Theoretical Calculations on the Torsion Potential of Peroxynitrite Anion in Aqueous Solution

Abstract

Solvent effects have been calculated on the torsion potential of the peroxynitrite anion (ONOO-) in aqueous solution, using the continuum solvent SCI−PCM method at the B3LYP/6-311+G* level, and by a combined (ab initio or DFT)/Monte Carlo simulation method at the MP2 and B3LYP levels using the 6-311+G* basis set. On the basis of the SCI−PCM calculations, the relative internal energy hardly changes for the cis and trans conformers upon solvation, but the difference in the solute−solvent interaction energy raises the free energy of the trans conformer relative to the cis to 4.3 kcal/mol, as compared to the gas-phase value of 3.1 kcal/mol at T = 298.15 K. Geometry distortion upon solvation is small, and subsequent changes in the relative thermal corrections for the different conformers are also small in general. B3LYP/Monte Carlo calculations at T = 310 K result in a decrease of the relative trans free energy to 1.8−2.5 kcal/mol when changes in the geometry upon solvation were considered, and the atomic charges were fit to the correlated charge distribution. All these results are in basic accord with the interpretation of the experimental in-solution Raman spectrum of ONOO-, in favor of the cis form by Tsai et al. (J. Am. Chem. Soc. 1994, 116, 4115). The calculations, however, call the attention to the different trend of the solvent effect on the cis−trans free energy separation for peroxynitirite anion, when using the SCI−PCM and the Monte Carlo methods, and calculating the internal energy at the B3LYP/6-311+G* level. Aqueous solvation was calculated as decreasing the free energy of the barrier by 1.8 kcal/mol with the continuum dielectric solvent model, whereas the barrier decreased by up to 7 kcal/mol on the basis of B3LYP/MC and MP2/MC calculations. Accordingly, the free energy of the barrier was predicted as 18−24 kcal/mol for peroxynitirite anion in aqueous solution. The present study suggests that on the basis of combined (ab initio or DFT)/Monte Carlo calculations, solute thermodynamics and solution structure can be characterized in accord with available experimental data, if the solute geometry is previously optimized using a continuum solvent method, the atomic charges are fitted to a correlated charge distribution, and thermal corrections are considered even from gas-phase calculations.