The O−O bond breaking reactions of peroxynitrous acid and methyl peroxynitrite, ROONO (R = H, Me), were investigated theoretically using the (U)CCSD/6-31+G*, (U)CCSD(T)/6-31+G*//(U)CCSD/6-31+G*, and CBS-QB3 methods. The OONO dihedral angle has a remarkably large influence on the barriers for cleavage of the O−O bonds, which influences the subsequent radical recombination to yield nitrates (RONO2). A barrier of ca. 18−19 kcal/mol is predicted for RO−ONO dissociation involving a 2A1-like NO2 fragment in transition states beginning from a cis-OONO conformation. This pathway is significantly favored relative to a 2B2-like transition state with a trans-ONOO conformation; the latter has a barrier of 33−34 kcal/mol. Notably, the favored cis-OONO pathway is “electronically correct” (because 2A1 NO2 is a N-centered radical), but “geometrically incorrect” for subsequent N−O bond formation to yield RONO2. The imperfect initial orientation of RO/NO2 for N−O bond formation rationalizes some escape of free radicals, in competition with low-barrier RO• and NO2 orientational motions followed by near-barrierless collapse to RONO2. For HOONO, the pathway for HONO2 formation may include a hydrogen-bonded intermediate, •OH···ONO•, earlier proposed as a source of one-electron processes occurring after O−O bond cleavage. The cis-ONOO rearrangement barrier is in accord with the experimental free energy of activation (18 ± 1 kcal/mol) for the rearrangement of peroxynitrous acid (HOONO) into nitric acid (HNO3). MeOONO has a similar rearrangement mechanism, although the pathways for its rearrangement lack any hydrogen-bonded intermediates.

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/6-311G 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 approximately 0 degrees or approximately 180 degrees ) 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 NO(2) fragments with either the ground (2)A(1) state or the excited (2)B(2) state of NO(2), depending on the starting ONOONO conformation. A cis-oriented NO(2) fragment correlates with the (2)A(1) state, whereas a trans-oriented NO(2) fragment correlates with the (2)B(2) state. Each NO(2) fragment that correlates with (2)A(1) lowers the O-O homolysis energy by approximately 15 kcal/mol, similar to the approximately 17-25 kcal/mol (2)A(1) --> (2)B(2) energy difference in NO(2). 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 (2)B(2) 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 O(2)NNO(2), whereas for cis,cis- and cis,trans-ONOONO, the initially formed NO(2) radical pairs can undergo further rearrangement to yield ONONO(2). 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 (18)O(2)) are likely from ONONO(2), not the OONO radical as reported.