Nucleophilic substitution reaction mechanisms: An atomic-molecular perspective on chemical speciation and transport properties in silicate melts
Nucleophilic substitution reaction mechanisms: An atomic-molecular perspective on chemical speciation and transport properties in silicate melts
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亲核取代反应机制：硅酸盐熔体中化学形态和输运特性的原子分子视角
DOI：
10.1016/j.chemgeo.2020.119818
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发表时间：
2020
期刊：
影响因子：
3.9
通讯作者：
G. Henderson
中科院分区：
文献类型：
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作者：
H. Nesbitt;G. Bancroft;G. Henderson
尽管亲核取代反应通常发生在含有C、Si、P和Ge中心四面体的气相和液相中，但目前还没有系统的尝试来确定亲核取代（SN）反应是否在硅酸盐熔体中进行。在这里，通过提供这样的分析来纠正这种疏忽。发生亲核取代反应所需的条件有：(1)亲核试剂（路易斯碱）的存在，在硅酸盐熔体中有桥氧（BO）、非桥氧（NBO -）和自由氧（O2 -）；(2)具有强亲电中心（即Si原子）的四面体（Q种）的存在；(3)含有五面体配位Si的Si过渡种（即VSi种）的存在；(4)四面体物种间反应速率较快。硅酸盐熔体满足所有条件。例如，在高温下，强亲核试剂NBO-存在于二元碱和碱土硅酸盐熔体中，主要是由于热搅拌，一些Si-NBO- m键断裂，产生亲核的Si-NBO-部分。该亲核试剂攻击相邻四面体的Si中心，形成单键do键，从而产生aVSi过渡物质。过渡种通过断裂位于过渡种极性对面的另一个单键do键而分解。已知三种类型的snr反应均涉及evsi过渡种。它们是NBO-BO交换反应（如Q3+ Q4→Q4+ Q3）、歧化反应（如2Q3→Q4+ Q2）和聚合反应（如Q3→Q4+ 1/2O2−）。H2O和OH−也是亲核试剂，它们与Q的反应是通过snr反应机制进行的，并可能导致熔体解聚。也可能发生H2O与BO和NBO的氢键，如在冰和水中，从而提高熔体中H2O的溶解度。后一种反应既不会使熔体解聚，也不会影响NBO/T值。熔体中Si和O的扩散率、阴离子电导率和化学形态（如Q物质丰度）是通过一种或另一种snc反应进行的，其中过渡物质在扩散率、电导率和粘度中起着关键作用。结合过渡态理论，snc反应机理解释了：(1)五面体配位硅（VSi）的形成及其对高硅玻璃的明显限制；(2)硅和氧在硅酸盐熔体中的扩散系数非常相似；(3) Eyring方程的“跳跃距离”（α），通过sn机制为~3.5 Å（即vsi过渡种的直径）；(4) Adam-Gibbs方程“合作区域”中涉及的最小单体单位数（即Q种）；(5)高达~50 mol% Na2O的Na硅酸盐玻璃中Q种的定量分布。
There has been no systematic attempt to determine if nucleophilic substitution (SN) reactions proceed in silicate melts, even though they commonly occur in gaseous and liquid phases containing C, Si, P, and Ge centered tetrahedra. The oversight is here rectified by providing such an analysis. Conditions required for nucleophilic substitution reactions to occur are: (1) the presence of nucleophiles (Lewis bases) which in silicate melts are bridging oxygen (BO), non-bridging oxygen (NBO−) and free oxygen (O2−); (2) the presence of tetrahedra (Q species) with strongly electrophilic centers (i.e., Si atoms); (3) the presence of Si transition species containing pentahedrally coordinated Si (i.e.,VSi species); (4) rapid reaction rates among tetrahedral species. All conditions are met for silicate melts. For example, the strong nucleophile, NBO−exists at high temperatures in binary alkali and alkaline earths silicate melts due primarily to thermal agitation whereby some Si-NBO-M bonds are ruptured to produce the nucleophilic Si-NBO−moiety. This nucleophile attacks the Si center of an adjacent tetrahedron to form a Sisingle bondO bond thereby producing aVSi transition species. The transition species decomposes by rupture of another Sisingle bondO bond located on the polar opposite side of the transition species. Three types of SNreaction are recognized and all involveVSi transition species. They are NBO-BO exchange reactions (e.g., Q3+ Q4→ Q4+ Q3), disproportionation reactions (e.g., 2Q3→ Q4+ Q2) and polymerization reactions (e.g., Q3→ Q4+ 1/2O2−). H2O and OH−are also nucleophiles and their reaction with Q species proceeds via SNreaction mechanisms, and may cause depolymerization of melts. Hydrogen bonding of H2O to BO and NBO may also occur, as in ice and water, thereby enhancing H2O solubilities in melts. These latter reactions should neither depolymerize melts nor affect NBO/T values.The diffusivity of Si and O in melts, anionic conductivity and chemical speciation (e.g. Q species abundances) proceed via one or other SNreaction, with the transition species assuming a critical role in diffusivity, conductivity and viscosity. The SNreaction mechanism, coupled with transition state theory, provides explanations for: (1) the formation of pentahedrally coordinated Si (VSi) and its apparent restriction to highly siliceous glasses; (2) the remarkably similar diffusivities of Si and O in silicate melts; (3) the ‘jump distance’ (α) of the Eyring equation, which by the SNmechanism is ~3.5 Å (i.e., diameter of theVSi transition species); (4) the minimum number of monomeric units (i.e., Q species) involved in the ‘cooperative region’ of the Adam-Gibbs equation; and (5) the quantitative distribution of Q species in Na silicate glasses up to ~50 mol% Na2O.
常压硅酸钾和硅酸锂玻璃中的五配位硅：温度和成分影响以及与碱金属硼酸盐和锗酸盐体系的类比
影响因子：
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作者：
Stebbins, Jonathan F.
通讯作者：
Stebbins, Jonathan F.