Correspondence/Rebuttal Cite This: Environ. Sci. Technol. XXXX, XXX, XXX-XXX
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Comment on “Molecular Mechanism of Dioxin Formation from Chlorophenol based on Electron Paramagnetic Resonance Spectroscopy”
R
ecently, Yang et al. claimed the EPR-spectroscopic identification of the “2,3,6-trichlorophenoxyl radical [(1)] on the surface of silica-supported copper(II)oxide” as well as the identification of the unsubstituted phenoxyl radical (2).1 Close inspection of the pivotal EPR spectra (Figures 3, 4) reveals that their assignment is in disagreement with the experimental observations: (1) Figure 3: The g-factors and overall widths of the spectra labeled “423 K” and “523 K” do not agree for 1 and 2.2 (2) Figure 4: The spectrum labeled “350 K” and the adjacent ones clearly show a superposition of several different spectra, rather than the spectrum of just a single radical. The intense center doublet of line separation of 1.9 G and g-factor of ≈2.0051 is unequivocally due to the 2,3,6trichloro-p-benzosemiquinone radical anion (3).2 (3) Figure 4, inset: Both spectra are labeled with a g-factor “2.0073”, but the origin of this number is mysterious because it contradicts with g ≈ 2.0051 of the central doublet signal of 3. The simulation of the ″350 K″ spectrum does not reproduce the experimental spectrum to any acceptable degree. The simulation represents the spectrum of a radical species having only three equivalent 35,37 Cl hfs of about 1.89 G. Apart from the fact that the chlorine hfs in 1 are not equivalent,2 where are the metaand para-hydrogen hfs expected for 1? Scrutinizing and simulating the multiline EPR spectra of Figure 4 of ref 1 discloses the presence of at least three radical species (Figure 1): (a) the trichloro-p-benzosemiquinone radical anion (3), (b) an unidentified radical (4) with g ≈ 2.0063 showing two hydrogen, three chlorine, and one copper hfs, and (c) possibly trichlorophenoxyl 1. UB3LYP/CBSB7 computations on a large variety of model Cu(I)-(chloro)phenoxyl and Cu(I)-(chloro)semiquinone(anion) radical complexes predict weak coordination of such radicals to the copper(I) center if a(63,65Cu) ≤ 3 G, disfavoring direct coordination via the oxygen radical center, for which a(63,65Cu) > 3 G.3 The OH-coordinated semiquinone model structure 5 (Figure 1) represents a prototypal candidate for the “unidentified radical” (4). In conclusion, the authors have by no means “successf ully monitored the hyperf ine splittings of TCPR [1]”. Given the fact that the EPR spectra are mainly due to radicals other than 1 (for 2 there is not even a hint), the mechanistic scheme in ref 1 (Figure 6) explaining the formation of dioxine products from further reaction of 1 remains speculation.
Figure 1. Experimental (blue, red) and simulated (black) EPR spectra #3 and #4 from bottom of Figure 4 of ref 1. Experimental spectra were reconstructed from digitized images of the spectra. Simulations are the superpositions of spectra 1(#), 3(×), 4(*) in ratios 30:18:52 (#3) and 6:21:73 (#4) with the hyperfine splittings (G) shown.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The author declares no competing financial interest.
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REFERENCES
(1) Yang, L.; Liu, G.; Zheng, M.; Zheng, Y.; Jin, R.; Wu, X.; Wu, Y. Molecular Mechanism of Dioxin Formation from Chlorophenol based on Electron Paramagnetic Resonance Spectroscopy. Environ. Sci. Technol. 2017, 51, 4999−5007. (2) (a) Landolt-Börnstein, New Series, Magnetic Properties of Free Radicals (Eds.: Fischer, H.; Hellwege, K. H.), Springer, Berlin, Vol. II/ 1, 1965; p 80; Vol. II/9c2, 1979, pp. 29−182; Vol. II/9d1, 1980, pp. 162−236; Vol. II/17e, 1988, pp. 36−193; Vol. II/17g, 1988, pp. 216− 248; Vol. II/26C 2004, pp. 341−532. (3) Kaim, W. The chemistry and biochemistry of the copper-radical interaction. J. Chem. Soc., Dalton Trans. 2003, 761−768 and references therein.
Hans-Gert Korth*
Institut für Organische Chemie, Universität Duisburg-Essen, Universitätsstr. 7, 45117 Essen, Germany © XXXX American Chemical Society
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DOI: 10.1021/acs.est.7b05126 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
