C?H/π‐Interaction‐Guided Self‐Assembly in π‐Conjugated Oligomers

We report CH/π hydrogen‐bond‐driven self‐assembly in π‐conjugated skeletons based on oligophenylenevinylenes (OPVs) and trace the origin of interactions at the molecular level by using single‐crystal structures. OPVs were designed with appropriate pendants in the aromatic core and varied by hydrocarbon or fluorocarbon tails along the molecular axis. The roles of aromatic π‐stack, van der Waals forces, fluorophobic effect and CH/π interactions were investigated on the theromotropic liquid crystallinity of OPV molecules. Single‐crystal structures of hydrocarbon OPVs provided direct evidence for the existence of CH/π interactions between the π‐ring (H‐bond acceptor) and alkyl C? H (H‐bond donor). The four important crystallographic parameters, d_c?x=3.79 Å, θ=21.49°, φ=150.25° and d_Hp?x=0.73 Å, matched in accordance with typical CH/π interactions. The CH/π interactions facilitate the close‐packing of mesogens in x–y planes, which were further protruded along the c axis producing a lamellar structure. In the absence of CH/π interactions, van der Waals interactions drove the assembly towards a Schlieren nematic texture. Fluorocarbon OPVs exhibited smectic liquid‐crystalline textures that further underwent Smectic A (SmA) to Smectic C (SmC) phase transitions with shrinkage up to 11 %. The orientation and translational ordering of mesogens in the liquid‐crystalline (LC) phases induced H‐ and J‐type molecular arrangements in fluorocarbon and hydrocarbon OPVs, respectively. Upon photoexcitation, the H‐ and J‐type molecular arrangements were found to emit a blue or yellowish/green colour. Time‐resolved fluorescence decay measurements confirmed longer lifetimes for H‐type smectic OPVs relative to that of loosely packed one‐dimensional nematic hydrocarbon‐tailed OPVs.     

Assembly,Thermodynamics, and Structure of a Two‐Wheeled Composite of a Dumbbell‐Shaped Molecule and Cylindrical Molecules with Different Edges

A carbonaceous dumbbell was able to spontaneously glue two tubular receptors to form a unique two‐wheeled composite through van der Waals interactions, thus forcing the wheel components into contact with each other at the edges. In the present study, two tubular receptors with enantiomeric carbon networks were assembled on the dumbbell joint, and the handedness of the receptors was discriminated, thus leading to the self‐sorting of homomeric receptors from a mixture of enantiomeric tubes. The crystal structures of the composites revealed the structural origins of the molecular recognition driven by van der Waals forces as well as the presence of a columnar array of C₁₂₀ molecules in a 1:1 composite.     

The molecular and crystal structure of bipolar,mesogenic biphenyls: a comparison of similar compounds HO(CH 2) 6 OC 6 H 4 C 6 H 4 R with R = cyano and nitro terminal groups

The crystal and molecular structures of 4,4′-disubstituted biphenyls of formula HO(CH₂)₆ OC₆H₄C₆H₄ R with R = cyano and nitro terminal groups have been determined including co-crystallized materials of both of the compounds. The extended molecules arrange in an antiparallel fashion with dipole-dipole interactions, exhibiting a sheet-like structure for the compound with the cyano terminal group, and for the other two materials an extended head to tail string-like structure caused by dipolar interaction, also packed in sheets in an antiparallel fashion by van der Waals interactions. Knowledge of the interaction of the compounds in the crystalline state serves as a prerequisite for the determination of interactions in the liquid crystalline state of the same or similar compounds.     

Ionic Liquids: Dissecting the Enthalpies of Vaporization

We calculate the heats of vaporisation for imidazolium‐based ionic liquids [C_nmim][NTf₂] with n=1, 2, 4, 6, 8 by means of molecular dynamics (MD) simulations and discuss their behavior with respect to temperature and the alkyl chain length. We use a force field developed recently. The different cohesive energies contributing to the overall heats of vaporisations are discussed in detail. With increasing alkyl chain length, the Coulomb contribution to the heat of vaporisation remains constant at around 80 kJ mol^?1, whereas the van der Waals interaction increases continuously. The calculated increase of about 4.7 kJ mol^?1 per CH₂‐group of the van der Waals contribution in the ionic liquid exactly coincides with the increase in the heats of vaporisation for n‐alcohols and n‐alkanes, respectively. The results support the importance of van der Waals interactions even in systems completely composed of ions.     

Polymeric μ‐bromo‐μ‐pyridine‐3‐carboxyl­ato‐κ3O,O′:N‐mercury(II)

The asymmetric unit of the title polymeric complex, [HgBr(C₆H₄NO₂)]_n or HgBr(nic), contains mercury coordinated via two Br atoms [Hg—Br = 2.6528 (9) and 2.6468 (9) Å], two carboxyl­ate O atoms, which form a characteristic four‐membered chelate ring [Hg—O = 2.353 (6) and 2.478 (7) Å], and an N atom [Hg—N = 2.265 (5) Å], in the form of a very irregular (3+2)‐coordination polyhedron. The pronounced irregularity of the effective Hg (3+2)‐coordination is a result of the rigid stereochemistry of the nicotinate ligand. According to the covalent and van der Waals radii criteria, the strongest bonds are Hg—Br and Hg—N. These covalent interactions form a two‐dimensional poly­mer. The puckered planes are connected by van der Waals interactions, and there are only two intermolecular C—H⋯O hydrogen bonds [3.428 (10) and 3.170 (10) Å].     

Assembly,Thermodynamics, and Structure of a Two‐Wheeled Composite of a Dumbbell‐Shaped Molecule and Cylindrical Molecules with Different Edges

A carbonaceous dumbbell was able to spontaneously glue two tubular receptors to form a unique two‐wheeled composite through van der Waals interactions, thus forcing the wheel components into contact with each other at the edges. In the present study, two tubular receptors with enantiomeric carbon networks were assembled on the dumbbell joint, and the handedness of the receptors was discriminated, thus leading to the self‐sorting of homomeric receptors from a mixture of enantiomeric tubes. The crystal structures of the composites revealed the structural origins of the molecular recognition driven by van der Waals forces as well as the presence of a columnar array of C₁₂₀ molecules in a 1:1 composite.     

2‐(6‐Bromopyridin‐3‐yl)‐4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolane and (6‐bromopyridin‐3‐yl)boronic acid,new bifunctional building blocks for combinatorial chemistry

The first comparative study between two new heterocyclic boron derivatives, viz. a (6‐bromo­pyridin‐3‐yl)­boronic ester, C₁₁H₁₅BBrNO₂, and (6‐bromo­pyridin‐3‐yl)­boronic acid, C₅H₅BBrNO₂, shows a small but not significant difference in their C—B bond lengths, which cannot explain the experimentally observed difference in their stabilities. The crystal packing of the boronic ester consists principally of van der Waals interactions, while the boronic acid mol­ecules interact in their crystal through hydrogen bonds.     

Long-Range van der Waals Interactions in Density Functional Theory

The early difficulties in accounting for long-range van der Waals interactions in the framework of density functional theory (DFT) have been overcome to a certain extent in recent works by several groups, and those interactions can be computed numerically. In this paper a derivation of the analytical form of the attractive van der Waals interaction between two neutral atoms with polarizabilities α₁ and α₂ at large distance R, namely E _int=−C ₆ α₁ α₂/R ⁶ is performed within the context of DFT. Use is made of the properties of the Coulomb correlation hole, and it is shown that nonlocal Coulomb correlations are responsible for long-range dispersion interactions.     

The molecular conformation of pentan‐3‐one studied in cholic acid pentan‐3‐one solvate

The crystal structure of cholic acid–pentan‐3‐one (1/1), C₅H₁₀O·C₂₄H₄₀O₅, has been determined in order to deduce the molecular conformation of the small volatile ketone. Data were collected at 100 K to a resolution of (sin θ)/λ = 0.91 Å⁻¹. The structure contains a hydrogen‐bonded cholic acid host network, forming only van der Waals interactions with the guest pentan‐3‐one molecules. The ketone molecules are disordered on general positions, with two clearly identifiable conformations. The majority conformer exhibits approximate C₂ symmetry and is similar to that recently observed by microwave spectroscopy in the gas phase.     

1‐[5‐(4,5‐Dimethyl‐1,3,2‐dioxaborolan‐2‐yl)thiophen‐2‐yl]ethanone and 4‐(4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolan‐2‐yl)benzaldehyde

In both title compounds, C₁₀H₁₃BO₃S, (I), and C₁₃H₁₇BO₃, (II), the molecules adopt nearly planar conformations. The crystal packing of (I) consists of a supramolecular two‐dimensional network with a herringbone‐like topology formed by self assembly of centrosymmetric pairs of molecules linked via dipole–dipole interactions. The crystal structure of (II) consists of a supramolecular two‐dimensional network built up from centrosymmetric pairs of molecules viaπ–π interactions. These pairs of molecules are self‐organized in an offset fashion related by a symmetry centre, generating supramolecular ribbons running along the [101] direction. Neighbouring ribbons are stacked via complementary van der Waals and hydrophobic methyl–methyl interactions.     

Jahn–Teller Disproportionation Induced Exfoliation of Unit-Cell Scale ϵ-MnO2

Exfoliation of non-layered (structurally) bulk materials at the nanoscale is challenging because of the strong chemical bonds in the lattice, as opposed to the weak van der Waals (vdW) interactions in layered materials. We propose a top-down method to exfoliate ϵ-MnO₂ nanosheets in a family of charge-ordered La_1−xAE_xMnO₃ (AE=Ca, Sr, Ba) perovskites, taking advantage of the Jahn–Teller disproportionation effect of Mn³⁺ and bond-strength differences. ϵ-MnO₂ crystallized into a nickel arsenide (NiAs) structure, with a thickness of 0.91 nm, displays thermal metastability and superior water oxidation activity compared to other manganese oxides. The exfoliation mechanism involves a synergistic proton-induced Mn³⁺ disproportionation and structural reconstruction. The synthetic method could also be potentially extended to the exfoliation of other two-dimensional nanosheet materials with non-layered structures.     

Relative influence of noncovalent interactions on the melting points of a homologous series of 1,2‐dibromo‐4,5‐dialkoxybenzenes

Crystal structures are presented for two members of the homologous series of 1,2‐dibromo‐4,5‐dialkoxybenzenes, viz. those with decyloxy and hexadecyloxy substituents, namely 1,2‐dibromo‐4,5‐bis(decyloxy)benzene, C₂₆H₄₄Br₂O₂, (II), and 1,2‐dibromo‐4,5‐bis(hexadecyloxy)benzene, C₃₈H₆₈Br₂O₂, (III). The relative influences which halogen bonding, π–π stacking and van der Waals interactions have on these structures are analysed and the results compared with those already found for the lightest homologue, 1,2‐dibromo‐4,5‐dimethoxybenzene, (I) [Cukiernik, Zelcer, Garland & Baggio (2008). Acta Cryst. C 64 , o604–o608]. The results confirm that the prevalent interactions stabilizing the structures of (II) and (III) are van der Waals contacts between the aliphatic chains. In the case of (II), weak halogen C—Br...(Br—C)′ interactions are also present and contribute to the stability of the structure. In the case of (III), van der Waals interactions between the aliphatic chains are almost exclusive, weaker C—Br...π interactions being the only additional interactions detected. The results are in line with commonly accepted models concerning trends in crystal stability along a homologous series (as measured by their melting points), but the earlier report for n = 1, and the present report for n = 10 and 16, are among the few providing single‐crystal information validating the hypothesis.     

Weak C—H...Cl—Pd interactions toward conformational polymorphism in trans‐dichloridobis(triphenylphosphane)palladium(II)

A new triclinic polymorph of the title compound, [PdCl₂(C₁₈H₁₅P)₂], has two independent molecules in the unit cell, with the Pd atoms located on inversion centres. One molecule has an eclipsed conformation, whereas the second molecule adopts a gauche conformation. The molecules with a gauche conformation are involved in weak intermolecular C—H...Cl—Pd interactions with symmetry‐related molecules. It is suggested that C—H...Cl—Pd interactions are mainly responsible for the existence of conformational differences, which contribute to the polymorph formation. In the crystal, there are layers of eclipsed and gauche molecules separated by normal van der Waals interactions.     

2‐Chloro­phenyl 3‐nitro­benzene­sulfonate and 2,4‐di­chloro­phenyl 3‐nitro­benzene­sulfonate: supramole­cular aggregation through C–H⋯O, π–π and van der Waals interactions

In 2‐chloro­phenyl 3‐nitro­benzene­sulfonate, C₁₂H₈ClNO₅S, and 2,4‐di­chloro­phenyl 3‐nitro­benzene­sulfonate, C₁₂H₇Cl₂NO₅S, weak C—H⋯O interactions generate S(5), S(6) and (7) rings. The supramolecular aggregation is completed by the presence of π–π interactions and intermolecular van der Waals short contacts.     

Modelisation of the Association Mechanism of a Series of Huperzine Derivatives Used for Alzheimer Disease with Human Serum Albumin: Effect of the Magnesium Cation

The role of the Mg²⁺ cation on huperzine molecule binding (drugs used for Alzheimer disease) on human serum albumin (HSA) was studied by affinity chromatography. The thermodynamic data corresponding to this binding were determined for a wide range of Mg²⁺ concentrations (x). For each solute, the huperzine binding on HSA was divided into two Mg²⁺ concentration regions. For a low x value, below x_c (1.2 mM), the binding decrease with x. For x above x_c the hydrophobic effect and van der Waals interactions between the huperzine molecule and the HSA implied a decrease in its binding. These results showed that for patients with Alzheimer disease, an Mg²⁺ supplementation during treatment with these huperzine molecules can increase the active pharmacological molecule concentration.     

Two 9‐[(E)‐nitrophenylvinyl]‐9H‐carbazoles

The crystal structures of 9‐[(E)‐(4‐nitrophenyl)vinyl]‐9H‐carbazole and 9‐[(E)‐(3‐nitrophenyl)vinyl]‐9H‐carbazole, both C₂₀H₁₄N₂O₂, are determined mainly by van der Waals forces and π–π interactions between the carbazole and benzene systems. However, the packing modes are different. In the 4‐nitro derivative, the molecules in the weakly bound stack are related by a unit‐cell translation, while in the 3‐nitro derivative there are centrosymmetric pairs of molecules joined by π–π interactions and also pairs of molecules, related by another centre of symmetry, connected by eight relatively short C—H...O interactions.     

(S)‐3‐(Ammoniomethyl)‐5‐methylhexanoate (pregabalin)

The title compound, C₈H₁₇NO₂, exists as a zwitterion, adopting a propeller conformation. Molecules self‐assemble to form a hydrogen‐bonded layer parallel to the ab crystallographic plane connected by N⁺—H...O⁻ and C—H...O⁻ hydrogen bonds. These layers are stacked along the c axis and are stabilized by van der Waals interactions.     

Spectroscopy of DABCO-rare-gas and DABCO-DABCO van der Waals complexes

The excited electronic origin bands of several DABCO containing van der Waals complexes have been observed via (1+1) resonance enhanced multi-photon ionization. Sharp resonances with widths of about 3 cm^–1 are seen for DABCO-Rg _n=1,2,3 (Rg is Ar, Kr or Xe), for the DABCO-DABCO dimer and for DABCO-DABCO-Ar. The origins of the rare-gas complexes are blue shifted with respect to the monomer origin. Broad features originating from DABCO-Rg _n complexes with highn, appear to higher energies than the complex origins, with widths of 120 cm^–1.     

2,6‐Di­phenylthiapyran‐4‐one

In the title compound, 2,6‐di­phenyl­thia­cyclo­hexan‐4‐one, C₁₇H₁₆OS, mirror site symmetry is retained by the mol­ecule in the solid state in the absence of C—H?X hydrogen bonds. The crystal structure is stabilized by van der Waals interactions, the shortest S?O and C?O contacts being 3.567 (2) and 3.512 (3) Å, respectively.     

Synthetic Nanosheets of Natural van der Waals Heterostructures

Creation of new van der Waals heterostructures by stacking different two dimensional (2D) crystals on top of each other in a chosen sequence is the next challenge after the discovery of graphene, mono/few layer of h ‐BN, and transition‐metal dichalcogenides. However, chemical syntheses of van der Waals heterostructures are rarer than the physical preparation techniques. Herein, we demonstrate the kinetic stabilization of 2D ultrathin heterostructure (ca. 1.13–2.35 nm thick) nanosheets of layered intergrowth SnBi₂Te₄, SnBi₄Te₇, and SnBi₆Te₁₀, which belong to the Sn_m Bi_2n Te_{3n +m} homologous series, by a simple solution based synthesis. Few‐layer nanosheets exhibit ultralow lattice thermal conductivity (κ _lat) of 0.3–0.5 W m⁻¹ K⁻¹ and semiconducting electron‐transport properties with high carrier mobility.