The Koelsch radical (also known as Koelsch's radical and 1,3-bisdiphenylene-2-phenylallyl or α,γ-bisdiphenylene-β-phenylallyl, abbreviated BDPA) is a

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

that is an unusually stable

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...

-centered

radical Radical may refer to: Politics and ideology Politics *Radical politics, the political intent of fundamental societal change *Radicalism (historical), the Radical Movement that began in late 18th century Britain and spread to continental Europe and ...

, due to its

resonance Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...

structures.

Properties

BDPA is an unusually stable

compound due to the extent to which its

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...

s are delocalized through

structures. The unpaired electron is located predominantly at the 1 and 3 positions.

Steric Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...

effects from the

biphenyl Biphenyl (also known as diphenyl, phenylbenzene, 1,1′-biphenyl, lemonene or BP) is an organic compound that forms colorless crystals. Particularly in older literature, compounds containing the functional group consisting of biphenyl less one ...

substituents also contribute to the compound's stability.

Uses

BDPA and closely related compounds are used as molecular standards in

electron paramagnetic resonance 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 spin ...

(EPR) and

electron nuclear double resonance Electron nuclear double resonance (ENDOR) is a magnetic resonance technique for elucidating the molecular and electronic structure of paramagnetic species.Kevan, L and Kispert, L. D. ''Electron Spin Double Resonance Spectroscopy'' Interscience: New ...

(ENDOR) experiments, and as a polarizing agent in

dynamic nuclear polarization 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 ...

(DNP) nuclear magnetic resonance (NMR) experiments. Because BDPA itself is

hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, th ...

, derivatives have been developed that are more soluble in

aqueous solution An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be re ...

History

The compound was first synthesized by

C. Frederick Koelsch Charles Frederick Koelsch (31 January 1907 - 24 December 1999) was an American organic chemist who spent his faculty career at the University of Minnesota. Education and academic career Koelsch was born in Boise, Idaho in 1907 in a family of G ...

while he was a

postdoctoral fellow A postdoctoral fellow, postdoctoral researcher, or simply postdoc, is a person professionally conducting research after the completion of their doctoral studies (typically a PhD). The ultimate goal of a postdoctoral research position is to p ...

Harvard University Harvard University is a private Ivy League research university in Cambridge, Massachusetts. Founded in 1636 as Harvard College and named for its first benefactor, the Puritan clergyman John Harvard, it is the oldest institution of higher le ...

in the 1930s. He attempted to publish a paper describing the compound, but the paper was rejected on the grounds that the described properties, particularly stability, were unlikely to be those of a radical. Subsequent experimental evidence and

quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...

calculations suggested his interpretation of the original experiment was correct, resulting in the publication of the paper in 1957, nearly 25 years after the original experiments. Although the original report described stability on the order of years, modern experiments suggest that this family of compounds, while unusually stable for radicals, shows measurable degradation in months after preparation.

References

{{reflist, 30em Aromatic compounds Free radicals