Quenching isopropanol

Let us now discuss some of the characteristics of this quenching with mercaptans and disulfides. Interestingly, both sulfur derivatives are equally effective in inhibiting the photoreduction and are in fact interconverted during the reaction. The same equilibrium mixture of mercaptan and disulfide is obtained regardless of which was initially added to the reaction mixture. Furthermore, there appears to be no appreciable consumption of the sulfur compounds/64 When benzophenone is irradiated in the presence of isopropanol (OD) and mercaptan, isopropanol containing two deuterium atoms is isolated,... 

Bimolecular quenching of the excited states of metal complexes generally involves electron transfer or energy transfer processes ( 1). Recently, however, Pt2(pop)4 " has been found to undergo a photochemical reaction involving atom abstraction as a primary photoprocess (.26). The reaction involves the catalytic conversion of isopropanol to acetone ... 

Our kinetic investigation of this reaction provides compelling evidence for an atom abstraction mechanism. Quenching of the triplet excited state of 1 by various alcohols occurs only when an a-hydrogen is present ( 19). No quenching occurs with -butanol or triphenyl-carbinol. Furthermore, completely deuterated isopropanol yields a kinetic isotope effect of 1.5 (Table II). 

The enantiomer excess depends on the solvent system isopropanol-water < ethanol-water methanol-water. The best values are 42% in 75% ethanol-water at 25°C and 40% in 75% methanol-water. The reaction mechanism was investigated kinetically. In general, photoreaction takes place via either a dynamic quenching mechanism or a static quenching mechanism, as shown in Scheme 23. 

Table 1. Decay rate constants (ki and k2) and rate constants for quenching of the triplet state of dyes K1-K3 and K4 by the nitroxyl radical in isopropanol solutions (kq) and of the long-lived component of the triplet state in the presence of DNA (kq(DNA)) in aqueous buffer solution (cdna = 2.5 x 10 mol 1 )...

The Kl and K4 triplet states were produced in isopropanol by triplet-triplet energy transfer from 1,2-benzanthracene. As the result, we obtained estimated values of kq for the triplet states of Kl and K4 in isopropanol, which were 1.5 x 10 and 3 x lO 1 mol" s , respectively. For K4, the kq value is typical for quenching by the mechanism of acceleration of intersystem crossing to the ground state (2) [17]. The higher value of the constant for Kl permits assumption of contribution of the charge transfer mechanism (3) to quenching of its triplet state. 

For dye K2 with methoxy group as the meso-substituent, the relatively high quantum yield to the triplet state permitted experiments with direet photoexcitation of the dye. We studied quenching of the triplet state of dye K2 by the radical in aqueous medium (phosphate buffer, pH 7, 20 mmol 1 ) and in organic solvents alcohols (methanol, isopropanol, hexanol) and dimethyl sulfoxide. Table 2 presents the rate constants for quenching of the K2 triplet state by the radical (cr = 0-7 x 10 mol l ). 

Future Areas of Research. There are certain future experi-ments that are very important for further understanding of the work reported here. The most important ones concern lifetime measurements of the quenching of isopropanol. Secondly, the mechanism of quenching must be understood. Electron paramagnetic resonance experiments will be helpful here. More bulk photolyses and other organic quenchers need to be studied, especially variable size quenchers, in order to help understand diffusional processes in these zeolites. We are presently studying the surfaces of these zeolites with ion scattering spectrometry and secondary ion mass spectrometry. The preliminary results indicate that we... 

To a solution of 0.608 g (2.98 mmol) of 84% cc (9/ )-7-cB-alcohol 50 and 0.83 mL (5.96 mmol) of EtjN in 50 mL of dry CH,cl2 at — 78 CC was slowly added 0.431 g (2.98 mmol) of phenylsulfcnyl chloride. The reaction mixture is slowly warmed to r.t. and then quenched by the addition of aq NatlCO,. The layers are separated and the aqueous layer is back extracted twice with CH,CI2. The combined organic extracts were washed with aq NH4CI, dried over MgSQ4 and concentrated. Chromatography (silica gel. petroleum cthers/Et20 3 2) affords 50 as a 1 1 mixture of scalemic diastereomers yield 0.891 g (96%). The individual diastereomers are isolated by semipreparative HPLC. An 8 92 (84% ee) mixture of one of the diastereomers was resolved by HPLC [Pirkcl Type 1-A column (4% isopropanol in Skellysolve B. 0.7 mL/min)]. 

Figure 17. Time dependence of the multiplet polarization PM/AM in the product 22 upon quenching of the radical R (21 ) by an olefin Q solvent isopropanol. For further explanation, see Chart XI and the text. (Left) Constant concentration (3.3 x 10-3M) of the quencher CH2=CPh2, variable initial radical concentration (open circles, [R ] = 11.6 x 10 5 M stars, [R ] = 2.1 x 10-5 M). (Right) Variable concentration of the quencher CH2=CHCN (open circles, [Q] = 3.4 x 10 4M stars, [Q] = 7.5 x 10 4M), constant initial radical concentration ([R ] = 10.9 x 10 5 M). [Adapted from S. N. Batchelor and H. Fischer, J. Phys. Chem., 100, 9794 (1996) with permission. Copyright 1996 American Chemical Society.]...

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