Spin forbidden reactions

In chemistry, the selection rule (also known as the transition rule) formally restrict certain reactions, known as spin-forbidden reactions, from occurring due to a required change between two differing quantum states. When a reactant exists in one spin state and the product exists in a different spin state, the corresponding reaction will have an increased activation energy when compared to a similar reaction in which the spin states of the reactant and product are isomorphic. As a result of this increased activation energy, a decreased rate of reaction is observed.

In quantum mechanics and particle physics, spin is an intrinsic form of angular momentum carried by elementary particles, composite particles (hadrons), and atomic nuclei.

A singlet state is a molecular electronic state such that all electron spins are paired. In a triplet state the electron spins are no longer paired; that is, they are parallel (same spin). In organic molecules, such as with singlet and triplet oxygen interconversion, intersystem crossing can be used to understand the movement between the singlet ground state, singlet excited state, and the triplet excited state.

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