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Excited States and photodebromination of selected polybrominated diphenyl ethers: computational and quantitative structure--property relationship studies.

Luo J, Hu J, Wei X, Li L, Huang X - Int J Mol Sci (2015)

Bottom Line: This paper presents a density functional theory (DFT)/time-dependent DFT (TD-DFT) study on the lowest lying singlet and triplet excited states of 20 selected polybrominateddiphenyl ether (PBDE) congeners, with the solvation effect included in the calculations using the polarized continuum model (PCM).The results obtained showed that for most of the brominated diphenyl ether (BDE) congeners, the lowest singlet excited state was initiated by the electron transfer from HOMO to LUMO, involving a π-σ* excitation.Based on the previously reported kinetic data for the debromination by ultraviolet (UV) and sunlight, obtained QSPR models exhibited a reasonable evaluation of the photodebromination reactivity even when the BDE congeners had same degree of bromination, albeit different patterns of bromination.

View Article: PubMed Central - PubMed

Affiliation: Guizhou Provincial Key Laboratory for Information System of Mountainous Areas and Protection of Ecological Environment, Guizhou Normal University, Guiyang 550001, China. luojin@gznu.edu.cn.

ABSTRACT
This paper presents a density functional theory (DFT)/time-dependent DFT (TD-DFT) study on the lowest lying singlet and triplet excited states of 20 selected polybrominateddiphenyl ether (PBDE) congeners, with the solvation effect included in the calculations using the polarized continuum model (PCM). The results obtained showed that for most of the brominated diphenyl ether (BDE) congeners, the lowest singlet excited state was initiated by the electron transfer from HOMO to LUMO, involving a π-σ* excitation. In triplet excited states, structure of the BDE congeners differed notably from that of the BDE ground states with one of the specific C-Br bonds bending off the aromatic plane. In addition, the partial least squares regression (PLSR), principal component analysis-multiple linear regression analysis (PCA-MLR), and back propagation artificial neural network (BP-ANN) approaches were employed for a quantitative structure-property relationship (QSPR) study. Based on the previously reported kinetic data for the debromination by ultraviolet (UV) and sunlight, obtained QSPR models exhibited a reasonable evaluation of the photodebromination reactivity even when the BDE congeners had same degree of bromination, albeit different patterns of bromination.

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Visualized geometries of BDE-154 (A) and BDE-183 (B) in the triplet excited state.
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ijms-16-01160-f003: Visualized geometries of BDE-154 (A) and BDE-183 (B) in the triplet excited state.

Mentions: In the present work, the DFT calculations (PCM/B3LYP/6-31G(d)) were employed to investigate the geometries of the lowest triplet excited states of the BDE congeners. Results show that the structures of the triplet excited states differ significantly from that of the BDE congeners in ground states. 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. Similar results can also be observed in the corresponding gas-phase calculations. This is in agreement with the previous report that photodebromination of BDE-209 occurred mostly at the para-position [43]. Fang et al. [40] also reported that the photo-reactivity of bromines at various positions of phenyl rings decreased in the following order: ortho > para for lower brominated PBDEs (e.g., BDE-47), while for higher brominated congeners, the regioselectivity of photodebromination was not significant.


Excited States and photodebromination of selected polybrominated diphenyl ethers: computational and quantitative structure--property relationship studies.

Luo J, Hu J, Wei X, Li L, Huang X - Int J Mol Sci (2015)

Visualized geometries of BDE-154 (A) and BDE-183 (B) in the triplet excited state.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4307296&req=5

ijms-16-01160-f003: Visualized geometries of BDE-154 (A) and BDE-183 (B) in the triplet excited state.
Mentions: In the present work, the DFT calculations (PCM/B3LYP/6-31G(d)) were employed to investigate the geometries of the lowest triplet excited states of the BDE congeners. Results show that the structures of the triplet excited states differ significantly from that of the BDE congeners in ground states. 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. Similar results can also be observed in the corresponding gas-phase calculations. This is in agreement with the previous report that photodebromination of BDE-209 occurred mostly at the para-position [43]. Fang et al. [40] also reported that the photo-reactivity of bromines at various positions of phenyl rings decreased in the following order: ortho > para for lower brominated PBDEs (e.g., BDE-47), while for higher brominated congeners, the regioselectivity of photodebromination was not significant.

Bottom Line: This paper presents a density functional theory (DFT)/time-dependent DFT (TD-DFT) study on the lowest lying singlet and triplet excited states of 20 selected polybrominateddiphenyl ether (PBDE) congeners, with the solvation effect included in the calculations using the polarized continuum model (PCM).The results obtained showed that for most of the brominated diphenyl ether (BDE) congeners, the lowest singlet excited state was initiated by the electron transfer from HOMO to LUMO, involving a π-σ* excitation.Based on the previously reported kinetic data for the debromination by ultraviolet (UV) and sunlight, obtained QSPR models exhibited a reasonable evaluation of the photodebromination reactivity even when the BDE congeners had same degree of bromination, albeit different patterns of bromination.

View Article: PubMed Central - PubMed

Affiliation: Guizhou Provincial Key Laboratory for Information System of Mountainous Areas and Protection of Ecological Environment, Guizhou Normal University, Guiyang 550001, China. luojin@gznu.edu.cn.

ABSTRACT
This paper presents a density functional theory (DFT)/time-dependent DFT (TD-DFT) study on the lowest lying singlet and triplet excited states of 20 selected polybrominateddiphenyl ether (PBDE) congeners, with the solvation effect included in the calculations using the polarized continuum model (PCM). The results obtained showed that for most of the brominated diphenyl ether (BDE) congeners, the lowest singlet excited state was initiated by the electron transfer from HOMO to LUMO, involving a π-σ* excitation. In triplet excited states, structure of the BDE congeners differed notably from that of the BDE ground states with one of the specific C-Br bonds bending off the aromatic plane. In addition, the partial least squares regression (PLSR), principal component analysis-multiple linear regression analysis (PCA-MLR), and back propagation artificial neural network (BP-ANN) approaches were employed for a quantitative structure-property relationship (QSPR) study. Based on the previously reported kinetic data for the debromination by ultraviolet (UV) and sunlight, obtained QSPR models exhibited a reasonable evaluation of the photodebromination reactivity even when the BDE congeners had same degree of bromination, albeit different patterns of bromination.

Show MeSH
Related in: MedlinePlus