Unexpected observation of carbonyl oxide radical cations from peroxy radical cations. A new isomerism of oxygen‐containing radical cations

DFT computational studies with B3LYP/6-31G(d) level theory has been carried out for peroxy radical cations which are formed by the reaction of molecular oxygen with aliphatic and aromatic olefin radical cations. The calculations have led us to find rearrangement reactions of the peroxy radical cations to the isomeric carbonyl oxide radical cations. The results show that the peroxy radical cations in which their positive charges at the olefinic carbons cannot be stabilized due to the structural requirements have a great tendency to undergo the rearrangement. Thus, 2-methylpropene radical cation does not afford a peroxy radical cation formed by the addition of molecular oxygen at 2-position of the olefinic carbon, and DFT calculations to locate the structure were found to give a rearranged ethyl methyl carbonyl oxide radical cation, which is significantly more stable than the peroxy radical cation. It was also found that carbonyl oxide radical cations, depending on the structures, are as stable as, or even more stable than, the isomeric peroxy radical cations which are formed by the addition of molecular oxygen to the aromatic olefin radical cations derived from trans-stilbene and β-methylstyrene. The results also indicate that the C–O bonds are shorter than the O–O bonds in the alkyl carbonyl oxide radical cations examined, which suggests that oxygen lone pair delocalization to the carbonyl carbon play an important role for the determinaton of the structures and their stability. Copyright © 2012 John Wiley & Sons, Ltd.