Binding energies of hexahydrated alkaline earth metal ions, M2+(H2O)6, M = Mg, Ca, Sr, Ba: evidence of isomeric structures for magnesium.
Binding energies of hexahydrated alkaline earth metal ions, M2+(H2O)6, M = Mg, Ca, Sr, Ba: evidence of isomeric structures for magnesium.
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DOI：
10.1021/ja983232v
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发表时间：
1999-03
影响因子：
15
通讯作者：
S. E. Rodriguez-Cruz;R. A. Jockusch;E. Williams
中科院分区：
文献类型：
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作者：
S. E. Rodriguez-Cruz;R. A. Jockusch;E. Williams
为了更详细地了解散装溶液中的离子化学，通常采用的一种策略是研究气相溶剂化离子的结构和能量学。虽然在过去的二十年里，人们对单电荷金属离子的溶剂化进行了广泛的研究，但对双电荷和三电荷金属离子的溶剂化却知之甚少。溶剂化的二价金属离子现在可以很容易地产生使用电喷雾电离（ESI）。这引起了人们对通过实验3 - 5和理论获得这些离子的热化学信息的极大兴趣。Kebarle及其同事利用平衡实验测量了位于各种二价金属离子的第二溶剂化层的水分子的水合作用的吉布斯自由能最近，Posey及其同事将ESI与激光光破碎质谱结合，研究了第二溶剂化壳中与甲醇的二价过渡金属配体配合物利用黑体红外辐射解离（BIRD）实验测定了溶剂壳层水分子围绕Ni2+和Ca2+离子的结合能本文介绍了六水合碱土金属离子Mg2+、Ca2+、Sr2+和Ba2+的BIRD动力学。在低温下，从这些动力学数据得到的结合能与金属离子（Mg2+ > Ca2+ > Sr2+ > Ba2+）的半径直接相关。相反，在较高温度下，结合能遵循Ca2+ > Mg2+ > Sr2+ > Ba2+的趋势。这是水合二价金属离子存在两种不同气相结构的第一个直接证据。实验使用外部电喷雾电离源傅立叶变换质谱仪进行，该质谱仪已在前面描述过采用纳米电喷雾技术，从~10−4 M的金属氯盐水溶液中生成水合碱土M2+(H2O)n离子。离子被加载到电池中5 s，在此期间，氮气（10−6 Torr）被引入。在Sr2+ > Ca2+ > Mg2+压力下研究了大量选择离子的解离动力学。这一趋势与基于金属离子半径的预期反应性是一致的（附着在较大金属离子上的水分子的结合强度预计较低）。对于六水合离子，在低温下可以观察到相同的速率常数顺序。与此形成鲜明对比的是，高温下的顺序为Ba2+ > Sr2+ > Mg2+ > Ca2+；Mg2+(H2O)6的阿伦尼乌斯曲线的斜率在80°C左右变化！在高温下，五水合物的BIRD速率常数与六水合物（Ba2+ > Sr2+ > Mg2+ > Ca2+）的BIRD速率常数具有相同的变化趋势因此，这些解离动力学不遵循基于离子半径的预期趋势。Mg2+-(H2O)6的阿伦尼乌斯曲线斜率的变化表明该离子存在两种不同的气相结构。在所有温度下的一级行为（> - 80%前体离子耗尽）表明存在一种或几种相互转化的结构。图1六水合和七水合碱土金属离子的黑体红外辐射解离得到的阿伦尼乌斯图；Mg2+（〇）、Ca2+（•）、Sr2+(*)、Ba2+（）。由动力学数据主方程建模得到的实测零压极限Arrhenius参数和结合能Eo如表1所示。低温Eo值随着阳离子尺寸的减小而增加。在高温下，第6个水分子与Mg2+的结合能（21.1 kcal/mol）小于低温时的结合能（23.5 kcal/mol）。它也略低于较大的钙离子（21.6 kcal/mol）。对于Mg2+(H2O)6，为了拟合低温下的动力学数据，在模型中使用了异常大的辐射速率常数。其原因尚不清楚，但这表明低温下Eo的误差可能比报道的要大。高温下意想不到的结合能排序与Kebarle及其同事最近在170-225°c的平衡实验中报道的ΔH值一致。最近由Pavlov等人计算的密度功能理论（DFT）已被用于评估水合二价镁离子和钙离子M2+(H2O)m-(H2O)n的结构，其中m和n分别对应第一层和第二层的水分子数。这些结构的连续水结合能（ΔEbinding）也包含在表1中。为了进一步研究Mg2+(H2O)6的这种意外反应性，我们还对所有四种六水合二价金属进行了DFT计算研究了三种不同类型的结构，其中六个水分子要么全部位于第一壳层，要么分布在金属周围的第一和第二溶剂化壳层之间。使用Pavlov等人的命名法，研究的结构是M2+(H2O)6, M2+(H2O)5(H2O)，15和M2+(H2O)4(H2O)2。计算表明，所有金属的稳定性顺序为M2+(H2O)6 > M2+(H2O)5(H2O) > M2+(H2O)4(H2O)2。每种高能量结构与M2+(H2O)6构象之间的能量差如表2所示。表中还包括巴甫洛夫等人8对镁和钙的数值。在这个理论水平上，所有三种六水合Mg2+结构的能量都在5千卡/摩尔以内。六水合钡离子的几个类似的能量结构也被发现。然而，在这些实验的温度范围内，没有观察到多种气相六水合钡结构的证据。表2 B3LYP M2+(H2O)5（H2O）和M2+(H2O)4(H2O)2之间的能量差异和最稳定的M2+(H2O)6结构虽然实验证据表明Mg2+(H2O)6有两种不同的结构，但这些同分异构体的确切性质尚不清楚。一种可能是同分异构体的第一层和第二层有不同数量的水分子。DFT计算结果表明，所研究的三种六水合Mg2+结构的K能量均在5 kcal/mol以内（表2）。在标准条件下，Mg2+(H2O)5（H2O）和Mg2+(H2O) 4(H2O)2的熵大于Mg2+(H2O)6。Ca2+， Sr2+和Ba2+六水合物的情况正好相反。Mg2+(H2O)6的异常熵可能是由于6个水分子聚集在小得多的金属离子周围。这表明高温结构对应于双壳层结构。另一种可能的解释是其中一个同分异构体是盐桥结构（方案1）。这种盐桥结构在理论上的半经验PM3水平上是稳定的，但在理论上的密度泛函水平上则不稳定。形成盐桥所必需的质子转移过程预计在水合Mg中比在较大的金属水合物中更容易发生。综上所述，给出了Mg2+(H2O)6存在两种不同异构体结构的证据。这是水合金属指示物的两种异构体结构的第一个实验证据。水与Mg2+(H2O)7和Mg2+(H2O)6（低温）的结合能符合阳离子大小的预期趋势。Mg2+(H2O)6（高温）的这些值不存在。计算表明，M2+(H2O)6最稳定的结构是所有水分子都在内壳层的结构。然而，要明确地阐明这些结构，还需要进一步的实验和更高水平的计算。
One strategy that has commonly been used to gain a more detailed understanding of ion chemistry in bulk solution is to investigate the structure and energetics of solvated ions in the gas phase. While solvation of singly charged metal ions has been studied extensively during the last two decades,1 significantly less information about doubly and triply charged metal ions is known. Solvated divalent metal ions can now be readily produced using electrospray ionization (ESI).2–5 This has led to a significant interest in obtaining thermochemical information about these ions with both experiment3–5 and theory.6–8 Kebarle and co-workers measured the Gibbs free energies of hydration for water molecules located in the second solvation shell of a variety of divalent metal ions using equilibrium experiments.3 More recently, Posey and co-workers combined ESI with laser photofragmentation mass spectrometry to study divalent transition metal–ligand complexes with methanol in the second solvation shell.4 Binding energies of inner solvent shell water molecules around Ni2+ and Ca2+ ions have been determined from blackbody infrared radiative dissociation (BIRD) experiments.5 Here, BIRD kinetics of the hexahydrated alkaline earth metal ions, Mg2+, Ca2+, Sr2+, and Ba2+, are presented. At low temperatures, binding energies obtained from these kinetic data are directly correlated with the radii of the metal ions (Mg2+ > Ca2+ > Sr2+ > Ba2+). In contrast, the binding energies at higher temperatures follow the trend Ca2+ > Mg2+ > Sr2+ > Ba2+. This is the first direct evidence for two distinct gas-phase structures for a hydrated divalent metal ion. Experiments were performed using an external electrospray ionization source Fourier transform mass spectrometer that has been described previously.9 Hydrated alkaline earth M2+(H2O)n ions were generated from ~10−4 M aqueous solutions of the metal chloride salts using nanoelectrospray. Ions are loaded into the cell for 5 s during which time N2 gas (10−6 Torr) is introduced. Dissociation kinetics of the mass-selected ion are investigated at pressures Sr2+ > Ca2+ > Mg2+. This trend is consistent with the expected reactivity based on the ionic radii of the metals (water molecules attached to larger metal ions are expected to be less strongly bound). For the hexahydrated ions, this same ordering of rate constants is observed at low temperatures. In striking contrast, the order is Ba2+ > Sr2+ > Mg2+ > Ca2+ at higher temperatures; the slope of the Arrhenius plot for Mg2+(H2O)6 changes around 80°C! BIRD rate constants for the pentahydrated species have the same trend as that observed for the hexahydrated species at high temperature (Ba2+ > Sr2+ > Mg2+ > Ca2+).13 Thus, these dissociation kinetics do not follow the trend expected based on ionic radii. The change in the slope of the Arrhenius plot for Mg2+-(H2O)6 shows the presence of two distinct gas-phase structuresfor this ion. The first-order behavior (>80% precursor ion depletion) at all temperatures indicates the presence of one structure or several structures that interconvert. Figure 1 Arrhenius plots obtained from blackbody infrared radiative dissociation data of the hexa- and heptahydrated alkaline earth metal ions; Mg2+ (○), Ca2+ (•), Sr2+ (*), and Ba2+ (▵). The measured zero-pressure limit Arrhenius parameters and the binding energies (Eo) obtained from master equation modeling of the kinetic data are given in Table 1. The low-temperature Eo’s consistently increase with decreasing cation size. At high temperatures, the binding energy of the sixth water molecule to Mg2+ (21.1 kcal/mol) is less than that at low temperatures (23.5 kcal/mol). It is also slightly less than that for the larger calcium cation (21.6 kcal/mol). For Mg2+(H2O)6, anomalously large radiative rate constants were used in the modeling in order to fit the kinetic data at low temperature. The reason for this is not clear, but this suggests that the error in Eo at low temperature may be larger than reported. The unexpected ordering of binding energies at high temperature is consistent with ΔH values recently reported by Kebarle and co-workers from equilibrium experiments at 170–225°C.3a Recent density functional theory (DFT) calculations by Pavlov et al.8 have been used to evaluate structures for hydrated divalent magnesium and calcium ions, M2+(H2O)m-(H2O)n, where m and n correspond to the number of water molecules in the first and second shell, respectively. The successive water binding energies (ΔEbinding) reported for these structures are also included in Table 1. Table 1 Measured Zero-Pressure Limit Arrhenius Parameters and EO Values Obtained from Master Equation Modeling for Loss of Water from M2+(H2O)6 Ionsa To further investigate this unexpected reactivity of Mg2+(H2O)6, we also performed DFT calculations on all four hexahydrated divalent metals.14 Three different types of structures were studied in which the six water molecules are either all in the first shell or distributed between the first and second solvation shells around the metals. By the use of the nomenclature of Pavlov et al., the structures investigated were M2+(H2O)6, M2+(H2O)5(H2O),15 and M2+(H2O)4(H2O)2. Calculations indicate that the order of stability is M2+(H2O)6 > M2+(H2O)5(H2O) > M2+(H2O)4(H2O)2 for all of the metals. The energy differences between each of the higher energy structures and the M2+(H2O)6 conformation are reported in Table 2. Also included in the table are the values of Pavlov et al.8 for magnesium and calcium. At this level of theory, all three hexahydrated Mg2+ structures have energies within 5 kcal/mol. Several similar energy structures for hexahydrated barium ions were also found. However, no evidence of multiple gas-phase hexahydrated barium structures was observed over the temperature range of these experiments Table 2 B3LYP Energy Differences between M2+(H2O)5(H2O) and M2+(H2O)4(H2O)2 and the Most Stable M2+(H2O)6 Structurea Although the experimental evidence for two distinct structures for Mg2+(H2O)6 is clear, the exact nature of these isomers is not. One possibility is that the isomers have different number of water molecules in the first and second shell. The results of DFT calculations show that all three hexahydrated Mg2+ structures investigated have 0 K energies within 5 kcal/mol (Table 2). The entropies of Mg2+(H2O)5(H2O) and Mg 2+(H2O)4(H2O)2 under standard conditions are higher than that of Mg2+(H2O)6. The opposite is true for the Ca2+, Sr2+, and Ba2+ hexahydrates. The anomalous entropy for Mg2+(H2O)6 is presumably due to the crowding of the six water molecules around the much smaller metal ion. This would suggest that the higher temperature structure corresponds to a two-shell structure. Another possible explanation is that one of the isomers is a salt-bridge structure (Scheme 1). Such a salt-bridge structure was found to be stable at the semiempirical PM3 level of theory, but not with the density functional levels of theory applied. The proton-transfer process necessary for the formation of a salt-bridge is expected to occur more readily for hydrated Mg than for the larger metal hydrates.3b Scheme 1 In conclusion, evidence for two distinct isomeric structures of Mg2+(H2O)6 is presented. This is the first experimental evidence for two isomeric structures of hydrated metal dications. Binding energies of water to Mg2+(H2O)7 and Mg2+(H2O)6 (low temperature) follow the expected trend in cation size. These values for Mg2+(H2O)6 (high temperature) do not. Calculations suggest that the most stable structure for M2+(H2O)6 is one in which all of the water molecules are in the inner shell. However, further experiments and higher level calculations are necessary to definitively elucidate these structures.