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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Silicate Glass and Mineral Dissolution: Calculated Reaction Paths and Activation Energies for Hydrolysis of a Q~3 Si by H_3O~+ Using Ab Initio Methods
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Silicate Glass and Mineral Dissolution: Calculated Reaction Paths and Activation Energies for Hydrolysis of a Q~3 Si by H_3O~+ Using Ab Initio Methods

机译：硅酸盐玻璃和矿物质溶解度：从头算方法计算的H_3O〜+水解Q〜3 Si的反应路径和活化能

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Molecular orbital energy minimizations were performed with the B3LYP/6-31G(d) method on a [((OH)_3SiO)_3SiOH-(H_3O~+) centre dot 4(H_2O)] cluster to follow the reaction path for hydrolysis of an Si-O-Si linkage via proton catalysis in a partially solvated system. The Q~3 molecule was chosen (rather than Q~2 or Q~1) to estimate the maximum activation energy for a fully relaxed cluster representing the surface of an Al-depleted acid-etched alkali feldspar. Water molecules were included in the cluster to investigate the influence of explicit solvation on proton-transfer reactions and on the energy associated with hydroxylating the bridging oxygen atom (O_(br)). Single-point energy calculations were performed with the B3LYP/6-311+G(d,p) method. Proton transfer from the hydronium cation to an O_(br) requires sufficient energy to suggest that the Si-(OH)-Si species will occur only in-trace quantities on a silica surface. Protonation of the O_(br) lengthens the Si-O_(br) bond and allows for the formation of a pentacoordinate Si intermediate (~([5])Si). The energy required to form this species is the dominant component of the activation energy barrier to hydrolysis. After formation of the pentacoordinate intermediate,hydrolysis occurs via breaking the ~([5])Si-(OH)-Si linkage with a minimal activation energy barrier. A concerted mechanism involving stretching of the ~([5])Si-(OH) bond,proton transfer from the Si-(OH_2)~+ back to form H_3O~+,and a reversion of ~([5])Si to tetrahedral coordination was predicted. The activation energy for Q~3Si hydrolysis calculated here was found to be less than that reported for Q~3Si using a constrained cluster in the literature but significantly greater than the measured activation energies for the hydrolysis of Si-O_(br) bonds in silicate minerals. These results suggest that the rate-limiting step in silicate dissolution is not the hydrolysis of Q~3Si-Obr bonds but rather the breakage of Q~2 or Q~1Si-O_(br) bonds.
机译：使用B3LYP / 6-31G（d）方法在[[（（OH）_3SiO）_3SiOH-（H_3O〜+）中心点4（H_2O）]簇上进行分子轨道能量最小化，以追踪水解氢的反应路径。在部分溶剂化的系统中通过质子催化进行Si-O-Si连接。选择Q〜3分子（而不是Q〜2或Q〜1）来估计完全弛豫的簇的最大活化能，该簇代表贫铝酸蚀长石的表面。将水分子包括在该簇中，以研究显式溶剂化对质子转移反应以及与桥接氧原子（O_（br））羟基化相关的能量的影响。用B3LYP / 6-311 + G（d，p）方法进行单点能量计算。质子从水合氢阳离子转移到O_（br）需要足够的能量，以表明Si-（OH）-Si物种只会在二氧化硅表面上以痕量出现。 O_（br）的质子化延长了Si-O_（br）键，并允许形成五配位的Si中间体（〜（[5]）Si）。形成该物质所需的能量是活化能屏障对水解的主要组成部分。在形成五配位中间体之后，通过以最小的活化能垒破坏〜（[5]）Si-（OH）-Si键来进行水解。协同机制涉及拉伸〜（[5]）Si-（OH）键，质子从Si-（OH_2）〜+转移回形成H_3O〜+，以及〜（[5]）Si还原为四面体的协调被预测。使用文献中的约束簇，发现此处计算的Q〜3Si水解活化能小于Q〜3Si报道的活化能，但显着大于测得的硅酸盐中Si-O_（br）键水解的活化能。矿物质。这些结果表明，硅酸盐溶解的限速步骤不是Q〜3Si-Obr键的水解，而是Q〜2或Q〜1Si-O_（br）键的断裂。

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