Disproportionation of Bromous Acid HOBrO by Direct O-Transfer and via Anhydrides O(BrO)2 and BrO鈥揃rO2. An Ab Initio Study of the Mechanism of a Key Step of the Belousov鈥揨habotins

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• 作者：Rainer Glaser ; Mary Jost
• 刊名：The Journal of Physical Chemistry A
• 出版年：2012
• 出版时间：August 16, 2012
• 年：2012
• 卷：116
• 期：32
• 页码：8352-8365
• 全文大小：647K
• 年卷期：v.116,no.32(August 16, 2012)
• ISSN：1520-5215

文摘

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) + HBrO₃ (3), reaction (R4鈥?/a>), 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鈥揜4 of the FKN mechanism of the Belousov鈥揨habotinsky reaction and considering the acidities of HBr and HOBrO₂. 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)(HOBrO₂), 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 Br₂O₃ shows that O(BrO)₂ is prone to isomerization to the mixed anhydride 8, BrO鈥揃rO₂, and to dissociation to 9, BrO, and 10, BrO₂, and their radical pair 11. Hence, three possible paths from O(BrO)₂ to the products of disproportionation, HOBr and HOBrO₂, are discussed: (1) hydrolysis of O(BrO)₂ along a path that differs from its formation, (2) isomerization of O(BrO)₂ to BrO鈥揃rO₂ followed by hydrolysis, and (3) O(BrO)₂ dissociation to BrO and BrO₂ 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 螖G_act(SMD) = 13.6 kcal/mol for the process 2路2a 鈫?[14a]^{鈥?/sup> 鈫?12a corresponds to the reaction rate constant k₄ = 667.5 M鈥? s鈥? 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 HOBrO₂ via transition state structure 14b. The transition state structure 14b is much more stable than 14a, and, hence, the formation of the 鈥渟ymmetrical anhydride鈥?from bromous acid becomes an irreversible reaction for all practical purposes because 7 will instead be hydrolyzed as a 鈥渕ixed anhydride鈥?to afford HOBr and HOBrO₂. The mixed anhydride 8, BrO鈥揃rO₂, does not play a significant role in bromous acid disproportionation.}