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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2004-07, Vol.108 (27), p.5864-5871
• Main Authors: Zhao, Yilei, Houk, K. N, Olson, Leif P
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp048661w

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.