A comparison of orbital interactions in the additions of phosphonyl and acyl radicals to double bonds

Calculation of the barriers for addition of the H₂P(=O)̇ and HC(=O)̇ radicals to alkenes, at the CCSD(T)/aug-cc-pVDZ//BHandHLYP/6- 311G** level, indicates that both radicals display ambiphilic behaviour. For the HC(=O)̇ radical this behaviour occurs because a secondary orbital interaction of the type π*_COHOMO acts in conjunction with the primary SOMOHOMO interaction to balance the SOMO→LUMO interaction. For the H₂P(O)̇ radical, on the other hand, the much higher-lying LUMO (the σ*_P-O orbital) allows for only minimal secondary interaction, and this radical's ambiphilic behaviour is therefore reflective of a balance between SOMO→LUMO and SOMO → HOMO interactions.