A reaction in which the number of electron pairs is conserved, for example:
CH4(1A1) + H(2S)CH3(2A'2) + H2(1Σ+g) Source:
PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1947 (https://doi.org/10.1351/pac199971101919)