Part 2—Quantum Chemistry Studies of Peroxynitrous Acid (HOONO)

The reaction of the hydroxyl radical (OH) with the nitrogen dioxide radical (NO2) can form one of two products: peroxynitrous acid (HOONO), or nitric acid (HONO2). Because HOONO is a temporary reservoir for OH and NO2, while HONO2 is a permanent reservoir, the branching ratio of these two reaction channels is atmospherically significant. Previous experiments have measured the OH-stretch cavity ringdown spectra of HOONO and HONO2 formed from OH + NO2 to assess the branching ratio. However, the torsional modes of HOONO are coupled to the OH-stretch, leading to a series of sequence bands that complicate the OH-stretch spectrum. These sequence bands must be accounted for in order to obtain a quantitatively correct branching ratio. This chapter describes a 3-dimensional potential energy surface and dipole moment surface for HOONO to study the torsion/torsion coupling to the OH stretch. Geometries and energies were computed at the CCSD(T)/cc-pVTZ level of theory and basis, as a function of the HOON and OONO dihedral angles and the OH bond length. Dipole moments were then calculated at the CCSD/aug-cc-pVDZ level of theory and basis. The resulting surfaces were used to compute the wavefunctions, energies, and two simulated spectra of HOONO: the OH stretch spectrum, and the HOON and OONO torsional spectrum. These data were used in conjunction with the previous cavity ringdown spectra to obtain the true branching ratio for the two channels of OH + NO2, and to make assignments in the matrix spectrum of HOONO. 46 Introduction The reaction of the hydroxyl radical (OH) with nitrogen dioxide (NO2) is a very important reaction in the troposphere. As stated in Chapter 1, HOx and NOx radicals catalyze tropospheric ozone formation, causing air pollution and health hazards. Both OH and NO2 are formed from industrial and automotive emissions: OH from the oxidation of methane, and NO2 directly from combustion engine and industrial plant emissions. Because of the large impact on air quality, it is important to understand how OH and NO2 will react with each other in the atmosphere. Until thirty years ago, it was assumed that OH + NO2 formed only nitric acid (HONO2). 57 However, Robertshaw et al. noticed that their data did not fit to a single termolecular falloff curve. They suggested the possibility of two reaction pathways, one of which was a “weakly bound state.” Spectroscopic studies by Lee (matrix IR) identified this product as peroxynitrous acid (HOONO). Thus, there are two reaction pathways for OH + NO2: 2 2 OH + NO HONO  (3.1) 2 OH + NO HOONO   (3.2) The geometry and energetics of HOONO has been determined by many theoretical and experimental studies. 16, 42, 43, 62-72 The ground state of HOONO is bound by 19.6 kcal mol below the dissociation limit to OH + NO2. In contrast, HONO2 is 47.2 kcal mol below the dissociation limit (Figure 3.1).