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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Disproportionation of bromous acid HOBrO by direct o-transfer and via anhydrides O(BrO) _2 and BrO-BrO _2. An Ab initio study of the mechanism of a key step of the Belousov-Zhabotinsky oscillating reaction
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Disproportionation of bromous acid HOBrO by direct o-transfer and via anhydrides O(BrO) _2 and BrO-BrO _2. An Ab initio study of the mechanism of a key step of the Belousov-Zhabotinsky oscillating reaction

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The results are reported of an ab initio study of the thermochemistry and of the kinetics of the HOBrO disproportionation reaction 2HOBrO (2) ? HOBr (1) + HBrO _3 (3), reaction (R4′), in gas phase (MP2(full)/6-311G*) and aqueous solution (SMD(MP2(full)/6-311G*)). The reaction energy of bromous acid disproportionation is discussed in the context of the coupled reaction system R2-R4 of the FKN mechanism of the Belousov-Zhabotinsky reaction and considering the acidities of HBr and HOBrO _2. The structures were determined of ten dimeric aggregates 4 of bromous acid, (HOBrO) _2, of eight mixed aggregates 5 formed between the products of disproportionation, (HOBr)(HOBrO _2), and of four transition states structures 6 for disproportionation by direct O-transfer. It was found that the condensation of two HOBrO molecules provides facile access to bromous acid anhydride 7, O(BrO) _2. A discussion of the potential energy surface of Br _2O _3 shows that O(BrO) _2 is prone to isomerization to the mixed anhydride 8, BrO-BrO _2, and to dissociation to 9, BrO, and 10, BrO _2, and their radical pair 11. Hence, three possible paths from O(BrO) _2 to the products of disproportionation, HOBr and HOBrO _2, are discussed: (1) hydrolysis of O(BrO) _2 along a path that differs from its formation, (2) isomerization of O(BrO) _2 to BrO-BrO _2 followed by hydrolysis, and (3) O(BrO) _2 dissociation to BrO and BrO _2 and their reactions with water. The results of the potential energy surface analysis show that the rate-limiting step in the disproportionation of HOBrO consists of the formation of the hydrate 12a of bromous acid anhydride 7 via transition state structure 14a. The computed activation free enthalpy ΔG _(act)(SMD) = 13.6 kcal/mol for the process 2·2a → 14a ? → 12a corresponds to the reaction rate constant k _4 = 667.5 M ~(-1) s ~(-1) and is in very good agreement with experimental measurements. The potential energy surface analysis further shows that anhydride 7 is kinetically and thermodynamically unstable with regard to hydrolysis to HOBr and HOBrO _2 via transition state structure 14b. The transition state structure 14b is much more stable than 14a, and, hence, the formation of the "symmetrical anhydride" from bromous acid becomes an irreversible reaction for all practical purposes because 7 will instead be hydrolyzed as a "mixed anhydride" to afford HOBr and HOBrO _2. The mixed anhydride 8, BrO-BrO _2, does not play a significant role in bromous acid disproportionation.

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