Mechanisms of Peroxynitrous Acid and Methyl Peroxynitrite, ROONO (R = H, Me), Rearrangements:  A Conformation-Dependent Homolytic Dissociation

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 (RONO 2 ). A barrier of ca. 18−19 kcal/mol is predicted for RO−ONO dissociation involving a 2 A 1 -like NO 2 fragment in transition states beginning from a cis -OONO conformation. This pathway is significantly favored relative to a 2 B 2 -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 2 A 1 NO 2 is a N-centered radical), but “geometrically incorrect” for subsequent N−O bond formation to yield RONO 2 . The imperfect initial orientation of RO/NO 2 for N−O bond formation rationalizes some escape of free radicals, in competition with low-barrier RO• and NO 2 orientational motions followed by near-barrierless collapse to RONO 2 . For HOONO, the pathway for HONO 2 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 (HNO 3 ). MeOONO has a similar rearrangement mechanism, although the pathways for its rearrangement lack any hydrogen-bonded intermediates.