Ring-Stacking Water Clusters: Morphology and Stabilities

The structures and interaction energies of water clusters with ring stacking motifs are studied by using ab initio calculations. The structures of the water clusters are constructed by stacking either single rings or multi-rings of tetramer, pentamer, and hexamer. We found that, in the single-ring-stacking motif, the most stable isomers exhibit an alternative clockwise-anticlockwise stacking pattern. We also show that four-layer single-ring-stacking isomers are not energetically favorable in comparison with those of two-layer multi-ring-stacking isomers. The relative stability of the isomers is also analyzed in terms of H-bond strength and elastic distortions of the water molecules.     

Desymmetrized Leaning Pillar[6]arene

In this work, a novel version of macrocyclic arenes, namely leaning pillar[6]arenes, was discovered and it can be considered as a tilted version of a pillar[6]arene with two hydroxy/alkoxy functionalities removed. Through a facile two‐step synthetic approaches, in conjunction with a diversity of post‐modification possibilities, a series of leaning pillar[6]arenes, with good cavity adaptability and enhanced guest‐binding capability, was synthesized, and their self‐assembly in single‐crystal states is presented. DFT calculations demonstrated that the lower rotational barrier of unsubstituted phenylene rings, the uneven electron density centered at the leaning phenyl rings, and the polarization effect along the edge generated by the hydrogen‐bond‐induced orientation of hydroxy groups greatly affected the host‐guest properties, and meanwhile provided an intuitive explanation for the pillar‐like and rigid structure of traditional pillar[6]arenes. Significantly, the crystal structure of cyclo‐oligomeric quinone was obtained by direct oxidation of leaning pillar[6]arenes.     

Statistical Equivalence of Quantum Chemical Methods for Energy Distribution in Water Clusters

Methods of quantum chemistry are instrumental in understanding molecular structures and properties. However, the results demonstrate significant variability, which is difficult to predict and rationalize. The fundamental question is whether some molecular systems exhibit properties invariant with respect to the computational method. The idea explored here is that collective properties of statistical ensembles should be more robust than characteristics of individual molecules and their arbitrary sets. This effect is demonstrated for the complete set of hydrogen-bond topologies of the dodecahedral water cluster (H₂O)₂₀. Non-Gaussian energy distributions produced by various methods have the same functional form despite strong differences in mean values and standard deviations. The conclusion is tested on methods of different complexity and origin employing a number of criteria. A linear mapping between the energies produced by different methods is discussed. The significance of the results is in establishing a collective equivalence property of quantum chemical methods.     

Hydrogen‐Bond Cooperative Effects in Small Cyclic Water Clusters as Revealed by the Interacting Quantum Atoms Approach

The cooperative effects of hydrogen bonding in small water clusters (H₂O)_n (n=3–6) have been studied by using the partition of the electronic energy in accordance with the interacting quantum atoms (IQA) approach. The IQA energy splitting is complemented by a topological analysis of the electron density (ρ( r )) compliant with the quantum theory of atoms‐in‐molecules (QTAIM) and the calculation of electrostatic interactions by using one‐ and two‐electron integrals, thereby avoiding convergence issues inherent to a multipolar expansion. The results show that the cooperative effects of hydrogen bonding in small water clusters arise from a compromise between: 1) the deformation energy (i.e., the energy necessary to modify the electron density and the configuration of the nuclei of the isolated water molecules to those within the water clusters), and 2) the interaction energy (E_int) of these contorted molecules in (H₂O)_n. Whereas the magnitude of both deformation and interaction energies is enhanced as water molecules are added to the system, the augmentation of the latter becomes dominant when the size of the cluster is increased. In addition, the electrostatic, classic, and exchange components of E_int for a pair of water molecules in the cluster (H₂O)_n?1 become more attractive when a new H₂O unit is incorporated to generate the system (H₂O)_n with the last‐mentioned contribution being consistently the most important part of E_int throughout the hydrogen bonds under consideration. This is opposed to the traditional view, which regards hydrogen bonding in water as an electrostatically driven interaction. Overall, the trends of the delocalization indices, δ(Ω,Ω′), the QTAIM atomic charges, the topology of ρ( r ), and the IQA results altogether show how polarization, charge transfer, electrostatics, and covalency contribute to the cooperative effects of hydrogen bonding in small water clusters. It is our hope that the analysis presented in this paper could offer insight into the different intra‐ and intermolecular interactions present in hydrogen‐bonded systems.     

N‐Doping of Polyaromatic Capsules: Small Cavity Modification Leads to Large Change in Host–Guest Interactions

To gain insight into the host functions of a nanocavity encircled by both polyaromatic panels and heteroatoms, nitrogen‐doped polyaromatic capsules were successfully synthesized from metal ions and pyridine‐embedded, bent anthracene‐based ligands. The new capsules display unique host–guest interactions in the isolated cavities, which are distinct from those of the undoped analogues. Besides the inclusion of Ag⁺ ions, the large absorption change of fullerene C₆₀ and altered emission of a BODIPY dimer are observed upon encapsulation by the present hosts. Moreover, the N‐doped capsule exhibits specific binding ability toward progesterone and methyltestosterone, known as a natural female and synthetic male hormone, respectively, in water.     

First systematic investigation of C-H...Cl hydrogen bonding using inorganic supramolecular synthons: lamellar, stitched stair-case, linked-ladder, and helical structures

Using a group of six neutral M(II)Cl(2)-containing coordination compounds as building blocks, the first systematic investigation of C-H...Cl hydrogen-bonding interactions was performed. Single-crystal X-ray structural analyses of four new compounds (pseudo-tetrahedral Co(II) and Zn(II); distorted trigonal bipyramidal Zn(II)) authenticate the metal coordination geometry. To provide a unified view of the presence of noncovalent interactions in this class of compounds, we have re-examined the packing diagram of two previously reported compounds (a distorted square-pyramidal Cu(II) complex and a trans-octahedral Co(II) complex). The organic ligands of our choice comprise bidentate/tridentate pyrazolylmethylpyridines and an unsymmetrical tridentate pyridylalkylamine. This systematic investigation has allowed us to demonstrate the existence of versatile C-H...Cl(2)M interactions and to report the successful application of such units as inorganic supramolecular synthons. Additional noncovalent interactions such as C-H...O and O-H...Cl hydrogen bonding and pi-pi stacking interactions have also been identified. Formation of novel supramolecular architectures has been revealed: 2D lamellar (p-cyclophane) and 3D lamellar, 3D "stitched staircase" (due to additional hydrogen-bonding interactions by water tetramers, with an average O-O bond length in the tetramer unit of 2.926 A, acting as "molecular clips" between staircases), 3D linked ladder, and single-stranded 1D helix.     

Water–Water and Water–Solute Interactions in Microsolvated Organic Complexes

A structural study of microsolvated clusters of β‐propiolactone (BPL) formed in a pulsed molecular jet expansion is presented. The rotational spectra of BPL–(H₂O)_n (n=1–5) adducts have been analyzed by broadband microwave spectroscopy. Unambiguous identification of the structures has been achieved using isotopic substitution and experimental measurements of the cluster dipole moment. The observed structures are discussed in terms of the different intermolecular interactions between water molecules and between water and BPL, which include n–π* interactions involving the lone pairs of electrons on water oxygen atoms and the antibonding orbital of the BPL carbonyl group. The changes induced in the structures of the water hydrogen‐bonding network by complexation to BPL indicate that water clusters adopt specific configurations to maximize their links to solute molecules.     

Self-assembled 1,2-bis[4-(4-(10-decyloxy)phenylazo)]benzoylhydrazine dimer and its hydrogen-bonded complexes

We report the synthesis and investigation of a new self-assembled benzoylhydrazine-based compound, namely 1,2-bis[4-(4-(10-decyloxy)phenylazo)]benzoylhydrazine and their hydrogen-bonded complexes with different carboxylic acid derivatives, of which some exhibit liquid crystalline properties and some are non-mesogens. The conversion of the non-liquid crystalline target compound that is free from carboxylic acid to liquid crystalline complexes containing various carboxylic acids can probably be rationalised through the breaking of intermolecular hydrogen bonding within the supramolecular assembly by carboxylic acid dopants. The enhancement of liquid crystalline properties of benzoylhydrazine compounds with respect to various carboxylic dopants is documented. Other salient features can be exemplified by the formation of discotic columnar phase as shown by the azo-linkage containing 4-(4′-(10-decyloxy)phenylazo)benzoic acid-doped complex.     

Molecular and supramolecular ionic aggregates HxOyz in organic and organometallic crystalline hydrates

Ionic aggregates of the form H_xO_y^z (z≠ 0) have been characterized during an analysis of 245 crystal structures extracted from the Cambridge Structural Database [Allen (2002). Acta Cryst. B 58 , 380–388]. A systematic nomenclature is proposed for these species. Three modes of hydrogen bonding are described, characterized in part by the distance between contiguous O atoms: normal (NHB; O...O = 2.6–3.0 Å), charge assisted (CAHB; O...O = 2.5 Å) and molecular (MHB; O...O = 2.4 Å). The three modes are consistent with previous reports, our experimental results, and quantum‐chemical‐optimized geometries and energetics. No evidence is presented concerning the possible existence or stability of these aggregates in solution. Rather, emphasis is placed on the necessity in crystal structure analysis to develop thoroughly existing hydrogen‐bonded networks, ignorance of which can lead to erroneous crystal structure models and other physico‐chemical data associated with composition and charge balance.     

Hydrogen Bond Cooperativity and the Three‐Dimensional Structures of Water Nonamers and Decamers

Broadband rotational spectroscopy of water clusters produced in a pulsed molecular jet expansion has been used to determine the oxygen atom geometry in three isomers of the nonamer and two isomers of the decamer. The isomers for each cluster size have the same nominal geometry but differ in the arrangement of their hydrogen bond networks. The nearest neighbor O? O distances show a characteristic pattern for each hydrogen bond network isomer that is caused by three‐body effects that produce cooperative hydrogen bonding. The observed structures are the lowest energy cluster geometries identified by quantum chemistry and the experimental and theoretical O? O distances are in good agreement. The cooperativity effects revealed by the hydrogen bond O? O distance variations are shown to be consistent with a simple model for hydrogen bonding in water that takes into account the cooperative and anticooperative bonding effects of nearby water molecules.     

Synthesis of a New Self-Assembly Liquid Crystalline Polymer Through Intermolecular Hydrogen Bonding

A new side chain liquid crystalline polymer with biphenyl mesogenic group has been prepared. This polymer forms a bilayer smectic A phase through intermolecular hydrogen bonding according to the result of X-ray diffraction. Its phase transition was studied lining DSC and polarized optical microscopy. Infrared spectroscopic measurements show that cyclic dimeric hydrogen bonds of carboxylic acids are embedded in the bilayer.     

Hydrogen-bonded 1D and 2D Assemblies of Tetraiso-butyl-resorcin[4]arene in the Crystalline State

1 INTRODUCTION There is continuing interest in the assembly of molecular capsules based on concomitant formation of multiple hydrogen bonds between smaller mole- cular components[1]. A particularly attractive buil- ding block is calix[4]resorcinarenes with eight pen- dant hydroxyl functional groups[2]. In a crystal engi- neering design strategy for molecular self-assembly, cocrystallization of C-methylcalix[4]resorcinarenes with nitrogen-donor molecules such as pyridines in the presence …     

Supramolecular architectures featuring stereoisomeric cluster complexes of the [Re6(μ3-Se)8]2+ core

The [Re₆(μ₃-Se)₈]²⁺ core-containing cluster complexes of the general formula [Re₆(μ₃-Se)₈(PEt₃)₄L₂]²⁺ (both trans- and cis-isomers) site-differentiated with inert PEt₃ and functional L ligands that are capable of hydrogen bonding or secondary (with respect to primary coordination with the cluster core) metal-ligand coordination interactions have been prepared. The applications of such stereospecific cluster isomers as building blocks for supramolecular construction have been studied. A great variety of multicluster arrays mediated by intercluster hydrogen bonding or cluster ligand coordination with secondary metal ions have been obtained and structurally characterized. The findings from this research clearly establish the superior utility of these novel building blocks for the creation of structurally sophisticated architectures and possibly functional materials.     

Guest–Host Hydrogen Bonding in Structure H Clathrate Hydrates

Molecular dynamics simulations of the structure H (sH) clathrate of tert‐butylmethylether show the prevalence of ether–water hydrogen bonding (see picture), absent in the neo‐hexane sH clathrate. This affects guest–cage dynamics and host–water dielectric relaxation dynamics. The ¹³C and ¹H NMR relaxation times for both guests are measured, and the differences are explained in terms of guest–host interactions in the two clathrates.