We have studied the oxidation of sulfur dioxide by dissolved oxygen in highly dilute solutions with a new differential optical absorption technique. We measured the rate of oxidation catalyzed by iron(III) over a wide range of pH, ionic strength, and in the presence of various organic materials. The studies indicate that noncomplexing organic molecules are highly inhibiting at “high” pH values of 5 and above and are not inhibiting at “low” pH values of 3 and below. Furthermore, the order of the reaction with respect to iron is different in the two pH regimes. This suggests that the mechanism of this reaction differs in the two pH regimes and is probably a free radical chain at high pH and a nonradical mechanism at low pH. Some of the mechanisms proposed in the literature are discussed in the light of these new data. None of the proposed mechanisms give completely satisfactory agreement with the data. We propose a modified free radical chain mechanism for the high pH regime, which correctly predicts the organic inhibitions. For the low pH regime, mechanisms proposed by Conklin and Hoffmann (1988) and by Hoffmann and Jacob (1984) give fair agreement with the pH data and correctly predict the self‐inhibition, the sulfate inhibition, and the ionic strength inhibition. In view of the new data we believe that the iron(III) catalyzed reaction in tropospheric clouds can be a major contributor to the rate of sulfate formation, but there will be significant inhibition of this process by formic acid in some situations.

Highly efficient and unique enantiodifferentiating Z-E photoisomerization of cyclooctene (1) was achieved through singlet photosensitization with chiral polyalkyl benzenepolycarboxylates. Conversion dependence of optical yields and an attempted kinetic resolution of racemic (E)-isomer IE unequivocally showed that the enantiodifferentiating step is not the initial quenching of chiral sensitizer with enantiomeric IE but the rotational relaxation of planar 1Z to twisted singlet cyclooctene (1p) within the singlet exciplex with chiral sensitizer. Fluorescence quenching of some benzenetetracarboxylates with 1Z and IE provides further evidence for the intervention of an exciplex intermediate, showing a new emission at a longer wavelength, and also for the insignificant enantiodifferentiation in the quenching step, affording nearly diffusion-controlled quenching rate constants around 1010 s"1 for IE. This novel enantiodifferentiating'photosensitizing system via exciplex, in combination with low-temperature irradiation, not only affords the highest optical yields up to 53% but also exhibits unusual temperature-switching behavior of product chirality especially with the o-dicarboxylate sensitizers. The activation parameters obtained in the temperature-dependence study indicate that the temperature-switching phenomenon is attributed to the unequal activation entropies, or frequency factors, for the enantiodifferentiating relaxations of 1Z to (R)-and (S)-1 P within the exciplex, for which the dynamic structural changes in the relaxation process may be responsible.

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Various types of modified nickel catalysts were prepared and their enantioface-differentiating (asymmetric) abilities were examined. It was found that the presence of aluminum or related metal compounds in the catalyst was unfavorable for the effective enantioface-differentiating catalyst. The modification with a solution containing tartaric acid and inorganic salt gave a high enantioface-differentiating ability to Raney nickel and reduced nickel catalyst. Among the modified nickel catalysts examined, the Raney nickel modified with tartaric acid and sodium bromide gave the best result with respect to the hydrogenation activity and enantioface-differentiating ability (optical yield 88%). NaBr adsorbed on Raney nickel was found to inhibit the nonenantioface-differentiating hydrogenation.

Cyclodextrins (CDs), a series of cyclic oligomers of Dglucopyranose, are known to accommodate a variety of organic guests in their chiral hydrophobic cavities, forming inclusion compounds. Consequently, they have been applied widely to biomimetic chemistry as well as to separation science and technology, including chiral re~ognition.'-~ In sharp contrast to the well-documented stoichiometric and catalytic thermal asymmetric syntheses utilizing the CD's chiral c a v i t~, ' ,~,~-~* little effort appears to have been devoted to the study of asymmetric photochemistry in the chiral ~a v i t y ? , '~-'~ Only a few stoichiometric asymmetric photoreactions of CD complexes have hitherto been investigated in the solid state.I3-l5 We have reported recently that the direct irradiation at 185 nm of a 1:l complex of (a-cyclooctene (1Z) with p-CD in the solid state or in water suspension leads to prompt Z-E isomerization affording apparent photostationary-state (pss) En mixtures, but that the (@-isomer (1E) obtained is almost racemic: optical purity (op)