| Abstract
| - 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 forcleavage 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 fragmentin transition states beginning from a cis-OONO conformation. This pathway is significantly favored relativeto 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 initialorientation of RO/NO2 for N−O bond formation rationalizes some escape of free radicals, in competitionwith low-barrier RO• and NO2 orientational motions followed by near-barrierless collapse to RONO2. ForHOONO, 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-ONOOrearrangement barrier is in accord with the experimental free energy of activation (18 ± 1 kcal/mol) for therearrangement of peroxynitrous acid (HOONO) into nitric acid (HNO3). MeOONO has a similar rearrangementmechanism, although the pathways for its rearrangement lack any hydrogen-bonded intermediates.
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