The ground and excited states of HBrO2 [HOOBr, HOBrO, and HBr(O)O] and HBrO3 (HOOOBr and HOOBrO) isomers

Abstract

The HBrO2 isomers have been analyzed computationally to confirm the previous experimental assignments for HOOBr and HOBrO and to assist in future identification of the as yet unobserved HBr(O)O isomer. Optimized geometries of the HOOBr, HOBrO, and HBr(O)O isomers and the transition states connecting them were obtained at the CCSD(T)/O, H: aug-cc-pVTZ, Br: aug-cc-pVTZ-PP level of theory. The corresponding harmonic vibrational frequencies for the HOOBr, HOBrO, and HBr(O)O isomers are reported for all isotopologues considered in the experimental measurements, i.e., those involving hydrogen, deuterium, $$^{79}$$ Br, $$^{81}$$ Br, $$^{16}$$ O, and $$^{18}$$ O. The relative energetics of the stationary point geometries were determined through extrapolation of energies to the complete basis set limit. To explain the photodestruction observed experimentally for HOOBr and HOBrO, the three lowest low-lying singlet and triplet excited electronic states for each of the three isomers were computed using the equation-of-motion coupled-cluster with inclusion of single and double excitations (EOM-CCSD) and time-dependent density functional theory (TD-B3LYP and TD-CAM-B3LYP) approaches; all utilizing the all-electron aug-cc-pVTZ basis sets for all atoms. Multi-reference configuration interaction (MRCI)/aug-cc-pVTZ computations were carried out for the lowest singlet and lowest two triplet excited states. The vertical excitation energies for the low-lying excited states of the most stable isomer (HOOBr) are reported for the first time. The vibrational frequencies for the $$\hbox{HBrO}_{2}$$ isomers are used along with new anharmonic vibrational frequency computations (at the PBE0/aug-cc-pVTZ level of theory) and vertical excitation energies (at the TD-B3LYP/aug-cc-pVTZ, TD-CAMB3LYP/aug-cc-pVTZ, and EOM-CCSD/aug-cc-pVTZ levels of theory) for the $$\hbox {HBrO}_{3}$$ isomers, HOOOBr and HOOBrO, to determine that previously unassigned peaks in the experimental spectrum generated from HBr/O $$_{2}$$ photolysis in a Ne matrix belong to HOOOBr.