Vinyl peroxyl radicals in electron-irradiated acrylates. A molecular orbital study

The semiempirical quantum chemical methods MNDO, AM1 and PM3 were used to i nvestigate the performance of the Single excited Configuration Interaction (SCI) approximation and Multi-electron Configuration Interaction (MECI) tec hniques for calculating low energy excitation energies of vinylperoxyl radi cals. It was found that the low energy absorption line (460-590 MI, dependi ng on method used) can be obtained only from linear combination of singly e xcited microstates which are generated by promoting electrons from occupied pi-orbitals closest to the HOMO into unoccupied pi*-orbital closest to the LUMO. There are three dominant single excited configurations leading to th is absorption line and multi-excited configurations have only small influen ce on its calculated energy. The SCI formalism by AM1 as well as the PM3 me thods can be applied to calculate the low energy excited states of these ty pes of open-shell systems. (C) 1999 Elsevier Science B.V. All rights reserv ed.