A diradical in organic chemistry is a molecular species with two electrons occupying two degenerate molecular orbitals (MO)^{[1] [2]}. Organic chemistry is a discipline within Chemistry which involves the scientific study of the structure properties composition reactions, and preparation In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J In Chemistry, a molecular orbital (or MO) is a region in which an Electron may be found in a Molecule. They are known by their higher reactivities and shorter lifetimes. In a broader definition diradicals are even-electron molecules that have one bond less than the number permitted by the standard rules of valence ^{[3] [4]}. The electrons can pair up with opposite spin in one MO leaving the other empty. In Quantum mechanics, spin is a fundamental property of atomic nuclei, Hadrons and Elementary particles For particles with non-zero spin This is called a singlet state. In Theoretical physics, a singlet usually refers to a one-dimensional representation (e Alternatively each electron can occupy one MO with spins parallel to each other. This is called a triplet state. In Physics, '''spin''' is the Angular momentum intrinsic to a body as opposed to Orbital angular momentum, which is the motion of its Center of mass The related radical has just one free electron. In Chemistry, radicals (often referred to as free radicals) are atoms molecules or ions with Unpaired electrons on an otherwise Open shell The phrases singlet and triplet are derived from the appearance of diradicals in electron spin resonance: a singlet diradical displays a single peak in its spectrum and a triplet has its peak split into a central peak with two adjacent peaks. Electron paramagnetic resonance (EPR or electron spin resonance (ESR Spectroscopy is a technique for studying Chemical species that have one or more unpaired

The triplet state has total spin quantum number S = 1 and is paramagnetic ^[5]. In Atomic physics, the spin quantum number is a Quantum number that parameterizes the intrinsic Angular momentum (or spin angular momentum or simply Paramagnetism is a form of magnetism which occurs only in the presence of an externally applied magnetic field The singlet state has S = 0 and is diamagnetic. Diamagnetism is the property of an object which causes it to create a magnetic field in opposition of an externally applied Magnetic field, thus causing a repulsive effect The degeneracy of each state can be found with Hund's rule of maximum multiplicity: 2S + 1. Part of the list of Hund's rules, Hund's Rule of Maximum Multiplicity (1925 is an observational rule of atomic chemistry discovered by Friedrich Hund.

In molecules the free electrons can reside on one atom or on different atoms. A molecule can have a singlet state or triplet state with different energy and both states can inter-convert by a process called intersystem crossing. Intersystem crossing is a photophysical process An isoenergetic non-radiative transition between two Electronic states having different multiplicities. Phosphorescence is based on this principle. Phosphorescence is a specific type of Photoluminescence related to fluorescence.

Discrete molecules with a diradical nature are singlet oxygen and triplet oxygen. Singlet oxygen is the common name used for the two Metastable states of molecular Oxygen (O2 with higher energy than the ground state Triplet oxygen Triplet oxygen is the Ground state of the Oxygen molecule The Electron configuration of the molecule has two unpaired electrons occupying two degenerate Other important diradicals are carbenes and nitrenes. In Chemistry, a carbene is a highly reactive Organic molecule containing a Carbon atom with six valence electrons and having the general formula In Chemistry, a nitrene (R-N is the Nitrogen analogue of a Carbene. Lesser known diradicals are nitrenium ions and organic so-called non-Kekulé molecules in which the electrons reside on different carbon atoms. A nitrenium ion (also called aminylium ion) in Organic chemistry is a Reactive intermediate based on Nitrogen with both an electron Lone A non-Kekulé molecule is a conjugated Hydrocarbon that cannot be assigned classical Kekulé structures

External links

• Diradicals @ www. meta-synthesis. com Link

References

Compendium of Chemical Terminology (ISBN 0-86542-684-8 is a book published by IUPAC containing internationally accepted definitions for terms in Chemistry.

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