Properties of Organic Diradicaloid Molecules Predicted by Semiempirical Molecular Orbital Theory. A Practical Procedure

摘要

Semiempirical INDO and MNDO molecular orbital methods yield good results for both geometries and triplet-singlet (T-S) state ordering for a number of diradicals that have recently been examined experimentally. The putative pentamethylenepropane class of diradicals is computed to favor a triplet ground state, even though its members are formally disjoint. Diradicals 7-10 and bicyclic molecules 15-17, formally related to tetramethyleneethane by heteroatom bridging, show a range of electronic nature from strongly diradicaloid for 3,4-dimethylenecyclopentadie ne(7) to closed-shell zwitterionic for bicyclic 17. Phenoxy radicals linked by ethylenic units as diradical models (23-25) for oligomeric superparamagnetic polyradicals are found to interact weakly (small T-S gaps), regardless of connectivity. Keywords: Open-shell molecules; Organic magnetic molecules; Reprints. (aw)