MO‐LCAO‐Calculations on Polymethines. XXI. Electrono‐dative, electrono‐captive, and electrono‐ambident polymethine radicals

Abstract[N + 1]π‐polymethine radicals with nitrogen containing terminal groups (violenes) display electron transfer reactions. Modification of the simple chain structures results in distinct changes of the molecular electronegativity and, consequently, of the redox properties. Based on the formerly defined non‐bonding singly occupied MO (SOMO) of ideal polymethines, the various polymethine derivatives are classed into electrono‐dative and electrono‐captive polymethine radicals. Electrono‐dative radicals are prone to release electrons, whereas electrono‐captive radicals are prone to accept electrons. This classification reflects experimental results presented by HÜNIGand coworkers, which have been formerly discussed in terms of aromaticity rather than of MO theory. A third class of radicals embraces compounds in which the redox properties of the simple chain polymethine radicals are less changed upon structural modification. These radicals have been termed as electrono‐ambident radicals because they are both inclined to release and to accept an electron. HÜNIGand coworkers obviously found the first representatives of this series of radicals. Electronoambident radicals also encounter in VIEHE‐type radicals suggesting a more general definition of the latter ones.