Temperature and Density Effects on an SN2 Reaction in Supercritical Water
Temperature and Density Effects on an SN2 Reaction in Supercritical Water
复制标题
温度和密度对超临界水中 SN2 反应的影响
DOI：
10.1021/j100014a047
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发表时间：
1995
期刊：
影响因子：
--
通讯作者：
P. Rossky
中科院分区：
文献类型：
--
作者：
L. Flanagin;P. Balbuena;K. Johnston;P. Rossky
分子动力学计算机模拟是用来与热力学性质沿着反应坐标微观溶剂化的氯离子与氯甲烷的Sn 2反应作为温度和密度的函数。极端条件（例如，在1.0和1.3的温度下，密度分别降低到0.05和0.3）被认为是去除氯离子第一溶剂化层中一半水分子所必需的。随着温度的升高和密度的降低，Cl--水氢键的数目衰减得比配位数快。通过与吸附现象的类比，在三个区域中解释了局部溶剂密度相对于本体（聚集）的增大，这三个区域分别对应于气体、近临界和类液体密度。发现在第一配位球中的水分子的寿命比在环境条件下短约4倍; Cl-和水之间的氢键的寿命减少约6倍。的值，AE，和-TAS与从反应态到过渡态的转换解释的variationin平均配位数和氢键。温度变化和溶剂化作用的综合影响导致超临界条件下的速率常数比环境条件下增加9-12个数量级。
Molecular dynamics computer simulation is used to relate the thermodynamic properties along the reaction coordinate to microscopic solvation for the Sn2 reaction of the chloride ion with methyl chloride as a function of temperature and density. Extreme conditions (eg reduced densities of 0.05 and 0.3 for reduced temperatures of 1.0 and 1.3, respectively) are found to be necessary to remove half of the water moleculesin the first solvation shell aboutthe chloride ion. As the temperature is increased and density decreased, the number of Cl--water hydrogen bonds decays faster than the coordination number. By analogy to adsorption phenomena, augmentation in the local solvent density relative to the bulk (clustering) is interpretedin three regions corresponding to gas, near-critical, and liquid-like densities.· The lifetime of a water molecule in the first coordination sphere is found to be about 4 times shorter than under ambient conditions; the lifetime of a hydrogen bond between Cl-and water decreases by a comparable factor of about 6. The values of, AE, and—TAS associated with conversion from the reactantstate to the transition state are explained in terms of the variationsin the average coordination numbers and hydrogen bonding. The combined effects of changes in temperature and solvation leadto an increase in the rate constant by 9-12 orders of magnitude under supercritical conditions compared to ambient conditions.