Electron-induced reductive debromination of 2,3,4-tribromodiphenyl ether: a computational study

Abstract

To better understand the mechanism of the electron induced elimination of the bromide anion, we examined at the B3LYP/6-31+G(d) level electron capture by 2,3,4-tribromodiphenyl ether (BDE-21) followed by the release of the bromide anion and a radical. Both the geometry and energy of the BDE-21 neutral and its possible anionic states were studied. A significant relationship was found between the total energy and the length of the C-Br bonds by the analysis of the potential energy surface for the anionic states. Debromination preference for the bromine substituted positions was theoretically evaluated as meta-Br > ortho-Br > para-Br. The reaction profiles of the electron-induced debromination of BDE-21 demonstrated that, in general, the presence of a solvent makes the electron induced reductive debromination of BDE-21 significantly more advantageous, and the stabilization effect of the solvent on the reaction intermediates would make the electron attachment and dissociation relatively effective in comparison with the results from the gas-phase calculations.

Excited States and Photodebromination of Selected Polybrominated Diphenyl Ethers: Computational and Quantitative Structure—Property Relationship Studies

“…For the BDE congeners not symmetrically brominated on the two aromatic rings, the phenyl group with more Br substituents becomes more electron-deficient than the other one. Therefore, the electronic excitation from HOMO to LUMO of PBDEs would make the electron of electronic-rich ring transferred to the electron-deficient ring, resulting in a situation similar to the BDE anionic species which captured an additional electron in the LUMO [ 34 , 35 , 36 ]. Consequently, this transition of electrons into these anti-bond orbitals will reduce the C–Br bond order, and the C–Br bonds become weakened and easy to break in the excited states of BDE congeners.…”

“…For the BDE congeners under study (except for BDE-28), one of the C–Br bonds at a specific position is bent out of the aromatic ring plane with the angles from around 50 to 70 degrees that increases with a decrease in the number of Br, as shown in Figure 3 (visualized geometries of the BDE-154 and BDE-183 in triplet excited state are taken as examples, and the others can be found in Appendix A). The degrees of this bond bending in the triplet excited states of PBDEs are significantly larger than those in their anionic states, probably indicating that the cleavage of C–Br bonds for PBDEs would occur more easily by excitation than by electron attachment [ 34 , 35 ]. In addition, the singly occupied molecular orbital (SOMO) of the triplet excited states for the BDE congeners were examined.…”

“…The SOMO of the congeners (corresponding to the original HOMO in ground states) did not change significantly, leaving the π character. However, the SOMO + 1 (corresponding to the original LUMO in ground states) of the most BDEs is of the mixing characteristics of π and σ orbitals, or twisted σ character, which is believed to play an important role in the dissociation of halogen atoms from the halogenated compounds (e.g., PCDDs and PBDEs) induced by electron attachment [ 35 , 36 ]. From Table 2 , we can see that the bending of C–Br bonds in the triplet excited states occurred preferably at the para-position for higher brominated BDE congeners and at the ortho-position for lower brominated BDE congeners.…”

Theoretical Studies on Structures, Properties and Dominant Debromination Pathways for Selected Polybrominated Diphenyl Ethers

“…The transition states are significant important to determine the interconversion of BDE congeners. Luo et al studied the debromination pathway and transition state of BDE-21, which have the three bromine substituents on one phenyl ring [ 45 ].…”

“…The B3LYP functional (Becke exchange functional, three-parameter; Lee-Yang-Parr correlation functional) is one of the most widely used hybrid DFT methods [ 47 , 48 , 49 ]. The split valence 6-311+G(d) basis set, including the addition of diffuse functions, can deal with all electrons without considering the relativistic effect of Br [ 45 ]. Thus, the mixed basis set (genecp (6-311+G(d) for C, H, O atoms and SDD for Br atom)) was used to include the relativistic effect by use of the effective core potential for the inner electrons of Br.…”