In chemistry, reactions that involve a change in spin state are known as spin-forbidden reactions Such reactions show 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. A famous example of spin-forbidden reaction is the very slow reaction of with hydrocarbons.
The dissociation of nitrous oxide is a well-studied process:[1]
O atoms have a triplet ground state.
Methoxy cation has a triplet ground state. In a mass spectrometer, it dissociates into singlet products (formyl cation and H2):
Numerous spin-forbidden reactions are encountered in transition metal chemistry since many metal ions can adopt multiple spin states. For example, ferrous porphyrin complexes containing one axial donor are high spin ferrous. These complexes, which are represented by myoglobin and hemoglobin, bind CO to give singlet products:
Cobalt(I) dicarbonyl complexes of a trispyrazolylborate are diamagnetic. The corresponding monocarbonyls have triplet ground states.
The addition of CO to Fe(CO)4 is an example showing the slowing effect of spin-forbidden reaction takes place when Fe(CO)x is placed under CO pressure.[2]
When a reaction converts a metal from a singlet to triplet state (or vice versa):
Strong spin-orbital coupling can satisfy the 2nd condition. Parameter 1, however, can lead to very slow reactions due to large disparities between the metal complex's potential energy surfaces, which only cross at high energy leading to a substantial activation barrier.[3]
Spin-forbidden reactions formally fall into the category of electronically non-adiabatic reactions.[4] In general, potential energy surfaces fall into either the adiabatic and diabatic classification. Potential Energy Surfaces that are adiabatic rely on the use of the full electronic Hamiltonian, which includes the spin-orbit term. Those that are diabatic are likewise derived by solving the eigenvalues of the Schrödinger equation, but in this case one or more terms are omitted.[1]