Electron affinities of the bromine oxides BrOn, n = 1-4

Abstract

Bromine oxides are of significant interest due to their importance in atmospheric chemistry. Density functional theory (DFT) methods have been used in conjunction with a DZP++ (double-ζ plus polarization with diffuse functions) basis set to study the molecular geometries and total energies of BrOn and BrO- n (n = 1-4). The adiabatic electron affinity (EAad), the vertical electron affinity (EAvert) of the bromine oxide and the vertical detachment energy (VDE) of each anion are reported. Harmonic vibrational frequencies and zero point energies are also reported. Five different DFT methods were employed for comparison. Among these, the BHLYP method predicts the geometries and the vibrational frequencies in best agreement with available experimental data, while the the other methods do better in predicting the limited number of energetic quantities determined observationally. The predicted adiabatic electron affinities are 2.38 eV (BrO, experiment 2.35 eV), 2.36 eV (BrO2), 3.35 eV (BrOO), 4.32 eV (BrO3), 2.91 eV (BrOOO) and 5.28 eV (BrO4). The electronic ground state of BrOO- is a triplet (3A") state. Predicted Br–;O bond distances range from 1.61 (BrO3) to more than 2 Å for Br...O2 for the neutral molecules; and from 1.61 Å for BrO- to 1.82 Å for BrO- to more than 2 Å for Br-...O3 among the anions. The BrOO isomer (Cs symmetry) is predicted to lie 17–18 kcal mol-1 below the C2v symmetry OBrO structure. The asymmetric 3A" anion BrOO- analogously lies below OBrO-, in this case by 40–41 kcalmol-1. BrOOO (Cs symmetry) is predicted to lie 42-45 kcalmol-1 below the symmetric C3v BrO structure. Finally the asymmetric BrOOO- anion (C1 symmetry) is predicted to lie 10-13kcal mol-1 below symmetric C3v BrO- 3.