Theoretical study of cyclic radicals NOx (x = 2–6)

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.

Conformation-dependent state selectivity in O-O cleavage of ONOONO: an "inorganic cope rearrangement" helps explain the observed negative activation energy in the oxidation of nitric oxide by dioxygen

ONOONO has been proposed as an intermediate in the oxidation of nitric oxide by dioxygen to yield nitrogen dioxide. The O-O bond breaking reactions of this unusual peroxide, and subsequent rearrangements, were evaluated using CBS-QB3 and B3LYP/6-311G hybrid density functional theory. The three stable conformers (cis,cis-, cis,trans-, and trans,trans-ONOONO, based on the O-N-O-O dihedral angles of either approximately 0 degrees or approximately 180 degrees ) are predicted to have very different O-O cleavage barriers: 2.4, 13.0, and 29.8 kcal/mol, respectively. These large differences arise because bond breaking leads to correlation of the nascent NO(2) fragments with either the ground (2)A(1) state or the excited (2)B(2) state of NO(2), depending on the starting ONOONO conformation. A cis-oriented NO(2) fragment correlates with the (2)A(1) state, whereas a trans-oriented NO(2) fragment correlates with the (2)B(2) state. Each NO(2) fragment that correlates with (2)A(1) lowers the O-O homolysis energy by approximately 15 kcal/mol, similar to the approximately 17-25 kcal/mol (2)A(1) --> (2)B(2) energy difference in NO(2). Hence, this provides an unusual example of conformation-dependent electronic state selectivity. The O-O bond homolysis of cis,cis-ONOONO is particularly interesting because it has a very low barrier and arises from the most stable ONOONO conformer, and also due to obvious similarities to the well-known [3,3]-sigmatropic shift of 1,5-hexadiene, i.e., the Cope rearrangement. As an additional proof of our state selectivity postulate, a comparison is also made to breakage of the O-O bond of cis,cis-formyl peroxide, where no significant stabilization of the transition state is available because the (2)A(1) and (2)B(2) states of formyloxy radical are near-degenerate in energy. In the case of trans,trans-ONOONO, the O-O bond breaking transition state is a concerted rearrangement yielding O(2)NNO(2), whereas for cis,cis- and cis,trans-ONOONO, the initially formed NO(2) radical pairs can undergo further rearrangement to yield ONONO(2). It is proposed that previous spectroscopic observations of certain N=O stretching frequencies in argon-matrix-isolated products from the reaction of NO with O(2) (or (18)O(2)) are likely from ONONO(2), not the OONO radical as reported.