A Density Functional Study of the Structure, Vibrational Spectra, and Relative Energetics of XBrO₂ Isomers (Where X = H, Cl, and Br)

The geometries, vibrational spectra, and relative energies of HBrO 2, ClBrO 2, and BrBrO 2 isomers have been examined using various density functional (BLYP, SVWN, and B3LYP) methods. A comparison of the density functional results for HBrO 2 isomers with singles and doubles coupled-cluster theory which incorporates a perturbational estimate of the effects of connected triples excitation [CCSD(T)] shows that B3LYP results are in excellent agreement in predicting the geometries, vibrational spectra, and relative energies and should yield reasonable results for ClBrO 2 and BrBrO 2 isomers. The results also show interesting trends for HBrO 2, ClBrO 2, and BrBrO 2 isomers. The peroxide form, XOOBr, is found to be the lowest energy structure among the isomers. The heats of formation at 0 K for HOOBr, ClOOBr, and BrOOBr are estimated to be 8.6, 38.9, and 46.1 kcal mol -1, respectively. Increase in halogenation tends to destabilize the peroxide thermodynamically. We examine the implication for the formation of XBrO 2 isomers from atmospheric cross reactions of HO x, ClO x, and BrO x species.