Theoretical characterizations of the mechanism for the dimerization of monosilicic acid in basic solution.
Theoretical characterizations of the mechanism for the dimerization of monosilicic acid in basic solution.
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DOI：
10.1039/c3cp52117f
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发表时间：
2013-08
期刊：
影响因子：
--
通讯作者：
Hui Hu;Hua Hou;Zhen He;Baoshan Wang
中科院分区：
文献类型：
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作者：
Hui Hu;Hua Hou;Zhen He;Baoshan Wang
采用各种从头算方法，对单硅酸在碱性水溶液中初等二聚化反应的阴离子机理进行了全面表征。本文提出了许多关于硅酸盐低聚反应的新见解，该反应在溶胶-凝胶化学、沸石合成和水泥水化中具有重要意义。考虑到沿反应路线有数百种不同分子间和分子内氢键模式的构象，提出了二聚化反应的构象依赖性。揭示了五配位硅中间体的另一种水解理途径。绕道涉及六中心循环过渡态，这在能量上比众所周知的四中心除水步骤更可取。通过加入明确的水分子，可以大大降低四中心水裂解路径的激活势垒，甚至低于Si-O键形成的第一个势垒。相反，由于不利的几何畸变，额外的水分子对六中心绕道的影响较小。对二聚化机理的新认识可能对二氧化硅成核的初始阶段产生重大影响。
The anionic mechanisms for the elementary dimerization reaction of monosilicic acid in basic aqueous solution have been characterized comprehensively using various ab initio methods. Many new insights into the silicate oligomerization reaction, which is fundamentally important in sol-gel chemistry, zeolite synthesis, and cement hydration, are presented in this work. Conformational dependence of the dimerization reaction is proposed in view of hundreds of conformations with various inter- and intramolecular hydrogen bonding patterns along the reaction routes. An alternative water cleavage route from the five-coordinated silicon intermediate is revealed. The detour involves a six-center cyclic transition state, which is more preferable energetically than the well-known four-center water removal step. By including explicit water molecules, the activation barrier of the four-center water cleavage path can be reduced considerably to be even lower than the first barrier of the Si-O bond formation. In contrast, the six-center detour is less affected by the additional water molecules due to the unfavorable geometric distortion. The new understanding of the dimerization mechanism could have considerable impact on the initial stages of silica nucleation.