标题：水簇
1．水簇
水簇是两个或多个水分子通过氢键组装形成的具有特定构型和拓扑模式的聚集体。水在许多生命和化学过程中起着十分重要的作用。水的化学组成十分简单：一个氧原子和两个氢原子所组成的分子型化合物。然而它却体现出很多不同于其他化合物的物理化学性质。例如，根据热力学计算，以单分子存在的水其熔点和沸点应是-110 C和-85 C，远小于实际的0 C和100 C，由于水异常的熔沸点等特殊性质，人们推测在自然状态下水并非以单分子存在，而应以分子簇的形式存在。但直到1977年，Deky等利用红外光谱测试仪才首次证实了二元水簇的存在。
水是大量水分子的氢键聚集体。每一个水分子有两个氢原子和两对孤对电子，可以与其相邻的水分子形成二个、三个或四个氢键。这些氢键作用的详细结构信息是水的研究的关键之一。然而，由于水的微观复杂性，难以量化其结构信息，这给水的定量研究带来很大困难。水簇作为大量液态水的简化，可以为液体水性质的研究提供有效的途径。水簇的研究不仅可以了解水的一些不同寻常的行为，还为了探索水分子在生物体内的存在形式与所起到的作用以及设计新颖的功能材料。同时，水簇的研究也有助于揭开诸如酸雨的形成、云层的吸收和雨滴的成核等众多自然现象的谜团。
人们通过理论模拟和光谱实验方法对水簇进行了研究，可以揭示液态水分子簇存在的结构、稳定能量、力学机制等特点。近年来，随着X-射线衍射技术的进步，人们可以通过单晶衍射仪精确地测定晶格中水分子的位置，从而精确的描述水分子之间的氢键作用的各项参数，能够更清楚的了解水的各种物理化学行为。一般来说，水分子簇不可能孤立的存在，而是作为客体分子存在于某一种主体结构中。
2．不同结构的水簇
截至目前为止，已有很多例不同结构的水簇被表征，按照构成水簇的水分子个数，水簇可以分为小分子水簇、一维水簇、二维水簇和三维水簇。
2.1 小分子水簇小分子水簇也称低聚水簇，指的是由个数有限的水分子通过氢键形成孤立存在的簇状聚集体，簇与簇之间并没有通过氢键相连。
在目前所报道的小分子水簇中，被关注的较多是四元水簇与六元水簇。四元水簇是液态水和固态冰的二维模型中最简单的特例。如图1(a)所示，由于氢键的模式不同，四个水分子具有五种不同的连接模式，包括环状、心形、手鼓形、四面体状和线形。其中四元水环的能量更低，也更容易形成，因此所报道的四元水簇中，四元环状模式是最常见的。而与四元水簇不同，六元水簇代表了水簇从二维跨入三维。理论计算表明六元水分子簇可以有环状、书状、袋装、笼状和棱柱状等几种构型，其能量差异在0.7kcal/mol以内，如图1(b)所示。笼状是能量最低最稳定的一种构型，随后是书状和棱柱状，它们比笼状分别高0.1和0.2 kcal/mol；能量较高的是环状与袋装，它们比笼状分别高0.5 和0.7 kcal/mol。在所报道的水簇合物中，前两种居多，特别是以环状六元水簇最为常见。
  
图1 (a) 四元水簇的五种不同的连接模式(氢原子被忽略)；
(b) 六元水簇能量较低的五种构型
除了(H2O)4和(H2O)6以外，已有报道的小分子水簇还有(H2O)2 、(H2O)3 、(H2O)5 、(H2O)7 、(H2O)8 、(H2O)9 、(H2O)10 、(H2O)11 、(H2O)12 、(H2O)14 、(H2O)16 、(H2O)17 、(H2O)18 、(H2O)26 、(H2O)27等等。目前报道的最大的小分子水簇(H2O)27是在配合物[Co4(dpdo)12][H(H2O)27(CH3CN)12][PW12O40]3中发现的，它以质子化的形式H(H2O)27存在。如图2所示，[Co4(dpdo)12]和[PW12O40]离子形成一具有空洞的三维骨架，每一个空洞中，共存有12个(CH3CN)12分子和27个结晶水分子。其中26个水分子通过氢键相连形成(H2O)26笼状水簇。而剩下的一个水分子(O4W)进入(H2O)26水笼中并质子化，形成H(H2O)27离子。
  
图2. (a) H(H2O)27通过氢键主体与连接; (b) H(H2O)27内部连接模式
2.2 一维水簇一维水簇介于二维水簇和小分子水簇之间，可以认为是二维水簇的最简模型，同时也比小分子水簇更接近于固态冰或液态水的结构。它在某些生命和化学过程扮演重要角色。一维水簇可以分为水链、水带和水柱。一般水链指水分子首尾直接相连而形成的一维水簇，而水带则由一种或几种环状小分子水簇通过共用水分子相连而成，除一维无限外，另一维具有一定的宽度；如果水分子排列时，一维无限同时另两维均具有一定宽度，则称之为水柱。不过，由于近年所报道的一维水簇中存在不少结构比较复杂难以归属的模式，所以这样的分类其实是比较模糊的。
  
图3. (a) 一维带冠四聚体螺旋水带的内部连接模式；(b) 一维带冠四聚体螺旋水带连接主体的二维配位聚合层形成砖墙式超分子结构。
许多一维水簇显示了精美的结构。例如，A. D. Jana等人制备了配聚物{[Cu(mal)2](picH)2·5H2O}n（mal为丙二酸，pic为2 -氨基- 4 -甲基吡啶），其中含有一个带冠的四聚体螺旋水带。如图3所示，在该一维水簇中，相邻的四元水环通过共用一个水分子（O6）并扭转成86.44°的夹角相连，沿着c轴形成一维带状结构，而每个四元水环上下各接一个水分子（O5）形成冠状结构。而铜离子和丙二酸根通过配位间形成二维层状，该一维水簇通过氢键连接相邻的二维层形成三维的砖墙式超分子结构。
2.3 二维水簇从结构上看，二维水簇比一维水簇更接近于大量液态水或固态冰。例如在配合物Cd(H2O)2Ni(CN)4·4H2O存在二维水层，水分子中O···O的距离为2.821(9)和2.86(2) &Aring;，与液态水的相应距离相当(2.85&Aring;)。而在有机水合物bpedo·5H2O（bpdeo为反式-1,2-双(4-氧化吡啶基)-乙烯）存波浪形的在二维水层，其中O···O的平均距离为2.748 &Aring; (90K)和2.776 &Aring; (295K)，非常接近于固体冰Ih中的相应距离(2.759 &Aring;)。
某些二维水簇中可能形成很大的孔穴，如R. Carballo等报道了含有(H2O)18水环的大孔水层结构。如图4所示，该水层存在于配位聚合物[Cu(Hmal)(4pds)] ·6H2O中(4pds为4,4’-二硫代联吡啶，mal为苹果酸)，水层中含有有18个水分子形成的巨大水环，其平均孔径达11.76 &Aring;。该大孔水环和[Cu(Hmal)(4pds)]的二维配聚物主体形成了相互穿插的超分子结构。
  
图4. (a) 含有(H2O)18水环的二维水簇; (b) 二维水簇和主体相互穿插。
2.4 三维水簇三维水簇十分罕见，图5为仅有报道的三维水簇示意图。此三维水簇存在于下面的离子配合物中：[Zn(HL)(phen)2][Zn(L)(HL)(phen)]·13H2O [H2L = 甲基乳酸(methyllactic acid)；phen = 1,10-邻菲啰啉]。在此配合物中包含[Zn(L)(HL)(phen)]负离子, [Zn(HL)(phen)2]正离子和13个结晶水分子，两种不同的阴阳离子通过H键和π-π相互作用形成一维链，诱导水分子分布于其周围形成3D水簇结构。
  
图5. 在[Zn(HL)(phen)2][Zn(L)(HL)(phen)]·13H2O中三维水簇
2.5 水簇的拓扑模式随着越来越多的水簇被测定与表征，人们开始考虑如何来界定和描述水簇内部水分子的连接方式。L. Infantes等人重新检视剑桥晶体学数据库(CCDC)中的结构，发现在先前所合成的许多水合物中同样包含有各式各样的水簇，但这些化合物在最初报道时并没有提及水簇的存在。基于对CCDC数据库中水簇模式的分析，他们提出采用拓扑模型来描述水簇，这一方法主要将水簇分为五种不同的模式：(a)离散链状，代号为Dn，其中D为Discrete Chains，n为水簇内所形成的链具有的最大水分子个数。在Dn中，水分子呈线性连接模式，不包含环，同时也不是无限连接的；如果水分子存在分叉，则以所形成的最长链的水分子个数计为n。(b)离散环状，代号为Rn，其中R为Discrete Rings，n为水簇内所形成的环具有的最大水分子个数。在Rn中，水分子也是有限连接的，形成单个的环状形式。(c)一维无限链状，代号为Cn，其中C为Infinite Chains，n为所形成的一维无限链最小重复单元的水分子个数。(d)一维无限带状，代号为Tn(m)Ak，其中T为Infinite Tapes，n为形成的环所包含的水分子个数，m为环与环之间共用的水分子个数，Ak指无交集环之间所间隔的水分子个数。(e)二维无限层状，代号为Lm(r)n(s)···，其中L为Infinite Layers，m，n为平面内形成的环包含的水分子个数，r，s为环外与之有共用水分子的环的个数。
上述五种模式中，a、b为小分子水簇，c、d为一维水簇，e为二维水簇。虽然它们是基于有机宿主中水簇分析后得到的，但应可以被应用到不同主体中水簇的分析中。在前两种模式中，即线形和环状模式中，由于在定义水簇模式时，只考虑成链或成环所具有的最大水分子个数，因而会出现某些三元、四元和五元水簇均为D3模式，无法体现它们的差异。因此，袁良杰课题组提出了一种改进的命名规则：借鉴有机化合物中烷烃和环烷烃的命名规则，以最长的链或最大的环为水簇的主链(环)，而其它的水分子则看成是取代基，引入m(r)n(s)···Dn和m(r)n(s)···Rn来修订水簇的模式，这里m，n为取代水分子在水簇的主链(环)的取代位置，r，s为取代水分子的个数。例如，上述三种D3模式的水簇依次可以分别定义为：D3、2(1)D3和2(1)2(1)D3。另外，这种修订可以被扩展到一维链状模式的水簇中。如一维C3水链中最小重复单元为三个水分子，如果在第一个水分子上连接一个水分子，则可以定义为1(1)C3。类似的，如果一个水分子连接与第二个水分子，则可以定义为2(1)C3；如果第一个和第三个水分子分别连接一个水分子，则可以定义为1(1)3(1)C3。尽管这种改进的命名规则能够描述更多水簇的构成形式，但由于实际水分子连接模式的复杂性，随着更多复杂的水簇被合成和表征，很多水簇无法用上述简单的符号来定义。
3．主体设计与水簇
目前所报道的水簇大多存在于一些特殊的主体结构中，即水簇是作为主体结构的副产物被表征的，而关于水簇的结构与主体结构相关性的报道则较少涉及。例如图6所示，在有机化合物3-(4 -(1 H -咪唑-1-基)甲基苯基)丙烯酸及其衍生酯类为主体的水合物中，若主体为羧酸时，不能形成水簇结构。而若羧基上的氢原子被甲基所取代时，形成一维C2水链结构。当羧基上的氢原子被乙基所取代时，形成一维T4(2)水带结构。在这些水合物中，尽管主体的堆积模式相似，但由于取代基的不同，使得在主体堆积时所形成的一维纳米孔道内部微环境不同，进而导致形成不同结构的一维水簇。这样，通过调控主体有机分子上的取代基，就可以设计合成不同形貌的水簇结构。
  
图6. 通过调控有机主体上的取代基获得的不同一维水簇
中山大学的陈小明等通过通过选择相似的次级结构单元即二聚金属离子[Cd2(bpa)2]作为节点（bpa 为N,N’ -二(吡啶酰胺)吖嗪），以不同的羧酸配体作为连接，构筑出4个具有不同拓扑结构的配位聚合物。在这些配位聚合物主体中，存在着结构各异的一维水簇。而这些不同结构水簇的形成依赖于合适的次级结构单元和不同有机羧酸配体连接所成的配位聚合物主体。
  
图7 通过调节膦酸抗衡离子构造不同拓扑模式的一维水链
袁良杰课题组对以有机膦酸为主体构筑的水簇结构进行了详细研究，分别以磷酸、1-氨基-1, 1-乙叉双膦酸(AEDPH4)和1-氨基-1, 1, 3-丙叉三膦酸(APTPH6)为前驱物，通过[Ni(Im)6]阳离子和不同的膦酸阴离子进行氢键自组装，合成和表征了三种新的有机磷酸金属配合物。从这三个配合物中分别得到不同的一维水链结构：C8螺旋状水链，C4锯齿状水链，以及1(1)8(1)C14波状水链。这样，通过调节膦酸抗衡离子的结构，能获得不同结构的主体，从而可以构造出具有不同拓扑模式的水簇（见图7）。此外，陈硕平等首次发现了氨基双膦酸在水热条件和金属离子的催化下发生原位缩合反应，获得一类结构新颖的的双轮膦酸配体，并以这类双轮膦酸为前驱物，合成和表征了4个分别包含有复杂的一维T4(3)5(0)A0水柱、一维(H2O)24水柱、二维L5(4)5(5)15(14)水网和一维(H2O)36水柱的配合物。在配合物[Zn3(L1H)2(2,2'-bipy)3]·18H2O中，罕见的一维(H2O)36水柱和以双轮膦酸为主体构筑的一维配聚物链形成垂直穿插。由于双轮膦酸较氨基双膦酸有更多亲水的膦氧基团，因此形成的水簇的拓扑模式更为复杂，另外通过局部调节配合物中的某一个离子或配体结构，也能够改变水簇的拓扑模式。
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