Theoretical study of XONO2 (X=Br, OBr, O2Br): Implications for stratospheric bromine chemistry

The equilibrium structure, dipole moment, harmonic vibrational frequencies, and infrared intensities of XONO2 (X=Br, OBr, O2Br) are determined using density functional theory in conjunction with a TZ2P (triple zeta double polarized) basis set. The B3LYP functional was used since this has previously been shown to perform well for similar bromine compounds. The equilibrium geometry and vibrational spectra of BrONO2 are shown to be in good agreement with the experimental data and also with high-level coupled-cluster calculations. The vibrational spectrum of O2BrONO2 has been compared with that of the chlorine analog, O2ClONO2, for which some experimental data exist. The bonding in OBrONO2 is shown to be more similar to that in BrONO2. Using large atomic natural orbital basis sets, the singles and doubles coupled-cluster method that includes a perturbative correction for triple excitations, denoted CCSD(T), was employed to compute energies for three isodesmic reactions in order to determine heats of formation for OBrONO2 and O2BrONO2. Our best estimates are 36.7 and 38.7 kcal/mol, respectively. Finally, the possible formation of O2BrONO2 in the stratosphere by adduct formation and oxidation of OBrONO2 and the implications for stratospheric bromine chemistry are discussed.