Langmuir 1991, 7, 289-296
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Chemisorption of p-Dimethoxybenzene on Copper-Montmorillonite C. T. Johnston,**tT. Tipton,$ D. A. Stone,$ C. Erickson,+and S. L. Trabuet Department of Soil Science, University of Florida, Gainesville, Florida 32611-0150, and HQ AFESCIRDVS, Tyndall AFB, Florida 32403-6001 Received March 5, 1990. I n Final Form: July 12, 1990
In situ Fourier transform IR, UV-visible, and gravimetric data were obtained to characterize the single electron transfer reaction of 1,4-dimethoxybenzene (DMOR) on copper-montmorillonite (CuX). Upon dehydration, 0.8 molecule of DMOB is sorbed onto each unit cell of CuX. Sorption of DMOB produces a color change of the DMOB-CuX complex from pale blue to dark green typical of clay-mediated electron transfer reactions. This color change is manifested in the UV-visible spectrum by the growth of three intense bands at 400,451, and 464 nm, which are in good agreement with published electronic spectra of the DMOB cation. Several vibrational modes of DMOB were significantly perturbed by the formation of the radical cation which include a red-shift of the v19 band from 1509 to 1502 cm-I and a blue-shift of the ~ 1 mode 3 from 1230 to 1238 cm-l upon oxidation. These changes are in good agreement with the calculated force constants for the DMOB radical cation, which indicated a weakening of the C-C ring bonds and a strengthening of the C,inr-O bond. In addition to the perturbed fundamentals, a number of new bands were observed in the IR spectrum of the dry complex at 1302,1308,1342,1427,1438, and 1551 cm-l. Upon exposure of the DMOB-CuX complex to water vapor, the UV-visible bands at 400,451, and 463 nm and the IR bands at 1302, 1308, 1342, 1427, 1438 and 1551 cm-l were eliminated within 6 min. The DMOB fundamental bands at 1230 and 1509 cm-l were reduced in intensity after exposure to water vapor but remained present for several hours. Upon prolonged exposure to water vapor (>24 h), all of the DMOB and DMOB+ bands were eliminated. These data indicate that the formation of the radical cation is, in fact, reversible and that no significant dimerization or polymerization of DMOB occurs.
Introduction Single electron transfer (SET) reactions of unsaturated organic compounds on clay surfaces represent an important class of abiotic reactions that can enhance the degradation of toxic organic solutes in soils and sediments.'+ Mortland and Pinnivaia7-Io were among the first to show that simple arenes (e.g., benzene, toluene, and p-xylene) are chemisorbed onto transition-metal (e.g., Cu2+,Fe3+, and Ru"+)-exchanged montmorillonites under dry conditions forming strongly colored complexes whose molecular properties are quite distinct from those of the unperturbed arene. Although the earlier work was restricted to simple arenes, these reactions have since been extended to a much broader class of unsaturated organic compounds including dioxins," chloroanisoles,12 and ch10roethenes.l~Table I t IJniversity of Florida.
* Tyndall AFB.
(1)Mortland, M. M. Adu. Agron. 1970, 22, 75. (2) Voudrias, E. A.; Reinhard, M. Abiotic organic reactions at mineral
surfaces. In: Geochemical processes a t mineral surfaces; ACS Symposium Series 323; Davis J., Hayes, K. F., Eds.; American Chemical Society: Washington, DC, 1986; pp 462-486. (3) Chiou, C. T.; Shoup, T. D. Enuiron. Sci. Technol. 1985,19, 1196. (4) Zielke, R. C.; Pinnavaia, T. J.; Mortland, M. M. Adsorption and reactions of selected organic-moleculeson clay mineral surfaces. Reactions and movement of organic chemicals in soils; SSSA Special Publication Number 22; Sawhney, B. L., Brown, K., Eds.; Soil Science Society of America: Madison, WI, 1989; pp 81-98. (5)Sawhney, B. L., Cloys Clay Miner. 1985,33, 123. (6) Yamagishi, A. J. Coord. Chem. 1987, 16, 132. (7) Doner, H. E.; Mortland, M. M. Science 1969, 166, 1406. (8)Mortland, M. M.; Pinnavaia, T. J. Nature 1971, 229, 75. (9) Pinnavia, T. J.; Mortland, M. M. J. Phys. Chem. 1971, 75, 3957. (10) Pinnavia, T. J.; Hall, P. L.; Cady, S. S.; Mortland, M. M. J.Phys. Chem. 1974, 78, 994. (11)Boyd, S.A.; Mortland, M. M. Nature 1985, 316, 532. (12) Govindaraj, N.; Mortland, M. M.; Boyd, S. A. Enuiron. Sci. Technol. 1987, 22, 1119.
0743-7463/91/2407-0289$02.50/0
presents a partial listing of compounds that are known to participate in SET reactions on transition-metal-exchanged montmorillonites. It has been established that these reactions proceed through a single electron transfer (SET) from the unsaturated organic solute to the transition-metal c a t i ~ n . ~ J ~ J ~ - ~ ~ In order for the SET reaction to occur, the organic solute must coordinate with the transition-metal cation by assuming a direct ligand position. Under hydrated conditions, hydrophobic organic solutes have little, if any, affinity for the hydrophilic, interlamellar environment of the clay. Thus, minimal sorption of the hydrophobic solute occurs.3J7 Dehydration of the transition-metal-exchanged montmorillonite, however, enhances the sorption of the organic solute by removal of interlamellar water molecules which strongly compete for coordination sites around the metal cation. If the ionization potential of the organic solute is below =
