AB-INITIO AND NMR-STUDY OF PEROXYNITRITE AND PEROXYNITROUS ACID - IMPORTANT BIOLOGICAL OXIDANTS

The peroxy isomers of nitrate and nitric acid, peroxynitrite and perox ynitrous acid, are studied with ab initio and density functional metho ds. The results are compared to the observed Raman and N-15 NMR spectr a. The harmonic vibrational frequencies, NMR chemical shifts, and ener gies clearly favor cis ONOO- as the most stable and predominant ONOO- isomer. Peroxynitrite has a large rotational barrier of similar to 24 kcal/mol because of partial pi-bonding in the central bond. This is co nfirmed by a bond order of 1.5 for cis and trans ONOO- computed by ele ctron density analysis, Electron correlation is critical in accurately predicting the relative energies for this system, as Hartree-Fock pre dicts a lower tripler state. The intense, broad band in the solution R aman spectrum centered at 642 cm(-1) is predicted to be too low by 100 -150 cm(-1) if the vibration is the cis torsion. Resolution of this di screpancy is attempted by estimating the effects of solvent and anharm onicity, The results on ONOO- are compared to those for ONOOH, which i s adequately described by lower levels of theory. The planar cis-cis i somer of ONOOH is the lowest energy structure by 1-2 kcal/mol.