Institut für Chemie, Abteilung Physikalische und Theoretische Chemie, Universität Rostock, 18059 Rostock, Germany.
Department Life, Light & Matter, Universität Rostock, Albert-Einstein-Str. 25, 18059 Rostock, Germany.
Molecules. 2020 Oct 27;25(21):4972. doi: 10.3390/molecules25214972.
We explore quantum chemical calculations for studying clusters of hydroxyl-functionalized cations kinetically stabilized by hydrogen bonding despite strongly repulsive electrostatic forces. In a comprehensive study, we calculate clusters of ammonium, piperidinium, pyrrolidinium, imidazolium, pyridinium, and imidazolium cations, which are prominent constituents of ionic liquids. All cations are decorated with hydroxy-alkyl chains allowing H-bond formation between ions of like charge. The cluster topologies comprise linear and cyclic clusters up to the size of hexamers. The ring structures exhibit cooperative hydrogen bonds opposing the repulsive Coulomb forces and leading to kinetic stability of the clusters. We discuss the importance of hydrogen bonding and dispersion forces for the stability of the differently sized clusters. We find the largest clusters when hydrogen bonding is maximized in cyclic topologies and dispersion interaction is properly taken into account. The kinetic stability of the clusters with short-chained cations is studied for the different types of cations ranging from hard to polarizable or exhibiting additional functional groups such as the acidic C(2)-H position in the imidazolium-based cation. Increasing the alkyl chain length, the cation effect diminishes and the kinetic stability is exclusively governed by the alkyl chain tether increasing the distance between the positively charged rings of the cations. With adding the counterion tetrafluoroborate (BF) to the cationic clusters, the binding energies immediately switch from strongly positive to strongly negative. In the neutral clusters, the OH functional groups of the cations can interact either with other cations or with the anions. The hexamer cluster with the cyclic H-bond motive and "released" anions is almost as stable as the hexamer built by H-bonded ion pairs exclusively, which is in accord with recent IR spectra of similar ionic liquids detecting both types of hydrogen bonding. For the cationic and neutral clusters, we discuss geometric and spectroscopic properties as sensitive probes of opposite- and like-charge interaction. Finally, we show that NMR proton chemical shifts and deuteron quadrupole coupling constants can be related to each other, allowing to predict properties which are not easily accessible by experiment.
我们探索了量子化学计算，以研究尽管存在强烈静电排斥力但仍通过氢键动力学稳定的羟基官能化阳离子簇。在一项全面的研究中，我们计算了氨、哌啶、吡咯烷、咪唑、吡啶和咪唑阳离子的簇，这些阳离子是离子液体的主要组成部分。所有阳离子都被羟烷基链修饰，允许同种电荷的离子形成氢键。簇拓扑结构包括线性和环状簇，大小可达六元环。环结构表现出协同氢键，与排斥的库仑力相对抗，从而使簇具有动力学稳定性。我们讨论了氢键和色散力对于不同大小的簇稳定性的重要性。我们发现，当环状拓扑结构中氢键最大化且适当考虑色散相互作用时，会形成最大的簇。我们研究了具有短链阳离子的簇的动力学稳定性，涵盖了从硬到极化或具有附加官能团（如咪唑阳离子中酸性 C(2)-H 位）的不同类型的阳离子。随着烷基链长度的增加，阳离子效应减弱，动力学稳定性仅由增加阳离子的正电荷环之间距离的烷基链束缚控制。向阳离子簇中添加反离子四氟硼酸根（BF）时，结合能立即从强正变为强负。在中性簇中，阳离子的 OH 官能团可以与其他阳离子或阴离子相互作用。具有环状氢键动机和“释放”阴离子的六元环簇几乎与仅由氢键离子对构建的六元环簇一样稳定，这与最近检测到类似离子液体中存在两种氢键的红外光谱一致。对于阳离子和中性簇，我们讨论了作为相反电荷和同种电荷相互作用的敏感探针的几何和光谱性质。最后，我们表明 NMR 质子化学位移和氘核四极矩耦合常数可以相互关联，允许预测实验不易获得的性质。
