Tunell Ingvar, Lim Carmay
Institute of Biomedical Sciences, Academia Sinica, Taipei 115, Taiwan.
Inorg Chem. 2006 Jun 12;45(12):4811-9. doi: 10.1021/ic0519741.
Many of the group IA and IIA metal ions, such as Na+, K+, Mg2+, and Ca2+, play crucial roles in biological functions. Previous theoretical studies generally focus on the number of water molecules bound to a particular (as opposed to all) alkali or alkaline earth cations and could not establish a single preferred CN for the heavier alkali and alkaline earth ion-water complexes. Crystal structures of hydrated Na+, K+, and Rb+ also cannot establish the preferred number of inner-shell water molecules bound to these cations. Consequently, it is unclear if the gas-phase CNs of group IA metal hydrates increase with increasing ion size, as observed for the group IIA series from the Cambridge Structural Database, and if the same factors govern the gas-phase CNs of both group IA and IIA ion-water complexes. Thus, in this work, we determine the number of water molecules directly bound to the series of alkali (Li+, Na+, K+, and Rb+) and alkaline earth (Be2+, Mg2+, Ca2+, Sr2+, and Ba2+) metal ions in the gas phase by computing the free energy for forming an isolated metal-aqua complex as a function of the number of water molecules at 298 K. The preferred gas-phase CNs of group IA hydrates appear insensitive to the ion size; they are all 4, except for Rb+, where a CN of 6 seems as likely. In contrast, the preferred gas-phase CNs of the group IIA dications increase with increasing ion size; they are 4 for Be2+, 6 for Mg2+ and Ca2+, and 7 for Sr2+ and Ba2+. An entropic penalty disfavors a gas-phase CN greater than 4 for group IA hydrates, but it does not dictate the gas-phase CNs of group IIA hydrates. Instead, interactions between the metal ion and first-shell water molecules and between first-shell and second-shell water molecules govern the preferred gas-phase CNs of the group IIA metal hydrates.
许多第IA族和第IIA族金属离子，如Na⁺、K⁺、Mg²⁺和Ca²⁺，在生物功能中起着关键作用。先前的理论研究通常聚焦于与特定（而非所有）碱金属或碱土金属阳离子结合的水分子数量，且无法为较重的碱金属和碱土金属离子 - 水络合物确定单一的优选配位数。水合Na⁺、K⁺和Rb⁺的晶体结构也无法确定与这些阳离子结合的内壳层水分子的优选数量。因此，尚不清楚第IA族金属水合物的气相配位数是否会像从剑桥结构数据库观察到的第IIA族系列那样随离子尺寸增大而增加，以及相同的因素是否支配第IA族和第IIA族离子 - 水络合物的气相配位数。因此，在本工作中，我们通过计算在298 K下形成孤立金属 - 水络合物的自由能作为水分子数量的函数，来确定气相中与一系列碱金属（Li⁺、Na⁺、K⁺和Rb⁺）和碱土金属（Be²⁺、Mg²⁺、Ca²⁺、Sr²⁺和Ba²⁺）金属离子直接结合的水分子数量。第IA族水合物的优选气相配位数似乎对离子尺寸不敏感；除了Rb⁺的配位数似乎可能为6外，其他的都是4。相比之下，第IIA族二价阳离子的优选气相配位数随离子尺寸增大而增加；Be²⁺为4，Mg²⁺和Ca²⁺为6，Sr²⁺和Ba²⁺为7。熵罚不利于第IA族水合物的气相配位数大于4，但它并不决定第IIA族水合物的气相配位数。相反，金属离子与第一壳层水分子之间以及第一壳层和第二壳层水分子之间的相互作用支配着第IIA族金属水合物的优选气相配位数。
