Criegee中间体CH_3CHOO与H_2O反应机理及酸催化效应

Reaction Mechanism of Criegee Intermediate CH_3CHOO with H_2O and the Acid Catalytic Effect

采用CCSD(T)//B3LYP/6-311+G(d,p)方法研究了H2O及甲酸等6种有机酸对CH3CHOO与H2O加成反应的催化作用。结果表明,非催化反应存在双质子迁移和加成反应2条通道,其中加成反应为优势通道。其加成机理为H2O中OH加到CH3CHOO的α-C上,同时H2O中另一个H迁移到CH3CHOO的端O上。催化剂H2O及有机酸以氢键复合物的形式参与反应促进了H质子转移,可降低基元反应能垒和表观活化能,且催化效应与有机酸的强度成正比。例如,当分别用H2O(p Ka=15.7)、甲酸(p Ka=3.75)和草酸(p Ka=1.23)催化时,生成syn-HAHP的基元反应能垒由非催化的69.12 k J?mol-1分别降至40.78、18.88和10.61 k J?mol-1。非催化反应具有正的表观活化能,而所有催化反应则均具有负的表观活化能。

The catalytic effect of H2O and six kinds of organic acids （e.g., formic acid） on the reaction of CH3CHOO with H2O is studied at the CCSD（T）//B3LYP/6-311＋G（d,p） level. The results reveal that two possible channels exist as the double proton transfer and addition, of which the latter dominates for the non-catalytic reactions. For the additions, the OH of water is added to the α-C of CH3CHOO, and the H atoms migrate to the end oxygen atoms. Catalysts such as H2O and organic acid can form a hydrogen-bonded complex with CH3CHOO, which promotes the H transfer and thus significantly reduces the elementary reaction energy barrier and apparent activation energy when compared with that of the non-catalytic reaction. The catalytic effect is proportional to the strength of the organic acids. For example, for the formation of syn-HAHP catalyzed by H2O （pK, = 15.7）, formic acid （pK, = 3.75） and oxalic acid （pKa = 1.23）, the energy barrier is reduced from 69.12 to 40.78, 18.88 and 10.61 k J·mol -1, respectively. In addition, the non-catalytic reaction has a positive activation energy, whereas the catalytic reactions have an negative apparent activation energy.