Spin chemistry

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Spin chemistry is a sub-field of chemistry and physics, positioned at the intersection of chemical kinetics, photochemistry, magnetic resonance and free radical chemistry, that deals with magnetic and spin effects in chemical reactions. Spin chemistry concerns phenomena such as chemically induced dynamic nuclear polarization (CIDNP), chemically induced electron polarization (CIDEP), magnetic isotope effects in chemical reactions, and it is hypothesized to be key in the underlying mechanism for avian magnetoreception [1] and consciousness. [2]


Radical-pair mechanism

The radical-pair mechanism explains how a magnetic field can affect reaction kinetics by affecting electron spin dynamics. Most commonly demonstrated in reactions of organic compounds involving radical intermediates, a magnetic field can speed up a reaction by decreasing the frequency of reverse reactions.


The radical-pair mechanism emerged as an explanation to CIDNP and CIDEP and was proposed in 1969 by Closs; Kaptein and Oosterhoff. [3]

Radicals and radical-pairs

Example radical: Structure of Hydrocarboxyl radical, lone electron indicated as single black dot Hydrocarboxyl radical.svg
Example radical: Structure of Hydrocarboxyl radical, lone electron indicated as single black dot

A radical is a molecule with an odd number of electrons, and is induced in a variety of ways, including ultra-violet radiation. A sun burn is largely due to radical formation from this radiation. The radical-pair, however, is not simply two radicals. This is because radical-pairs (specifically singlets) are quantum entangled, even as separate molecules. [1] More fundamental to the radical-pair mechanism, however, is the fact that radical-pair electrons both have spin, short for spin angular momentum, which gives each separate radical a magnetic moment. Therefore, spin states can be altered by magnetic fields.

Singlet and triplet spin states

The radical-pair is characterized as triplet or singlet by the spin state of the two lone electrons, paired together. The spin relationship is such that the two unpaired electrons, one in each radical molecule, may have opposite spin (singlet; anticorrelated), or the same spin (triplet; correlated). The singlet state is called such because there is only one way for the electrons’ spins to anticorrelate (S), whereas the triplet state is called such because the electron's spin may be correlated in three different fashions, denoted T+1, T0, and T−1.

Simple diagram of bonding between electrons with opposite spin Spin vzaimodeistvie.jpg
Simple diagram of bonding between electrons with opposite spin

Reaction kinetics and the Zeeman interaction

Spin states relate to chemical and biochemical reaction mechanisms because bonds can be formed only between two electrons of opposite spin (Hund's rules). Sometimes when a bond is broken in a particular manner, for example, when struck by photons, each electron in the bond relocates to each respective molecule, and a radical-pair is formed. Furthermore, the spin of each electron previously involved in the bond is conserved, [1] [3] which means that the radical-pair now formed is a singlet (each electron has opposite spin, as in the origin bond). As such, the reverse reaction, i.e. the reforming of a bond, called recombination, readily occurs. The radical-pair mechanism explains how external magnetic fields can prevent radical-pair recombination with Zeeman interactions, the interaction between spin and an external magnetic field, and shows how a higher occurrence of the triplet state accelerates radical reactions because triplets can proceed only to products, and singlets are in equilibrium with the reactants as well as with the products. [1] [3] [4]

Zeeman interactions can “flip” only one of the radical's electron's spin if the radical-pair is anisotropic, thereby converting singlet radical-pairs to triplets. [1]

Typical Reaction Scheme of the Radical-pair Mechanism, which shows the effect of alternative product formation from singlet versus triplet radical-pairs. The Zeeman and Hyperfine Interactions take effect in the yellow box, denoted as step 4 in the process Spin Chemistry RPM.png
Typical Reaction Scheme of the Radical-pair Mechanism, which shows the effect of alternative product formation from singlet versus triplet radical-pairs. The Zeeman and Hyperfine Interactions take effect in the yellow box, denoted as step 4 in the process

The Zeeman interaction is an interaction between spin and external magnetic field, and is given by the equation

where is the energy of the Zeeman interaction, is the Larmor frequency, is the external magnetic field, is the Bohr magneton, is Planck's constant, and is the g-factor of a free electron, 2.002319, which is slightly different in different radicals. [1]

It is common to see the Zeeman interaction formulated in other ways. [4]

Hyperfine interactions

Hyperfine interactions, the internal magnetic fields of local magnetic isotopes, play a significant role as well in the spin dynamics of radical-pairs. [1] [3] [4]

Zeeman interactions and magnetoreception

Because the Zeeman interaction is a function of magnetic field and Larmor frequency, it can be obstructed or amplified by altering the external magnetic or the Larmor frequency with experimental instruments that generate oscillating fields. It has been observed that migratory birds lose their navigational abilities in such conditions where the Zeeman interaction is obstructed in radical-pairs. [1]

Related Research Articles

Intersystem crossing

Intersystem crossing (ISC) is an isoenergetic radiationless process involving a transition between the two electronic states with different states spin multiplicity.

Dynamic nuclear polarization (DNP) results from transferring spin polarization from electrons to nuclei, thereby aligning the nuclear spins to the extent that electron spins are aligned. Note that the alignment of electron spins at a given magnetic field and temperature is described by the Boltzmann distribution under the thermal equilibrium. It is also possible that those electrons are aligned to a higher degree of order by other preparations of electron spin order such as: chemical reactions, optical pumping and spin injection. DNP is considered one of several techniques for hyperpolarization. DNP can also be induced using unpaired electrons produced by radiation damage in solids.

In nuclear magnetic resonance (NMR) spectroscopy, the chemical shift is the resonant frequency of an atomic 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.

Singlet oxygen Oxygen with all of its electrons spin paired

Singlet oxygen, systematically named dioxygen(singlet) and dioxidene, is a gaseous inorganic chemical with the formula O=O, which is in a quantum state where all electrons are spin paired. It is kinetically unstable at ambient temperature, but the rate of decay is slow.

CIDNP, often pronounced like "kidnip", is a nuclear magnetic resonance (NMR) technique that is used to study chemical reactions that involve radicals. It detects the non-Boltzmann (non-thermal) nuclear spin state distribution produced in these reactions as enhanced absorption or emission signals.

Electron paramagnetic resonance Technique to study materials that have unpaired electrons

Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy is a method for studying materials that have unpaired electrons. The basic concepts of EPR are analogous to those of nuclear magnetic resonance (NMR), but the spins excited are those of the electrons instead of the atomic nuclei. EPR spectroscopy is particularly useful for studying metal complexes and organic radicals. EPR was first observed in Kazan State University by Soviet physicist Yevgeny Zavoisky in 1944, and was developed independently at the same time by Brebis Bleaney at the University of Oxford.

Triplet oxygen Triplet state of the dioxygen molecule

Triplet oxygen, 3O2, refers to the S = 1 electronic ground state of molecular oxygen (dioxygen). It is the most stable and common allotrope of oxygen. Molecules of triplet oxygen contain two unpaired electrons, making triplet oxygen an unusual example of a stable and commonly encountered diradical: it is more stable as a triplet than a singlet. According to molecular orbital theory, the electron configuration of triplet oxygen has two electrons occupying two π molecular orbitals (MOs) of equal energy (that is, degenerate MOs). In accordance with Hund's rules, they remain unpaired and spin-parallel and account for the paramagnetism of molecular oxygen. These half-filled orbitals are antibonding in character, reducing the overall bond order of the molecule to 2 from a maximum value of 3 (e.g., dinitrogen), which occurs when these antibonding orbitals remain fully unoccupied. The molecular term symbol for triplet oxygen is 3Σ

In chemistry and physics, the exchange interaction is a quantum mechanical effect that only occurs between identical particles. Despite sometimes being called an exchange force in an analogy to classical force, it is not a true force as it lacks a force carrier.

In nuclear chemistry and nuclear physics, J-couplings are mediated through chemical bonds connecting two spins. It is an indirect interaction between two nuclear spins that arises from hyperfine interactions between the nuclei and local electrons. In NMR spectroscopy, J-coupling contains information about relative bond distances and angles. Most importantly, J-coupling provides information on the connectivity of chemical bonds. It is responsible for the often complex splitting of resonance lines in the NMR spectra of fairly simple molecules.

Zero field NMR

Zero- to ultralow-field (ZULF) NMR is the acquisition of nuclear magnetic resonance (NMR) spectra of chemicals with magnetically active nuclei in an environment carefully screened from magnetic fields. ZULF NMR experiments typically involve the use of passive or active shielding to attenuate Earth’s magnetic field. This is in contrast to the majority of NMR experiments which are performed in high magnetic fields provided by superconducting magnets. In ZULF experiments the dominant interactions are nuclear spin-spin couplings, and the coupling between spins and the external magnetic field is a perturbation to this. There are a number of advantages to operating in this regime: magnetic-susceptibility-induced line broadening is attenuated which reduces inhomogeneous broadening of the spectral lines for samples in heterogeneous environments. Another advantage is that the low frequency signals readily pass through conductive materials such as metals due to the increased skin depth; this is not the case for high-field NMR for which the sample containers are usually made of glass, quartz or ceramic.

Magnetobiology is the study of biological effects of mainly weak static and low-frequency magnetic fields, which do not cause heating of tissues. Magnetobiological effects have unique features that obviously distinguish them from thermal effects; often they are observed for alternating magnetic fields just in separate frequency and amplitude intervals. Also, they are dependent of simultaneously present static magnetic or electric fields and their polarization.

Quantum biology is the study of applications of quantum mechanics and theoretical chemistry to biological objects and problems. Many biological processes involve the conversion of energy into forms that are usable for chemical transformations, and are quantum mechanical in nature. Such processes involve chemical reactions, light absorption, formation of excited electronic states, transfer of excitation energy, and the transfer of electrons and protons in chemical processes, such as photosynthesis, olfaction and cellular respiration.

Bond cleavage, or bond fission, is the splitting of chemical bonds. This can be generally referred to as Dissociation (chemistry) when a molecule is cleaved into two or more fragments.

An electronic effect influences the structure, reactivity, or properties of molecule but is neither a traditional bond nor a steric effect. In organic chemistry, the term stereoelectronic effect is also used to emphasize the relation between the electronic structure and the geometry (stereochemistry) of a molecule.

Radical (chemistry) Atom, molecule, or ion that has an unpaired valence electron; typically highly reactive

In chemistry, a free radical is an atom, molecule, or ion that has at least one unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spontaneously dimerize. Most organic radicals have short lifetimes.

Nuclear magnetic resonance Spectroscopic technique based on change of nuclear spin state

Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field and respond by producing an electromagnetic signal with a frequency characteristic of the magnetic field at the nucleus. This process occurs near resonance, when the oscillation frequency matches the intrinsic frequency of the nuclei, which depends on the strength of the static magnetic field, the chemical environment, and the magnetic properties of the isotope involved; in practical applications with static magnetic fields up to ca. 20 tesla, the frequency is similar to VHF and UHF television broadcasts (60–1000 MHz). NMR results from specific magnetic properties of certain atomic nuclei. Nuclear magnetic resonance spectroscopy is widely used to determine the structure of organic molecules in solution and study molecular physics and crystals as well as non-crystalline materials. NMR is also routinely used in advanced medical imaging techniques, such as in magnetic resonance imaging (MRI).

Electron nuclear double resonance (ENDOR) is a magnetic resonance technique for elucidating the molecular and electronic structure of paramagnetic species. The technique was first introduced to resolve interactions in electron paramagnetic resonance (EPR) spectra. It is currently practiced in a variety of modalities, mainly in the areas of biophysics and heterogeneous catalysis.

Magnetochemistry is concerned with the magnetic properties of chemical compounds. Magnetic properties arise from the spin and orbital angular momentum of the electrons contained in a compound. Compounds are diamagnetic when they contain no unpaired electrons. Molecular compounds that contain one or more unpaired electrons are paramagnetic. The magnitude of the paramagnetism is expressed as an effective magnetic moment, μeff. For first-row transition metals the magnitude of μeff is, to a first approximation, a simple function of the number of unpaired electrons, the spin-only formula. In general, spin-orbit coupling causes μeff to deviate from the spin-only formula. For the heavier transition metals, lanthanides and actinides, spin-orbit coupling cannot be ignored. Exchange interaction can occur in clusters and infinite lattices, resulting in ferromagnetism, antiferromagnetism or ferrimagnetism depending on the relative orientations of the individual spins.

Fluorenylidene Chemical compound

9-Fluorenylidene is an aryl carbene derived from the bridging methylene group of fluorene. Fluorenylidene has the unusual property that the triplet ground state is only 1.1 kcal/mol lower in energy than the singlet state. For this reason, fluorenylidene has been studied extensively in organic chemistry.

Christiane Renate Timmel is a German chemist who is Director of the Centre for Advanced Electron Spin Resonance at the University of Oxford. Her group make use of electron-spin resonance to understand long-range structures in chemical and biological systems. Timmel was awarded the Tilden Prize on 2020 by the Royal Society of Chemistry for her contributions to electron-spin resonance.


  1. 1 2 3 4 5 6 7 8 Hore, P. J.; Mouritsen, Henrik (2016-01-01). "The Radical-Pair Mechanism of Magnetoreception". Annual Review of Biophysics. 45 (1): 299–344. doi:10.1146/annurev-biophys-032116-094545. PMID   27216936.
  2. Smith, J.; Zadeh Haghighi, H.; Salahub, D.; Simon, C. (2021). "Radical pairs may play a role in xenon-induced general anesthesia". Sci. Rep. 11 (1): 6287. doi: 10.1038/s41598-021-85673-w . PMC   7973516 . PMID   33737599.
  3. 1 2 3 4 Vyushkova, Maria (April 2011). "Basic Principles and Applications of Spin Chemistry" (PDF). www.nd.edu. Notre Dame University. Retrieved 5 December 2016.
  4. 1 2 3 "HP of Hisaharu Hayashi:Introduction to Dynamic Spin Chemistry". www015.upp.so-net.ne.jp. Retrieved 2016-12-05.