Prof. David Phillips, Director of the Ultrafast Laser Facility at The University of Hong Kong, will present the following seminar from the Pacific Rim Conference in Nanoscience (7-11 September 2004). The seminar will be available for viewing and discussion through the internanotech Community at http://nanotech.colayer.net/
Water-catalyzed dehalogenation reactions: building a nanoscale water solvated reaction system one molecule at a time
Bromoform is the most abundant source of organic bromine in the ocean and atmosphere and this makes it an attractive polyhalomethane to study. Ultraviolet excitation with 253.7 nm light (from a Hg lamp) of low concentrations (<10-6 M) of CHBr3 , CHBr2Cl, and CHCl2Br in aqueous solution led to complete conversion of the halogens into halide ions (bromide and/or chloride) with similar photo-quantum yields of about 0.43. How does the 253.7 nm photolysis of low concentrations of CHBr3 , CHBr2Cl, and CHCl2Br in water lead to complete conversion of the halogen atoms into bromide and/or chloride ion products and where does the energy come from to break all three carbon-halogen bonds?
We present a combined experimental and theoretical study of the photochemistry of CHBr3 in pure water and in acetonitrile/water mixed solvents that elucidates the reactions and mechanisms responsible for the photochemical conversion of the halogen atoms in CHBr3 into three bromide ions in water solution. Photochemistry experiments show 240 nm excitation of CHBr3 (about 9 x10 -5 M) in water leads to almost complete conversion into 3HBr leaving groups and CO (major product) and HCOOH (minor product) molecules. Picosecond time-resolved resonance Raman (ps-TR3 ) experiments and ab initio calculations indicate that water catalyzed O-H insertion/HBr elimination reaction of isobromoform and subsequent reactions of its products are responsible for the formation of the final products observed in the photochemistry experiments reported here.
Ab initio calculations (MP2/6-31G*) were done to study the isobromoform + nH2O->CHBr2OH + HBr + (n-1)H2O where n= 1,2,3; CHBr2 OH + nH2O->HBrCO + HBr + nH2O where n=0,1,2,3 and HBrCO + nH2O->CO + HBr + nH 2 O where n=0,1,2,3,4 reactions. IRC calculations were done to confirm the transition states connected the appropriate reactants and products. The relative energy profiles (in kcal/mol) for the reactions reveal that the barriers to reaction (e.g. from the reactant complexes to their respective transition state) become substantially smaller as the number of H2O molecules in the reaction system increase. This indicates water catalyzes these reactions. The reaction barrier decreases from 10.8 kcal/mol for one H2O molecule to 2.5 kcal/mol for three H2O molecules for the isobromoform + nH2O->CHBr2OH + HBr + (n-1)H2O reaction; from 17.6 kcal/mol for one H2O molecule to 2.25 kcal/mol for three H2O molecules for the CHBr2OH + nH2O->HBrCO + HBr + nH2O reaction and from 17.8 kcal/mol for one H2O molecule to 8.6 kcal/mol for three H2O molecules for the HBrCO + nH2O->CO + HBr + nH2O reaction. These results have important ramifications for the phase dependent behavior of polyhalomethane photochemistry and chemistry in water-solvated environments compared to gas phase reactions. A brief discussion is given for how this phase dependent behavior may influence the release and activation of halogens from polyhalomethanes in the natural environment. The solvation of the HBr leaving group and its spontaneous dissociation reaction into H+ and Br- ions helps catalyze several O-H insertion and HBr elimination reactions that also enable O-H and C-H bonds to be easily broken. This water-catalysis by solvation of a leaving group and its spontaneous dissociation into ions (e.g. H+ and Br- in the example studied here) should be of general interest for a wide range of chemical reactions occurring in water environments including some biological reactions.
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