally considered the smallest particle of a pure... In chemistry, the term transition metal (sometimes also called a transition element) has two possible meanings: It commonly refers to any element in the d-block of the periodic table, including zinc, cadmium and mercury. ... An ion is an atom or group of atoms that normally are electrically neutral and achieve their status as an ion by loss or addition of one or more electrons. ... Inorganic chemistry is the branch of chemistry concerned with the properties and reactions of inorganic compounds. ... Synthesis of copper(II)-tetraphenylporphine, a metal complex, from tetraphenylporphine and copper(II) acetate monohydrate. ... Pacific Northwest National Laboratorys high magnetic field (800 MHz, 18. ...

The basic physical concepts of the technique are analogous to those of NMR, but instead of the spins of the atom's nuclei, electron spins are excited. Because of the difference in mass between nuclei and electrons, weaker magnetic fields and higher frequencies are used, compared to NMR. For electrons in a magnetic field of 0.3 tesla, spin resonance occurs at around 10 GHz. In physics, spin refers to the angular momentum intrinsic to a body, as opposed to orbital angular momentum, which is the motion of its center of mass about an external point. ... Properties In chemistry and physics, an atom (Greek á¼„Ï„Î¿Î¼Î¿Ï‚ or Ã¡tomos meaning indivisible) is the smallest particle of a chemical element that retains its chemical properties. ... A semi-accurate depiction of the helium atom. ... Magnetic lines of force of a bar magnet shown by iron filings on paper In physics, magnetism is one of the phenomena by which materials exert an attractive or repulsive force on other materials. ... The magnitude of an electric field surrounding two equally charged (repelling) particles. ... Sine waves of various frequencies; the bottom waves have higher frequencies than those above. ... The tesla (symbol T) is the SI derived unit of magnetic flux density (or magnetic induction). ...

EPR is used in solid-state physics, for the identification and quantification of radicals (i.e., molecules with unpaired electrons), in chemistry, to identify reaction pathways, as well as in biology and medicine for tagging biological spin probes. Solid-state physics, the largest branch of condensed matter physics, is the study of rigid matter, or solids. ... In chemistry, radicals (often referred to as free radicals) are atomic or molecular species with unpaired electrons on an otherwise open shell configuration. ... Chemistry (from Greek Ï‡Î·Î¼ÎµÎ¯Î± khemeia[1] meaning alchemy) is the science of matter at the atomic to molecular scale, dealing primarily with collections of atoms, such as molecules, crystals, and metals. ... Biology (from Greek Î²Î¯Î¿Ï‚ Î»ÏŒÎ³Î¿Ï‚, see below) is the branch of science dealing with the study of living organisms. ... This article is about the field and science of medical practice and health care. ... A spin probe is a molecule with stable free radical character that carries a functional group. ...

Since radicals are very reactive, they do not normally occur in high concentrations in biological environments. With the help of specially designed nonreactive radical molecules that attach to specific sites in a biological cell, it is possible to obtain information on the environment of these so-called spin-label or spin-probe molecules. Drawing of the structure of cork as it appeared under the microscope to Robert Hook from Micrographia which is the origin of the word cell. Cells in culture, stained for keratin (red) and DNA (green). ...

To detect some subtle details of some systems, high-field-high-frequency electron spin resonance spectroscopy is required. While ESR is affordable for a medium-sized academic laboratory, there are few scientific centers in the world offering high-field-high-frequency electron spin resonance spectroscopy, among them ILL in Grenoble, France and the National High Magnetic Field Laboratory in Tallahassee, USA. Institut Laue-Langevin The Institut Laue-Langevin is an internationally-financed scientific facility, situated in Grenoble, France. ... Grenoble (Arpitan: GrasanÃ²bol) is a city and commune in south-east France, situated at the foot of the Alps, at the confluence of the Drac into the IsÃ¨re River. ... // Overview The National High Magnetic Field Laboratory (NHMFL) develops and operates high magnetic field facilities that scientists use for research in physics, biology, bioengineering, chemistry, geochemistry, biochemistry, materials science, and engineering. ... Tallahassee is the capital of Florida, a state of the United States of America. ...

1 EPR theory
2 EPR spectral parameters
- 2.1 The g factor
- 2.2 Resonance linewidth definition
3 External links
4 See also

EPR theory

EPR spectrometer

EPR spectrometer Faculty of Biotechnology Jagiellonian University File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... EPR spectrometer Faculty of Biotechnology Jagiellonian University File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... // Headline text Bold text:For Acoustic uses in spectrographs of sound waves, see below. ...

Units and constants

A magnetic Field is described by some constants and units:

Magnetic induction $bar{B}$ in teslas (T)
Magnetic flux density $bar{H}$ in amperes per metre (A/m)
$bar{B}$ and $bar{H}$ relationship:

$bar{B}=mu_0bar{H}$ Electromagnetic induction is the production of an electrical potential difference (or voltage) across a conductor situated in a changing magnetic field. ... The tesla (symbol T) is the SI derived unit of magnetic flux density (or magnetic induction). ... Current flowing through a wire produces a magnetic field (B, labeled M here) around the wire. ... A multimeter can be used to measure current The ampere (symbol: A) is the SI base unit of electric current. ... The metre, or meter (U.S.), is a measure of length. ...

The CGS unit for magnetic induction is the gauss (G) which is equivalent to 10^-4 T

Furthermore, in describing EPR, the following constants are very important: This article or section is in need of attention from an expert on the subject. ... Electromagnetic induction is the production of an electrical potential difference (or voltage) across a conductor situated in a changing magnetic field. ... Carl Friedrich Gauss was a German mathematician and physicist. ...

Planck's constant h = 6.63 x 10^-34 J s
Boltzmann constant k = 1.38 x 10^-23 J K^-1
Bohr magneton $μ$ _B = 9.27 x 10^-24 J T^-1

A commemoration plaque for Max Planck on his discovery of Plancks constant, in front of Humboldt University, Berlin. ... Ludwig Boltzmann The Boltzmann constant (k or kB) is the physical constant relating temperature to energy. ... In atomic physics, the Bohr magneton (symbol ) is named after the physicist Niels Bohr. ...

Basics

An electron has a magnetic moment. When placed in an external magnetic field of strength B₀, this magnetic moment can align itself parallel or antiparallel to the external field. The former is a lower energy state than the latter (this is the Zeeman effect), and the energy separation between the two is $Δ E$ = $g e μ B B 0$ , where g_e is the gyromagnetic ratio of the electron (see also the Landé g-factor) the ratio of its magnetic dipole moment to its angular momentum, and $μ$ _B is the Bohr magneton. To move between the two energy levels, the electron can absorb electromagnetic radiation of the correct energy: In physics, the magnetic moment or magnetic dipole moment is a measure of the strength of a magnetic source. ... The Zeeman effect (IPA ) is the splitting of a spectral line into several components in the presence of a magnetic field. ... In physics, the gyromagnetic ratio or LandÃ© g-factor is a dimensionless unit which expresses the ratio of the magnetic dipole moment to the angular momentum of an elementary particle or atomic nucleus. ... In physics, the LandÃ© g-factor, , relates the magnetic dipole moment to the angular momentum of a quantum state. ... In atomic physics, the Bohr magneton (symbol ) is named after the physicist Niels Bohr. ...

$Δ E$ = $h ν$ = g_e $μ$ _BB₀

and this is the fundamental equation of EPR spectroscopy. The paramagnetic centre is placed in a magnetic field and the electron caused to resonate between the two states; the energy absorbed as it does so is monitored, and converted into the EPR spectrum. Paramagnetism is the tendency of the atomic magnetic dipoles, due to quantum-mechanical spin, in a material that is otherwise non-magnetic to align with an external magnetic field. ...

A free electron (on its own) has a g value of 2.002319304386 (which is g_e, the electronic g factor). This means that for radiation at the commonly used frequency of 9.75 GHz (known as X-band microwave radiation, and thus giving rise to X-band spectra), resonance occurs at a magnetic field of about 0.34 tesla (3400 gauss). Download high resolution version (1574x1180, 75 KB)EPR fision, made by tukan File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ...

EPR signals can be generated by resonant energy absorption measurements made at different electromagnetic radiation frequencies $ν$ in a constant external magnetic field (i.e. you scan with a range of different frequency radiation whilst holding the field constant, like in an NMR experiment). Conversely, measurements can be provided by changing the magnetic field B and using a constant frequency radiation; due to technical considerations, this second way is more common. This means that an EPR spectrum is normally plotted with the magnetic field along the x-axis, with peaks at the field that cause resonance (whereas an NMR spectrum has peaks at the frequencies that cause resonance).

In practice a single, isolated, paramagnetic centre never occurs, but only a population of a large number of centres. If this population of centres is in thermodynamic equilibrium, its statistical distribution is described by the Boltzmann distribution. Ludwig Boltzmann Ludwig Boltzmann (February 20, 1844 – September 5, Austrian physicist famous for the invention of statistical mechanics. ...

Boltzmann distribution

$frac{n_{M_J+1}}{n_{M_J}}$ = exp $left ( -frac{E_{M_J+1}-{E_{M_J}}}{kT} right )$ =exp $left (-frac{Delta E}{kT}right )$

where $n_{M_J+1}$ is the number of paramagnetic centres occupying the $M J + 1$ level, k is the Boltzmann constant, and T is the temperature in kelvins. Ludwig Boltzmann The Boltzmann constant (k or kB) is the physical constant relating temperature to energy. ... The Kelvin scale is a thermodynamic (absolute) temperature scale where absolute zeroâ€”the lowest possible temperature where nothing could be colder and no heat energy remains in a substanceâ€”is defined as zero kelvin (0 K). ...

For X-band radiation ( $ν$ = 9.75 GHz) at room temperature, $frac{n_{M_J+1}}{n_{M_J}}$ = 0.998. Because the lower level has more electrons than the higher one, transitions from the lower to the higher level are more probable, which is why there is a net absorption of radiation.

EPR spectral parameters

The g factor

Knowledge of the g factor gives us information about the paramagnetic center's electronic structure. When an unpaired electron is in an atom, it feels not only the external magnetic field B₀ applied by the spectrometer, but also the effects of any local magnetic fields. Therefore, the effective field B_eff felt by the electron is The Landé g-factor, , is a multiplicative term in the lifting of the energy degeneracy in for an atom in a weak uniform external magnetic field. ...

B_eff = B₀(1 - σ)

where σ allows for the effects of the local fields (it can be positive or negative), and therefore the resonance condition is

$Δ E$ = $h ν$ = g_e $μ$ _BB_eff = g_e $μ$ _BB₀(1 - $σ$ )

The quantity g_e(1 - $σ$ ) is called the g factor, given the symbol g, so

$Δ$ E = h $ν$ = g $μ$ _BB₀

Given this last equation, you can measure g from the ESR experiment by measuring the field B₀ and the frequency $ν$ at which resonance occurs. If g differs from g_e (2.0023), this implies that the ratio of the electron's magnetic moment to its angular momentum has changed from the free electron value. Since the electron's magnetic moment is constant (it's the Bohr magneton), then the electron must have gained or lost angular momentum. It does this through spin-orbit coupling, and because the mechanisms of spin-orbit coupling are well understood, the magnitude of the change can be used to give information about the nature of the atomic or molecular orbital containing the electron.

In real life, electrons are normally associated with atoms. There are three important consequences of this. Firstly, the electron may gain or lose angular momentum (through spin-orbit coupling) which will affect the value of the g-factor — this often varies from 2.0023, especially in transition metal compounds. Secondly, this change in angular momentum is not the same for all orientations of the atom or molecule in an external magnetic field. In other words, the g-factor changes according to the orientation of the paramagnetic atom in the magnetic field—it is anisotropic. This anisotropy depends upon the electronic structure of the atom in question, and so can yield information about the atomic (or molecular) orbitals containing the unpaired electron. Thirdly, if the atom(s) which the electron is associated with has (or have) a non-zero nuclear spin, the magnetic field associated with this atom will affect the electron too. This leads to the phenomenon of hyperfine coupling, which is analogous to coupling in NMR in splitting the resonance signal into doublets, triplets and so forth. ...

Resonance linewidth definition

Resonance linewidths are defined in magnetic induction units B and are measured along the x axis, from line center to y value crossing chosen point of energy spectrum. These defined widths are called halfwidths and possess some advantages: for asymmetric lines values of left and right halfwidth can be given. Halfwidth $Δ B h$ is distance measured from center of line to the point in which absorption value has half of maximal absorption value in the center of resonance line. First inclination width $Delta B_{frac{1}{2}}$ is a distance from center of the line to the point of maximal absorption curve inclination. In a practical approach, full definition of linewidth is used. In the case of symmetric lines, halfwidth $Delta B_{frac{1}{2}} = 2Delta B_h$ , and full inclination width $Δ B m a x = 2Δ B 1 s$ Download high resolution version (1849x1412, 78 KB)EPR linewidths, made by tukan File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... This article needs to be cleaned up to conform to a higher standard of quality. ... In physics, absorption is the process by which the energy of a photon is taken up by another entity, for example, by an atom whose valence electrons make a transition between two electronic energy levels. ... // In physics, resonance is the tendency of a system to oscillate with high amplitude when excited by energy at a certain frequency. ...

External links

EPR Laboratory - list of EPR laboratories all over the world class

Contents

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