Conformation-dependent state selectivity in O-O cleavage of ONOONO: An inorganic cope rearrangement helps explain the observed negative activation energy in the oxidation of nitric oxide by dioxygen

ONOONO has been proposed as an intermediate in the oxidation of nitric oxide by dioxygen to yield nitrogen dioxide. The O-O bond breaking reactions of this unusual peroxide, and subsequent rearrangements, were evaluated using CBS-QB3 and B3LYP/6-311 G* hybrid density functional theory. The three stable conformers (cis,cis-, cis,trans-, and trans, trans-ONOONO, based on the O-N-O-O dihedral angles of either -0degrees or similar to180degrees) are predicted to have very different O-O cleavage barriers: 2.4, 13.0, and 29.8 kcal/mol, respectively. These large differences arise because bond breaking leads to correlation of the nascent NO2 fragments with either the ground (2)A(1) state or the excited B-2(2) state of NO2, depending on the starting ONOONO conformation. A cis-oriented NO2 fragment correlates with the (2)A(1) state, whereas a trans-oriented NO2 fragment correlates with the B-2(2) state. Each NO2 fragment that correlates with (2)A(1) lowers the O-O homolysis energy by similar to15 kcal/mol, similar to the similar to17-25 kcal/mol (2)A(1) --> B-2(2) energy difference in NO2. Hence, this provides an unusual example of conformation-dependent electronic state selectivity. The O-O bond homolysis of cis,cis-ONOONO is particularly interesting because it has a very low barrier and arises from the most stable ONOONO conformer, and also due to obvious similarities to the well-known {3,3}-sigmatropic shift of 1,5-hexadiene, i.e., the Cope rearrangement. As an additional proof of our state selectivity postulate, a comparison is also made to breakage of the O-O bond of cis,cis-formyl peroxide, where no significant stabilization of the transition state is available because the (2)A(1) and 2132 states of formyloxy radical are near-degenerate in energy. In the case of trans, trans-ONOONO, the O-O bond breaking transition state is a concerted rearrangement yielding O2NNO2, whereas for cis,cis- and cis,trans-ONOONO, the initially formed NO2 radical pairs can undergo further rearrangement to yield ONONO2. It is proposed that previous spectroscopic observations of certain N=O stretching frequencies in argon-matrix-isolated products from the reaction of NO with O-2 (or O-18(2)) are likely from ONONO2, not the OONO radical as reported.