Theoretical study on conformational conversion of 1,3-dioxane inside a capsular host

The conformational conversion of 1,3-dioxane guest encapsulated inside a cylindrical capsular host was investigated with PM3 method and single point energies were evaluated by B3LYP method. When entrapped in the capsule, the guest tumbles were slower than that in the free condition. The influences of the inner phase of the capsule on the guest conformational conversion were discussed in detail.     

Supramolecular self-assembly of water-soluble cavitands: investigated by molecular dynamics simulation

In this study, we have examined supramolecular self-assembly process of a hydrophobic guest with a water-soluble host known by the trivial name octa acid (OA). Two octa acids form a capsular assembly only in presence of a nonpolar guest(s). Size and shape of the guest control the stoichiometry of the capsular complex. Here, all atom molecular dynamics simulation has been utilized to investigate complex formation mechanisms of a nonpolar guest (nonylbenzene) with two OA cavitands. Nonylbenzene was encapsulated into the nonpolar cavity of OA capsule owing to solvophobic interactions. Upon encapsulation it was twisted and bent due to lack of free space within the capsule. These unusual forms obtained from the simulation study were in accord with experimental findings. The post-complexation attributes of the guest were regulated by the available free space within the OA and favorable non-covalent interactions between the guest and the walls of the OA capsule. In the identical simulation condition two OA cavitands did not form a capsule without a guest, thus indicating requirement of a guest during the self-assembly of OA cavitands.     

Highly selective recognition of diols by a self-regulating fine-tunable methylazacalix[4]pyridine cavity: guest-dependent formation of molecular-sandwich and molecular-capsule complexes in solution and the solid state

The molecular recognition of methylazacalix[4]pyridine (MACP-4; 1) towards various diols was investigated by using (1)H NMR spectroscopic and X-ray diffraction analysis. As a unique macrocyclic host molecule that undergoes conformational inversions very rapidly in solution, MACP-4 has been shown to self-regulate its conformation, through the formation of different conjugations of the four bridging nitrogen atoms with their adjacent pyridine rings, to form a cavity that best fits the guest species through intermolecular hydrogen-bond, C-H...pi, and pi-pi interactions between the host and guest. As a consequence, depending upon the diol structure and geometry, MACP-4 forms a 1:1 molecular sandwich, 2:1 molecular capsule, and 1:2 butterfly-layered complex with the guests. As a result of favorable enthalpy and entropy effects, MACP-4 exhibits excellent selectivity in the recognition of resorcinol, thus resulting in a very stable 1:1 sandwich complex with resorcinol with a binding constant of 6000 (M-1). The dynamic (1)H NMR spectroscopic study demonstrated that the 1,3-alternate conformation of the macrocyclic ring of the MACP-4resorcinol complex (13) is stable at low temperature (T<243 K), and its conformational inversion requires a larger activation energy (DeltaG++=(45.5+/-2.2) kJ mol(-1)). In the presence of an excess amount of resorcinol, however, the conformational inversion of the MACP-4resorcinol complex proceeds more readily with a decreased activation energy (DeltaG++=(33.5+/-1.5) kJ mol(-1)) owing most probably to the favorable enthalpy effect of the [3...1...3]++transition state.     

X-ray structural investigation of gossypol and its derivatives XXIII autoclathrates based on a mixed gossypol — 1,4-Dioxane matrix

Gossypol forms various complexes with the isomeric dioxanes. The clathrate with 1,4-dioxane is the only complex of gossypol in which the intrinsic symmetry of the gossypol molecule — the symmetry of a twofold axis — is retained. In this complex, two out of the three 1,4-dioxane molecules belonging to each gossypol molecule participate in the construction of a mixed H-bound gossypol -dioxane matrix, while the third molecule plays the part of guest, the guest molecules having no H-bonds with the host matrix and undergoing desolvation at 108–110°C.     

ITC and NMR analysis of the encapsulation of fatty acids within a water-soluble cavitand and its dimeric capsule

We report here NMR and Isothermal Titration Calorimetry studies of the binding of ionisable guests (carboxylate acids) to a deep-cavity cavitand. These studies reveal that the shortest guests favoured 1:1 complex formation, but the longer the alkyl chain the more the 2:1 host-guest capsule is favoured. For intermediate-sized guests, the equilibrium between these two states is controlled by pH; at low values the capsule containing the carboxylic acid guest is favoured, whereas as the pH is raised deprotonation of the guest favours the 1:1 complex. Interestingly, for one host–guest pair the energy required to decap the 2:1 capsular complex and form the 1:1 complex is sufficient to shift the pK_a of the guest by ~3–4 orders of magnitude (4.1–5.4 kcal mol^?1). The two largest guests examined form stable 2:1 capsules, with in both cases the guest adopting a relatively high energy J-shaped motif. Furthermore, these 2:1 complexes are sufficiently stable that at high pH guest deprotonation occurs without decapping of the capsule.     

Well-defined, organic nanoenvironments in water: the hydrophobic effect drives a capsular assembly

The synthesis of a water-soluble, deep-cavity cavitand is reported. A blend of molecular curvature and amphiphilicity, this molecule has a hydrophobic concave surface and a hydrophilic convex surface. As a result, in aqueous solution and in the presence of a guest molecule, the host self-assembles to form a capsular assembly with an interior cavity large enough to entrap steroidal guests.     

Structural alteration of hybrid supramolecular capsule induced by guest encapsulation

Heterofunctionalized C(2v) symmetrical cavitand 1 with 4-pyridylethynyl and 3-carbamoylphenyl groups in alternating arrangement was designed and synthesized. A 1:1 mixture of the cavitand 1 and a cis-coordinated palladium(II) or platinum(II) complex self-assembled into a hybrid supramolecular capsule via both metal-ligand coordination bonds and hydrogen bonds. Formation of the capsular assembly was confirmed by NMR spectroscopy and mass spectrometry. The hybrid capsule encapsulated the appropriate guest, the molecular sizes of which fit the size of the capsular cavity. Structural alteration of the hybrid capsule was induced by the guest encapsulation. A C(2h) structure for the encapsulation complex was assigned by 2D NMR spectra analysis. Thermodynamic and kinetic properties of the guest encapsulation were investigated. The kinetics of in/out guest exchange was strongly influenced by hydrogen bonding in the hybrid capsule.     

Synthesis and structure of the first per-substituted anthracene host: decakis(cyclopentylthio)anthracene

Abstract

The title compound, a novel host molecule, has been prepared both from mixtures of unsaturated fluorocarbons related to perfluoro-perhydroanthracene and, in an extension of a remarkable new reaction, perfluoroperhydroanthracene itself. The 1,4-dioxane inclusion compound of this anthracene-based host is triclinic, space group P 1, with a = 11.826(1), b = 15.297(1), c = 20.280(1) Å, α = 98.77(1), β = 99.85(1), and γ = 95.83(1)°, with two host, and ca. three 1,4-dioxane guest molecules per unit cell. The host molecule occupies a general position in the unit cell and its anthracene core is markedly non-planar; each of the three crystallographically independent six-membered rings possesses a shallow twist-boat conformation. The host's side-chain moieties display an asymmetrical conformational distribution of the abbabaabab type (a = above, b = below, mean core plane).     

Structure and Conformational Analysis of 5,5-Bis(bromomethyl)-2-phenyl-1,3-dioxane

The structure of 5,5-bis(bromomethyl)-2-phenyl-1,3-dioxane was investigated by the methods of 1Н, ¹³С NMR spectroscopy and X-ray analysis. The molecule exists in the chair conformation with the equatorial phenyl group. The routes of interconversion of the ring, free conformational energy and optimal conformation of the phenyl group were determined using computer modeling by the method of DFT (PBE/3ξ). The results are consistent with the data of X-ray analysis.     

New water-soluble organic capsules are effective in controlling excited-state processes of guest molecules

Synthesis, inclusion properties, and ability to control excited-state properties of two water-soluble hosts are presented. These hosts surround the guest molecule(s) by forming a capsular assembly. By constraining the guest and by providing very little free space, the host is able to alter the excited-state behavior of guest molecules. The excited-state chemistry and physics of guest molecules are distinctly different from those in organic solvents.     

Quantum-chemical study of 1,3-dioxane complexes with two water molecules

Methods PM3, RHF/6-31G(d), and MP2/6-31G(d)//RHF/6-31G(d) were used in calculation of the energy of formation of five 1,3-dioxane complexes with two water molecules formed through hydrogen bonds. The study of the conformational properties of the most stable associate revealed two routes of the chair-chair conformation isomerization. It was shown that the difference between the minima on the potential energy surface in this gase increased, and the barriersto the interconversion decreased as compared to the calculated values for the isolated molecule of 1,3-dioxane.     

Quinuclidine complex with alpha-cyclodextrin: a diffusion and 13C NMR relaxation study

The stability of an inclusion complex of quinuclidine with alpha-cyclodextrin in solution was investigated by NMR measurements of the translational diffusion coefficient. A 1:1 stoichiometry model yielded an association constant of 35 +/- 3 M(-1). The guest molecules exchange rapidly between the host cavity and the bulk solution. The reorientational dynamics of the guest and host molecules was studied using carbon-13 NMR relaxation at two magnetic fields. The relaxation of the host nuclei showed very little dependence on the guest-host concentration ratio, while the 13C spins in quinuclidine were sensitive to the solution composition. Using mole-fraction data, it was possible to extract the relaxation parameters for the bound and free form of quinuclidine. Relaxation rates of the guest molecule, free in solution, were best described by an axially symmetric model, while the data of the complex species were analyzed using the Lipari-Szabo method. Applying the axially symmetric model to the complexed quinuclidine indicated that the anisotropy of its reorientation in the bound form was increased.     

Restricted motion of guests confined in carceplexes and capsules

Guest orientation within carceplexes and capsules was determined qualitatively from NMR data, and the molecular mobility of guests was determined via coalescence of (1)H NMR signals. Both are highly dependent on guest size and shape, as is interconversion of twistomers. Incarceration of 1,4-thioxane results in a large (1.8 kcal/mol) constraint on thioxane's conformational mobility (chair-to-chair interconversion). Similar conformational constraints (1.6 kcal/mol) were determined for 1,4-dioxane both when incarcerated in carceplex 1b and when encapsulated reversibly in capsule 3b. Encapsulation-induced conformational constraints of this magnitude are unprecedented, and are particularly striking for the noncovalently linked capsules.     

Elucidating the mechanism of binding of chromate to cucurbit[6]uril: a DFT study

We report the binding of chromate, a toxic heavy metal ion to the macrocyclic host molecule, cucurbituril using density functional theory. Due to the anionic nature of the guest molecule and the portals of the host molecule, we propose that the binding mechanism should be assisted by cations. The calculated barrier for chromate binding to cucurbituril is found to be?~17?kcal?mol^?1. The large barrier can be attributed to portal opening of the host molecule, electrostatic repulsion between the guest molecule and the portals of the host molecule and the solvent re-organization around guest molecule.     

An Operative Electrostatic Slipping Mechanism along Macrocycle Flexibility Accelerates Guest Sliding during pseudo-Rotaxane Formation

A pseudo-rotaxane is a host−guest complex composed of a linear molecule encircled by a macrocyclic ring. These complexes can be assembled by sliding the host over the guest terminal groups. If there is a close match between the molecular volume of the flanking groups on the guest and the cavity size of the macrocycle, the slipping might occur slowly or even become completely hindered. We have previously shown that it is possible to overcome the restraints imposed by steric effects on the sliding process by integrating electrostatic attractive interactions during the slipping step. In this work, we extend our electrostatically assisted slipping approach (EASA) to a new host−guest system featuring a flexible macrocyclic ring and a series of asymmetric guests containing a cyclic tertiary ammonium group. Compelling evidence for pseudo-rotaxane formation is presented, along with thermodynamic and kinetic data. Experimental results suggests that the higher conformational flexibility of 24-crown-8 significantly increases the sliding rate, compared with the more rigid dibenzo-24-crown-8, without affecting complex stability. Furthermore, by combining the EASA and macrocycle flexibility, we were capable to slip a large eight-membered cyclic group across the 24-crown-8 annulus, setting a new limit on the ring molecular size that can pass through a 24-membered crown ether.     

Selective adsorption behaviors of guest molecules COR in the hexamer host networks at liquid/solid interface

Here,the selective adsorption behaviors of guest molecule COR in two hexamer host grids were investigated by means of scanning tunnelling microscope(STM).The assembled structures of small functional organic molecules TTBTA and TATBA were thermodynamically stable.Interestingly,the introduction of the guest molecule COR destroyed the original hexamer structure of TTBTA and combined with it to form a new triangular host-guest system.Different from TTBTA,the introduction of the guest molecule COR did not affect the six-membered ring structure of TATBA.Furthermore,the co-assembly structure of TTBTA/TATBA/COR was established and the guest molecule COR showed preferential adsorption to the TATBA host grid.Density functional theory(DFT) calculations had been performed to disclose the mechanism of the involved assemblies.     

Vibrational Spectroscopic Studies on the Dicycloheptylaminecobalt(II) Tetracyanonickellate(II) Host–Aromatic Guest Systems

Co(cycloheptylamine)₂Ni(CN)₄ host complex has been prepared in powder form and its vibrational spectra are investigated. The vibrational assignment of the cycloheptylamine in various phases (gas, solution, liquid and complex) are facilitated by means of DFT calculations performed on the free ligand molecule. The spectral data suggest that the prepared complex is similar in structure to the Hofmann-dma-type hosts. The sorption processes of some aromatic guests (benzene, toluene, o-, m-, p-xylene, 1,2-, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,4-dibromobenzene and naphthalene) in this host complex have also been examined at room temperature by gravimetric and spectroscopic measurements. The desorption of the benzene guest against time has been measured. The structure of the Co(cycloheptylamine)₂Ni(CN)₄ host changes on inclusion of the guest molecule and recovers after liberation.     

Dynamics of an inclusion complex of chloroform and cryptophane-E: evidence for a strongly anisotropic van der Waals bond

The motional dynamics of a van der Waals inclusion complex of cryptophane-E and chloroform has been investigated by a combined NMR exchange and relaxation study. The kinetics of exchange of chloroform between the bulk solution and the complex was investigated by means of proton EXSY measurements. The carbon-13 relaxation of the cryptophane-E host and of the bound chloroform guest was analyzed using the Lipari-Szabo "model-free" approach. For interpretation of the carbon-13 relaxation measurements for chloroform, the chemical-exchange process of complex formation and dissociation had to be taken into account in terms of the modified Bloch equations. It was found that the complex behaves as a single molecule without any significant guest chloroform motion inside the host's cavity.