Changjun Peng

Halogen bonding interactions in ion pairs versus conventional charge-assisted and neutral halogen bonds: a theoretical study based on imidazolium species

Halogen bonds in imidazolium-based ion pairs have attracted recent research interest, due to their importance in the fields of anion recognition and ionic liquids. According to our survey of the Cambridge Structural Database (CSD), a number of crystal structures involving these specific halogen bonds were extracted. In this work, three different types of halogen bonding interactions, i.e. ion-pair halogen bonds, charge-assisted and neutral halogen bonds, in a series of dimeric complexes of imidazolium species were systematically studied at the M06-2x and B3LYP levels of theory. Ion-pair halogen bonds, despite being considerably stronger, show similar structural and energetic characteristics to common charge-assisted and neutral halogen bonds. To gain a deeper understanding of these interactions, the atoms in molecules (AIM), noncovalent interaction index (NCI), and energy decomposition analysis (EDA) calculations were undertaken. Most halogen bonds in imidazolium-based ion pairs have some covalent character, while the other two kinds of halogen bonds are weak electrostatic interactions. The attraction of ion-pair halogen bonds arises dominantly from electrostatic forces, while dispersion interaction plays a minor role. These two terms, however, contribute almost equally to the attraction of neutral halogen bonds. In addition to electrostatic attraction, induction interaction, which corresponds to charge transfer and mixing terms, also plays an important role in ion-pair and charge-assisted halogen bonds. The results presented should assist in the development of potent imidazolium-based anion receptors and novel halogenated ionic liquids with promising properties.

Structures and Electronic Properties of Transition Metal-Containing Ionic Liquids: Insights from Ion Pairs

Transition metal-containing ionic liquids (TM-ILs) have attracted a great deal of attention in recent years, due to their unique physical and chemical properties. In this work, several representative TM-ILs, such as the cations consisting of silver(I) center coordinated by two n-alkylimidazole ligands ([(C(n)(im))Ag(mim)](+)) and the anions involving mercury(II) (HgCl3(-)), zinc(II) (ZnCl3(-)), and rhenium(VII) (ReO4(-)), were investigated using density functional theory calculations. First, the structural and energetic properties of the ion pairs for these TM-ILs have been examined in detail and compared with properties for conventional ILs. It was found that the interactions between the cations and anions, including hydrogen bonds and electrostatic interactions, in TM-ILs become weaker in strength than those in traditional ILs. In particular, the calculated geometric and energetic features compare fairly well with the experimental results, such as melting points and X-ray crystal structures of these TM-ILs. Then, the structures and energetics of ion-pair dimers for three ILs containing HgCl3(-), ZnCl3(-), and ReO4(-) were also explored, to gain a deeper understanding of the properties of TM-ILs. Finally, a survey of the Cambridge Structural Database (CSD) was undertaken to provide some crystallographic implications of TM-ILs.

Solvation of the Ca2UO2(CO3)3 Complex in Seawater from Classical Molecular Dynamics

Uranium from the sea provides a long-time supply guarantee of nuclear fuels for centuries to come, and the neutral Ca2UO2(CO3)3 complex has been shown to be the dominant species of uranium in seawater. However, the solvation and structure of the Ca2UO2(CO3)3 complex in seawater have been unclear. Herein we simulate the Ca2UO2(CO3)3 complex in a model seawater solution via classical molecular dynamics. We find that Na(+) and Cl(-) ions interact very differently with the neutral Ca2UO2(CO3)3 complex in seawater. Especially, one Na(+) ion is closely associated with the Ca2UO2(CO3)3 complex, thereby effectively making the complex have a +1 charge, while Cl(-) ions are much farther away. Hence, this work reveals the important role of Na(+) ions in affecting the solvation of the Ca2UO2(CO3)3 complex in seawater, which has implications in designing ligands to attract the Ca2UO2(CO3)3 complex to the sorbent.

The Interactions between Imidazolium-Based Ionic Liquids and Stable Nitroxide Radical Species: A Theoretical Study

In this work, the interactions between imidazolium-based ionic liquids and some stable radicals based on 2,2,6,6-tetramethylpiperidine-1-yloxyl (TEMPO) have been systematically investigated using density functional theory calculations at the level of M06-2x. Several different substitutions, such as hydrogen bonding formation substituent (OH) and ionic substituents (N(CH3)3(+) and OSO3(-)), are presented at the 4-position of the spin probe, which leads to additional hydrogen bonds or ionic interactions between these substitutions and ionic liquids. The interactions in the systems of the radicals containing ionic substitutions with ionic liquids are predicted much stronger than those in the systems of neutral radicals, resulting in a significant reduction of the mobility of ionic radicals in ionic liquids. To further understand the nature of these interactions, the natural bond order, atoms in molecules, noncovalent interaction index, electron density difference, energy decomposition analysis, and charge decomposition analysis schemes were employed. The additional ionic interactions between ionic radicals and counterions in ionic liquids are dominantly contributed from the electrostatic term, while the orbital interaction plays a major role in other interactions. The results reported herein are important to understand radical processes in ionic liquids and will be very useful in the design of task-specific ionic liquids to make the processes more efficient.

Ion-pair recognition based on halogen bonding: a case of the crown-ether receptor with iodo-trizole moiety

The development of halogen-bond-based ditopic receptors capable of binding simultaneously both a cation and an anion has attracted recent research interest. In this work, the crown-ether receptor 1, which consists of an iodo-trizole moiety for anion recognition through halogen bonding and a Lewis-basic center for cation binding, was investigated using density functional theory calculations. The structural and energetic features for the complexes of 1 with single cations, single halide anions, and ion pairs were explored. Intermolecular interactions in these complexes were systematically analyzed by the atoms in molecules and noncovalent interaction index methods. The presence of the coordinated cation significantly increases the anion-binding affinity, while the binding of halide anions has a slight influence on the cation-binding affinity. Anti-cooperative effects were found in the ion-pair recognition of 1, due to the strong attraction between the two counterions in the complexes. The solvent weakens the interaction strength considerably, and anti-cooperativity becomes very small in solvent. The results reported in this work are of fundamental importance in the design of ion-pair receptors based on halogen bonding.