Theoretical study of the potential stability of the peroxo nitrate radical

Low-Temperature Trapping of Intermediates in the Reaction of NO• with O2

The autoxidation of NO^• was studied in glass-like matrices of 2-methylbutane at 110 K and in a 8:3 v/v mixture of 2,2-dimethylbutane and n-pentane (rigisolve) at 80-90 K, by letting gaseous NO^• diffuse into these solvents that were saturated with O₂. In 2-methyllbutane, we observed a red compound. However, in rigisolve at 85-90 K, a bright yellow color appears that turns red when the sample is warmed by 10-20 K. The new yellow compound is a precursor of the red one and also diamagnetic. The UV-vis spectrum of the yellow compound contains a band which resembles that present in ONOO^-. Because the red and yellow intermediates are not paramagnetic, we postulate that O═N-O-O^• is in close contact with NO^•, or with another O═N-O-O^•. Diffusion of gaseous O₂ into rigisolve saturated with NO^• does not produce a color; however, a weak EPR signal (g = 2.010) is observed. This signal most likely indicates the presence of ONOO^•. These findings complement our earlier observation of a red color at low temperatures and the presence of ONOO^• in the gas phase (Galliker, B.; Kissner, R.; Nauser, T.; Koppenol, W. H. Chem. Eur. J. 2009, 15, 6161-6168), and they indicate that the termolecular autoxidation of nitrogen monoxide proceeds via the intermediate ONOO^• and not via N₂O₂.

Structure, stability, and properties of the trans peroxo nitrate radical: the importance of nondynamic correlation

We report a comparative single-reference and multireference coupled-cluster investigation on the structure, potential energy surface, and IR spectroscopic properties of the trans peroxo nitrate radical, one of the key intermediates in stratospheric NOX chemistry. The previous single-reference ab initio studies predicted an unbound structure for the trans peroxo nitrate radical. However, our Fock space multireference coupled-cluster calculation confirms a bound structure for the trans peroxo nitrate radical, in accordance with the experimental results reported earlier. Further, the analysis of the potential energy surface in FSMRCC method indicates a well-behaved minima, contrary to the shallow minima predicted by the single-reference coupled-cluster method. The harmonic force field analysis, of various possible isomers of peroxo nitrate also reveals that only the trans structure leads to the experimentally observed IR peak at 1840 cm(-1). The present study highlights the critical importance of nondynamic correlation in predicting the structure and properties of high-energy stratospheric NOx radicals.

NOx Catalyzed Pathway of Stratospheric Ozone Depletion: A Coupled Cluster Investigation

We report a theoretical investigation on the NOx catalyzed pathways of stratospheric ozone depletion using highly accurate coupled cluster methods. These catalytic reactions represent a great challenge to state-of-the-art ab initio methods, while their mechanisms remain unclear to both experimentalists and theoreticians. In this work, we have used the so-called "gold standard of quantum chemistry," the CCSD(T) method, to identify the saddle points on NOx-based reaction pathways of ozone hole formation. Energies of the saddle points are calculated using the multireference variants of coupled cluster methods. The calculated activation energies and rate constants show good agreement with available experimental results. Tropospheric precursors to stratospheric NOx radicals have been identified, and their potential importance in stratospheric chemistry has been discussed. Our calculations resolve previous conflicts between ab initio and experimental results for a trans nitro peroxide intermediate, in the NOx catalyzed pathway of ozone depletion.

Intermediates in the autoxidation of nitrogen monoxide

We have identified two intermediates in the autoxidation of NO*: ONOO*, which was detected by EPR spectroscopy at 295 K and atmospheric pressure in the gas phase, and ONOONO, a red substance produced at 113 K in 2-methylbutane. The red compound is diamagnetic and absorbs maximally at 500 nm. The ONOONO intermediate is unstable above the melting point of 2-methylbutane and rapidly converts to O2NNO2. From the semiquantitative determination of mole fractions present in the gas phase by EPR spectroscopy, we estimated the rate constants for the steps that lead to ONOO* and ONOONO, from the known overall rate constant of the autoxidation reaction, by assuming that a quasi-stationary mechanism applies. The rate constant for the rate-determining formation of ONOO* is about 3.1 x 10(-18) cm3 molecule(-1) s(-1) (or 80 s(-1) in mole fractions), the dissociation rate constant of ONOO* is about 6.5 x 10(3) s(-1), and ONOONO is formed with a rate constant of k=7.7 x 10(-14) cm3 molecule(-1) s(-1) (1.9 x 10(6) s(-1) in mole fractions). From these constants, we estimate that the equilibrium constant for the formation of ONOO* from NO* and O2 (K(ONOO*)) is 4.8 x 10(-22) cm3 molecule(-1) (1.2 x 10(-2)), and, therefore, DeltaG=+11.0 kJ mol(-1). In water, the Gibbs energy change is close to zero. The presence of ONOO* at steady-state concentrations under dioxygen excess may be important not only for reactions in the atmosphere, but especially for reactions in aerosols and biological environments, because the rate constant for formation in solution is higher than that in the gas phase, and, therefore, the half-life of ONOO* is longer.