Two-Dimensional Hydrogen-Bonding Patterns in a Series of Salts of Terephthalic Acid and the Cyclic Amines C_<i>n</i>H_{2<i>n</i>−1}NH₂, <i>n</i> = 3, 4, 5, 6, 7, 8, and 12

The assembly of two molecules into a specific dimensionality is one of the tenets to make new functional materials. Ammonium carboxylate salts, which make use of charge-assisted N−H···O hydrogen bonds, can form either one-dimensional or two-dimensional (1-D or 2-D) assemblies depending on the number of functional groups on the individual molecules. Terephthalic acid, with two carboxylic acid functional groups, and the cyclic amines with general formula CnH2n+1NH2 (n = 3, 4, 5, 6, 7, and 12), containing one amine functional group, undergo double proton transfer to form a 2-D network of hydrogen bonds. The hydrogen-bonded interactions between the functional groups result in ring-shaped patterns, and form two types of nodes between the cations and anions: Type II nodes have a repeating R43(10) ring, and type III nodes have alternating R42(8) and R44(12) rings. There are two types of 2-D hydrogen-bonded sheets, depending on the type of node used. The type V hydrogen-bonded sheet has nodes consisting of type II, and the type VI hydrogen-bonded sheet consists of type III nodes. Using the cation cyclopropylammonium and cycloheptylammonium, the type V assembly is observed, and with the cations cyclooctylammonium and cyclododecylammonium, the type VI assembly is observed. Cyclohexylammonium is a special case as it combines both types of assemblies into one structure. Cyclopropylammonium forms a different 2-D assembly, whereas the salt with cyclobutylammonium incorporates a water molecule to form a three-dimensional (3-D) assembly. The salt with cyclohexylammonium features a disorder of the cations, where the ammonium group occupies the equatorial and the less favored axial position in the ring. The frequency of the two types of 2-D assemblies is quantified using similar ammonium carboxylate salts retrieved from the Cambridge Structural Database.将两种分子组装为特定维度的结构，是制备新型功能材料的核心准则之一。利用电荷辅助N−H···O氢键的羧酸铵盐（ammonium carboxylate salts），可根据单个分子上的官能团数目，形成一维或二维（1-D或2-D）组装体。对苯二甲酸（terephthalic acid）带有两个羧酸官能团，而通式为CₙH₂ₙ₊₁NH₂（n=3、4、5、6、7、12）的环胺（cyclic amines）仅含一个胺基官能团，二者可发生双质子转移，形成二维氢键网络。官能团间的氢键相互作用会生成环状结构，并在阳离子与阴离子之间形成两类节点：II型节点带有重复出现的R43(10)环，III型节点则交替存在R42(8)与R44(12)环。根据所采用的节点类型，可分为两类二维氢键片层：V型氢键片层由II型节点构成，VI型氢键片层则由III型节点构成。使用环丙基铵（cyclopropylammonium）与环庚基铵（cycloheptylammonium）作为阳离子时，可观测到V型组装体；而使用环辛基铵（cyclooctylammonium）与环十二烷基铵（cyclododecylammonium）作为阳离子时，则会形成VI型组装体。环己基铵（cyclohexylammonium）属于特殊案例，其结构同时整合了两类组装体。环丙基铵会形成另一类独特的二维组装体，而环丁基铵（cyclobutylammonium）形成的盐会引入水分子，构筑出三维（3-D）组装体。环己基铵形成的盐存在阳离子无序现象，其铵基团在环结构中同时占据平伏位与能量更不占优的直立位。研究人员通过从剑桥结构数据库（Cambridge Structural Database）中检索得到的同类羧酸铵盐，对两类二维组装体的出现频率进行了定量分析。