The reaction of ethoxide ion with 0,S-dimethyl phenylphosphonothioate (la) proceeds with competitive displacements of the methylthio and methoxy ligands. Each displacement occurs with complete inversion of configuration. The two products, ethyl methyl phenylphosphonate (2ab) and O-ethyl S'-methyl phenylphosphonothioate (lb), respectively, react further with ethoxide ion to form diethyl phenylphosphonate (2bb). Displacement of the ethoxy ligand on 2ab or lb, which leads to racemization, competes with formation of 2bb in both of these reactions. The competitions favor displacement of methylthiolate over methoxide ion from la (3/1), methoxide over ethoxide ion from 2ab (6/1), and methylthiolate over ethoxide ion from lb (18/1). In addition, racemization of lb is 22 times faster than racemization of 2ab, and displacement of methylthiolate ion from lb is 65 times faster than displacement of methoxide ion from 2ab. The results rule out the possibility that methylthiolate ion is displaced in phosphonothioates with inversion stereochemistry simply because the retention pathway, seen in other related systems, is energetically blocked by the need for a high-energy isomerization process. The small preference for displacement of methylthiolate ion over methoxide ion from la is identified to be the result of a methylthio ligand having a larger relative intrinsic kinetic affinity to occupy either an axial position or an equatorial position in a pentacoordinate intermediate or transition state, and these affinities partially cancel.

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We used ultraviolet and heat (UV-heat) to costimulate HCl, HBr, and HI to yield Cl • , Br • , and I • for Hg 0 removal. Electron spin resonance (ESR) confirmed the radical formation. Cl • /Br • /I • were found to play the leading roles in Hg 0 removal, Br 2 /I 2 formed from the heat catalysis of HBr/HI also played key roles. The costs for removing 80% Hg 0 with UVheat/HCl, UV-heat/HBr, and UV-heat/HI were calculated to be 0.03, 2.4, and 36.9 USD/lb-Hg 0 , respectively. Rising heat catalysis temperature favored the Hg 0 removal. Declining flue gas residence time and adding NO and SO 2 impaired the Hg 0 removal by Cl • and Br • but did not include I • . Kinetics analyses indicated that UV increased the Hg 0 removal rate by 15.5 times for HCl, by 1.3 times for HBr, and by 0.2 time for HI. Radical quenching test with tertiary butanol (TBA) and CO 2 indicated that Cl • and Cl 2•− contributed almost 100% and <5% to Hg 0 removal, respectively. Density functional theory (DFT) calculation gives the reactivity order of oxidizing species toward Hg 0 , that is, Cl • > Br • > I • > I 2 > Br 2 > Cl 2 . The environmetnal implications were proposed finally.