Electron-water couplings, time dependence

Because the kinetic energy dissipation of an excess electron by surrounding water molecules plays an essential role during the formation of electron-radical pairs, the influence of the quantum polarization of water molecules and OH radical must be investigated in detail. Further experimental studies on the short-time dependence of vibronic couplings in aqueous environment would permit to understand the contribution of Jahn-Teller effects on the crossing of an elementary redox reaction with OH radical. 

Fig. 2 shows calculated Tim and Xm values for the 1 and 2 flash experiments assuming the PRE results from one exchangeable H2O. xm is in the range 10-20 ps this is the expected range for protolysis reactions of water coordinated to (+3) or (+4) ions. Measured Tim values in S2 range from <10 ps when T<17°C to near 50 ps at 30°C, with an extremely rapid temperature dependence in this region, which is believed to result from very small electron exchange couplings within the OEC (3). Measured Tim values have been used to calculate electron spin relaxation times xsi of the Mn ion that acts as the H relaxation trap (Fig. 3). Calculations assume an inner sphere dipolar interaction to a single H2O. Four situations were considered (1) that the trap consists of a Mn(lll) monomer (S=2, pe=4.9 BM), (2) a manganese(IV) monomer (S=3/2, pe=3.9 BM), and...

The easiest way to carry out heterophase polymerization is to mix water and a monomer, say styrene, which is able to undergo thermal polymerization, in a vessel at elevated temperatures. After a couple of hours, (the time depends on the temperature) the mixture becomes turbid because of the formation of polystyrene particles. Figure 19 shows a transmission electron microscopy picture of polystyrene particles obtained in this simple way. Although the amount of polystyrene formed was low and the reproducibility of this procedure very bad, a heterophase polymerization took place, and this offers a nice example of how easily heterophase polymerization can be carried out. 

The authors noticed no C-H/C-D isotope effect for the reaction of 13 with methanol and ferf-butanol, but saw a KIE k Jk = 1.4) for the O-H/O-D bond, suggesting that the stronger O-H bond is activated preferentially over the weaker C-H bonds (Pig. 12). In addition, the authors observed the formation of acetone upon the oxidation of tert-butanol. Upon comparison of rate constants (which have been normalized to account for the amount of hydrogens available for abstraction), tert-butanol reacts 50 times faster than cyclohexane. The authors propose a proton-coupled electron transfer event is responsible for the observed selectivity this complex represents a rare case in which O-H bonds may be homolyzed preferentially to C—H bonds. In further study, 13 was shown to oxidize water to the hydroxyl radical by PCET [95]. Under pseudo-first-order conditions, conversion of 13 to its one-electron reduced state was found to have a second-order dependence on the concentration of water, in stark contrast to the first-order dependence observed for aUphatic hydrocarbons and alcohols. Based on the theimoneutral oxidation of water (2.13 V v. NHE in MeCN under neutral conditions [96]) by 13 (2.14 V V. NHE in MeCN under neutral conditions) and the rate dependence, the authors propose a proton-coupled electron transfer event in which water serves as a base. While the mechanism for O-H bond cleavage of alcohols and water is not well understood in these instances, the capacity to cleave a stronger O-H bond in the presence of much weaker C-H bonds is a tremendous advance in metal-oxo chemistry and represents an exciting avenue for chemoselective substrate activation. 

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