| Abstract
| - The peroxy isomers of nitrate and nitric acid, peroxynitrite andperoxynitrous acid, are studied with ab initioand density functional methods. The results are compared to theobserved Raman and 15N NMR spectra.The harmonic vibrational frequencies, NMR chemical shifts, andenergies clearly favor cis ONOO- asthemost stable and predominant ONOO- isomer.Peroxynitrite has a large rotational barrier of ∼24kcal/molbecause of partial π-bonding in the central bond. This isconfirmed by a bond order of 1.5 for cis andtransONOO- computed by electron density analysis.Electron correlation is critical in accurately predictingtherelative energies for this system, as Hartree−Fock predicts a lowertriplet state. The intense, broad band inthe solution Raman spectrum centered at 642 cm-1 ispredicted to be too low by 100−150 cm-1 if thevibrationis the cis torsion. Resolution of this discrepancy isattempted by estimating the effects of solvent andanharmonicity. The results on ONOO- are compared tothose for ONOOH, which is adequately describedby lower levels of theory. The planarcis−cis isomer of ONOOH is the lowest energystructure by 1−2kcal/mol.
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