Hydration energies and structures of alkaline earth metal ions, M2+(H2O)n, n=5-7, M = Mg, Ca, Sr, and Ba
Hydration energies and structures of alkaline earth metal ions, M2+(H2O)n, n=5-7, M = Mg, Ca, Sr, and Ba
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DOI：
10.1021/ja9911871
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发表时间：
1999-09-29
影响因子：
15
通讯作者：
Williams, ER
中科院分区：
文献类型：
--
作者：
Rodriguez-Cruz, SE;Jockusch, RA;Williams, ER
用傅里叶变换质谱仪研究了水化碱土金属离子的蒸发过程。利用黑体红外辐射离解法，测量了水合二价金属离子M2+（H2O）（n）（M = Mg，Ca，Sr，n = 5-7，M = Ba，n = 4-7）中单个水分子的零压极限离解速率常数随温度的变化。从这些值中，零压力极限Arrhenius参数获得。通过使用主方程形式对解离动力学进行建模，确定阈值解离能（E-O）。这些反应应该具有可忽略的反向活化势垒;因此，E-0值应该近似等于0 K时的结合能或水合焓。对于七水和六水离子在低温下，结合能遵循离子半径的基础上预期的趋势：Mg > Ca > Sr > Ba。高温下六水合离子的结合能大小顺序为Ca > Mg > Sr > Ba。对于五水合离子观察到相同的顺序。四水合物质的碰撞解离实验导致与金属的大小直接相关的相对解离速率。这些结果表明存在两种异构体的六水合镁离子：低温异构体，其中六个水分子位于第一溶剂化壳层，和高温异构体的最可能的结构对应于四个水分子的内壳层和两个水分子的第二壳层。这些结果还表明，五水合镁离子具有在第一溶剂化壳层中具有四个水分子和在外壳层中具有一个水分子的结构。Ca ~（2+）、Sr ~（2+）和Ba ~（2+）的六水和五水团簇的离解动力学与所有水分子都位于第一溶剂化层的结构一致。
The evaporation of water from hydrated alkaline earth metal ions, produced by electrospray ionization, was studied in a Fourier transform mass spectrometer. Zero-pressure-limit dissociation rate constants for loss of a single water molecule from the hydrated divalent metal ions, M2+(H2O)(n) (M = Mg, Ca, and Sr for n = 5-7, and M = Ba for n = 4-7), are measured as a function of temperature using blackbody infrared radiative dissociation. From these values, zero-pressure-limit Arrhenius parameters are obtained. By modeling the dissociation kinetics using a master equation formalism, threshold dissociation energies (E-0,) are determined. These reactions should have a negligible reverse activation barrier; therefore, E-0 values should be approximately equal to the binding energy or hydration enthalpy at 0 K. For the hepta- and hexahydrated ions at low temperature, binding energies follow the trend expected on the basis of ionic radii: Mg > Ca > Sr > Ba. For the hexahydrated ions at high temperature, binding energies follow the order Ca > Mg > Sr > Ba. The same order is observed for the pentahydrated ions. Collisional dissociation experiments on the tetrahydrated species result in relative dissociation rates that directly correlate with the size of the metals. These results indicate the presence of two isomers for hexahydrated magnesium ions: a low-temperature isomer in which the six water molecules are located in the first solvation shell, and a high-temperature isomer with the most likely structure corresponding to four water molecules in the inner shell and two water molecules in the second shell. These results also indicate that the pentahydrated magnesium ions have a structure with four water molecules in the first solvation shell and one in the outer shell. The dissociation kinetics for the hexa- and pentahydrated clusters of Ca2+, Sr2+ and Ba2+ are consistent with structures in which all the water molecules ale located in the first solvation shell.