III: PROPERTIES OF COMPLEX SYSTEMS

Abstract

The molecular structures, vibrational frequencies, dissociation energies and electron affinities of the Br2/Br− 2, Br2O/Br2O−, Br2O2/Br2O− 2, Br2O3/Br2O− 3, and Br2O4/Br2O− 4 systems have been investigated using density functional theory (DFT) and hybrid Hartree-Fock/density functional theory. The methods used have been carefully calibrated against a comprehensive tabulation of experimental electron affinities, (Rienstra-Kiracofe, J. C., Tschumper, G. S., Shaefer, H. F., Nandi, S. and Ellison, G. B., 2002, Chem. Rev., 102, 231). Four different types of neutral/anion energetic difference are reported in this work: the adiabatic electron affinity (EAad), the zero-point corrected EAad (EAzero), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE). The basis set used in this work is of double-zeta plus polarization quality with additional s- and p-type diffuse functions, and is denoted as DZP++. The BHLYP method does well in reproducing the very limited experimental energetics, while the B3LYP method does well for the few known vibrational frequencies. The final predicted electron affinities with the BHLYP method are 3.41 eV (Br, experiment 3.36 eV), 3.02 eV (Br2, experiment 2.6 ± 0.2 eV), 3.27 eV (Br2O), 2.93 eV (Br2O2), 3.07 eV (Br2O3), and 2.54 eV (Br2O4). The global minimum structures for several of the larger dibromine oxides and their anions are unusual. For neutral Br2O2 the peroxide structure (BrOOBr) lies lowest, but for the anion a loosely bound Cs symmetry BrO-BrO− structure lies lowest for the hybrid functionals, while a C2 symmetry peroxide BrOOBr− structure lies lowest for the pure functionals. Furthermore, the C2 structures are found to exhibit an inverse symmetry breaking problem, and should be interpreted with caution. For neutral Br2O3, a chain structure Br-O3-Br lies lowest, while the complex Br···O3···Br− lies lowest for the negative ion. For neutral Br2O4, a chain structure Br-O4-Br lies lowest, while the complex BrO−···BrO3 lies lowest for the negative ion.