Paramagnetic Exchange Spin-Catalysis of theCis−TransIsomerization of Substituted Ethylenes

Abstract

Spin-catalysis of cis−trans isomerization reactions of substituted ethylenes by paramagnetic substances has been studied by ab initio calculations using an appropriate theoretical model: an internal rotation of the HCH group around the CC double bond in the ethylene molecule in the presence of O2 or NO molecules. It is shown that spin-catalysis of this reaction is caused by paramagnetic exchange interactions. An adiabatic singlet−triplet intersystem crossing was traced to a stabilization in the transition of the lowest triplet state at the ethylene region by intermolecular exchange and charge-transfer mixings. The creation of a chemical intermediate was verified for the catalysis by nitric oxide. The NO catalyst, when the N atom forms a chemical bond, leads to a more pronounced lowering of the activation energy than does the O2 catalyst.