On the clathrate structures of syndiotactic poly(m-methylstyrene)

The crystal structures of clathrate forms of syndiotactic poly(m-methylstyrene) containing guest molecules having different steric hindrance (CS₂, benzene and orto-dichlorobenzene) are presented. The structures are all characterized by polymer chains in s (2/1)2 helical conformation and guest molecules packed in an orthorhombic unit cell according to the space group Pcaa. All the presented clathrates belongs to β class indipendently from the dimensions of the guest molecule. In this aspect they differ both from clathrate forms of syndiotactic polystyrene, all belonging to α class, and from clathrate forms of syndiotactic poly(p-methylstyrene) that belong to α or β class according to the steric hindrance of the guest molecule.     

Comparative analysis of the clathrate forms of syndiotactic polystyrene,poly (p-methylstyrene) and poly (m-methylstyrene)

The crystal structures of the clathrate forms of syndiotactic polystyrene (s-PS), poly (p-methylstyrene) (s-PPMS) and poly (m-methylstyrene) (s-PMMS) containing guest molecules having widely different steric hindrance are compared in detail. Common features and differences concerning the packing of the chains, the shape and the dimensions of the cavities and the stability of the forms deprived of the guest molecules are pointed out. A new clathrate form of s-PPMS containing CS₂ is also described.     

Clathrates with tetrahydrofuran of styrene‐p‐methyl styrene co‐syndiotactic copolymers

Clathrates with tetrahydrofuran of styrene‐p‐methyl styrene co‐syndiotactic copolymers have been characterized by wide‐angle X‐ray scattering and differential scanning calorimetry. Stable clathrate structures with THF molecules have been observed in whole range of composition. In particular, syndiotactic polystyrene like and syndiotactic poly‐p‐methylstyrene like clathrate phases are obtained for p‐methylstyrene contents lower and higher than 35% by mol, respectively.     

Polymorphism of the co-crystalline forms of syndiotactic polystyrene with chloroform: Crystal structure of the δ clathrate

The crystal structure of the δ clathrate form of syndiotactic polystyrene (s-PS) containing CHCl₃, a molecule having a pivotal role in respect to the co-crystalline phases formation of this polymer, has been determined through X-ray diffraction data and molecular mechanics calculations. Analogously to all the other δ clathrate forms of s-PS, this structure presents a monoclinic unit cell (cell constants a = 1.77 nm, b = 1.32 nm, c = 0.78 nm and γ = 121.5°) in which the s(2/1)2 polymer helices and guest molecules are packed according the space group P2₁/a. At variance with all the other δ clathrate forms of s-PS whose crystal structure has been reported in the literature, probably due to the not planar shape of the chloroform guest molecule, in this structure guest molecules occupy each centrosymmetric cavity in a very low efficient way, giving rise to a disorder in the positioning of the guest molecules along the b + a/2 direction of the unit cell. A comparison with the ε type clathrate with the same guest, for which some preliminary results have been reported too, is also presented.     

Breaking the Tetra‐Coordinated Framework Rule: New Clathrate Ba8M24P28+δ (M=Cu/Zn)

A new clathrate type has been discovered in the Ba/Cu/Zn/P system. The crystal structure of the Ba₈M ₂₄P_28+δ (M =Cu/Zn) clathrate is composed of the pentagonal dodecahedra common to clathrates along with a unique 22‐vertex polyhedron with two hexagonal faces capped by additional partially occupied phosphorus sites. This is the first example of a clathrate compound where the framework atoms are not in tetrahedral or trigonal‐pyramidal coordination. In Ba₈M ₂₄P_28+δ a majority of the framework atoms are five‐ and six‐coordinated, a feature more common to electron‐rich intermetallics. The crystal structure of this new clathrate was determined by a combination of X‐ray and neutron diffraction and was confirmed with solid‐state ³¹P NMR spectroscopy. Based on chemical bonding analysis, the driving force for the formation of this new clathrate is the excess of electrons generated by a high concentration of Zn atoms in the framework. The rattling of guest atoms in the large cages results in a very low thermal conductivity, a unique feature of the clathrate family of compounds.     

Chemical separations by nanoporous crystalline samples of syndiotactic polystyrene

Chemical separations based on absorption phenomena into syndiotactic polystyrene samples including a nanoporous crystalline phase are described. These separation phenomena are due to the presence into the crystalline phase of cavities whose size and shape have been studied through sorption experiments relative to linear hydrocarbons and through calculation procedures relative to the available crystalline structures of different polymorphic and clathrate forms of syndiotactic polystyrene.     

Effect of Guest–Host Hydrogen Bonding on the Structures and Properties of Clathrate Hydrates

To provide improved understanding of guest–host interactions in clathrate hydrates, we present some correlations between guest chemical structures and observations on the corresponding hydrate properties. From these correlations it is clear that directional interactions such as hydrogen bonding between guest and host are likely, although these have been ignored to greater or lesser degrees because there has been no direct structural evidence for such interactions. For the first time, single‐crystal X‐ray crystallography has been used to detect guest–host hydrogen bonding in structure II (sII) and structure H (sH) clathrate hydrates. The clathrates studied are the tert‐butylamine (tBA) sII clathrate with H₂S/Xe help gases and the pinacolone + H₂S binary sH clathrate. X‐ray structural analysis shows that the tBA nitrogen atom lies at a distance of 2.64 Å from the closest clathrate hydrate water oxygen atom, whereas the pinacolone oxygen atom is determined to lie at a distance of 2.96 Å from the closest water oxygen atom. These distances are compatible with guest–water hydrogen bonding. Results of molecular dynamics simulations on these systems are consistent with the X‐ray crystallographic observations. The tBA guest shows long‐lived guest–host hydrogen bonding with the nitrogen atom tethered to a water HO group that rotates towards the cage center to face the guest nitrogen atom. Pinacolone forms thermally activated guest–host hydrogen bonds with the lattice water molecules; these have been studied for temperatures in the range of 100–250 K. Guest–host hydrogen bonding leads to the formation of Bjerrum L‐defects in the clathrate water lattice between two adjacent water molecules, and these are implicated in the stabilities of the hydrate lattices, the water dynamics, and the dielectric properties. The reported stable hydrogen‐bonded guest–host structures also tend to blur the longstanding distinction between true clathrates and semiclathrates.     

On the clathrate forms of syndiotactic poly(p-methylstyrene)

A classification for clathrate structures of syndiotactic poly(p-methylstyrene) in two classes (α and β), on the basis of X-ray diffraction analyses as well as of the different observed behavior with respect to the thermal and acetone treatment, is presented. A structural correlation between α and β class clathrates and forms I and II, respectively, is evidenced. It is also suggested that form I and form II could be the “emptied forms” of the α and β class clathrates, respectively. Similar steric hindrance of the molecules inducing clathrate structures, within each class has been observed.     

Crystal structures of syndiotactic poly(p-methylstyrene) clathrates with two guest molecules having very similar shape and volume

In the present paper we report on the clathrate structures of syndiotactic poly(p-methylstyrene) (s-PPMS) containing chlorobenzene (cell constants: a = 23.5 Å, b = 12.0 Å, c = 7.9 Å and γ = 112.4°; space group: P2₁/a) and toluene (cell constants: a = 19.5 Å, b = 13.5 Å, c = 7.9 Å and γ = 90°; space group: P2₁). Despite the extreme similarity between the shape and volume of these two molecules, they give rise to completely different clathrate structures, the first belonging to α class, the second belonging to β class. Moreover the clathrate form containing chlorobenzene represent the first case in which a α class clathrate form of s-PPMS contains two guest molecules in each cavity while the crystal structure here proposed for the s-PPMS/toluene clathrate represents a new example of chiral crystalline phase in which the polymer helices assume all the same chirality in the lattice. These results underline the fact that the choice of a particular structural organization in the process of clathrates’ formation of s-PPMS is not easily referable only to steric effects but seems to be dependant even on the chemical structure of the guest molecules. A comparison with syndiotactic polystyrene is also done.     

1,2‐dichloroethane conformation and molecular organization in syndiotactic polystyrene gels

The conformational equilibrium of 1,2‐dichloroethane (DCE) in syndiotactic polystyrene (sPS)/DCE gels has been investigated by using Fourier transform infrared spectroscopy. Results show that the fraction of DCE in the trans conformation (X_T) increases with increasing polymer concentration. From these X_T values, by assuming that the crystalline phase in the gel presents the clathrate structure, the amount of DCE included in the crystalline phase and the fraction of polymer included in the crystalline phase were evaluated as a function of the gel composition.     

Molecular‐Complex Formation of Syndiotactic Polystyrene with Stable Radical Molecules

Summary: The formation of a molecular‐complex crystalline phase of syndiotactic polystyrene (sPS) that contains a stable nitroxide radical compound, 2,2,6,6‐tetramethylpiperidinyl‐N‐oxyl (TEMPO), is confirmed by IR and electron spin resonance (ESR) spectroscopy, X‐ray diffractometry, and thermogravimetric analysis. Through a guest exchange procedure assisted by a plasticizing agent, the original guest (chloroform) contained in the starting clathrate phase is completely replaced by TEMPO. Although the conformational regularity of the sPS helices in the resultant crystalline phase that contains TEMPO is similar to that in the starting clathrate phase, the host lattice expands in the 010 direction. The guest TEMPO molecules exhibit a significantly broadened ESR signal because of their highly concentrated state in the complex crystalline phase.

Thermogravimetric measurement of a powder sample of the sPS/TEMPO complex.     

Effect of stereosequences on crystallinity and properties of zone‐drawn poly(vinyl alcohol) microfibrils

To effectively orient the molecular chains of novel syndiotactic poly(vinyl alcohol) (PVA) microfibrillar fiber (PVA fibril), a high‐temperature zone‐drawing method was adopted. The PVA fibrils were directly prepared from the saponification and in situ fibrillation without a spinning procedure. The maximum draw ratio of the PVA fibril increased with a decrease in the syndiotactic diad (r‐diad) content, indicating that the deformability of PVA molecules was lowered in higher syndiotactic PVA. Degree of crystal orientations up to 0.990 were achieved by stretching the PVA fibril with the r‐diad content of 65.1% and the original degree of crystal orientation of 0.902 at 250 °C close to its crystal melting temperature (T_m). When the same draw ratio was applied to the fibrils, a higher crystal orientation was achieved for the fibrils having higher syndiotacticity. Wide‐angle X‐ray data show that the longitudinal crystal sizes of drawn PVA fibrils were larger in higher syndiotacticities. The degree of crystal orientation, crystallinity, T_m, longitudinal crystal size, and tensile strength of the maximum drawn PVA fibril with a r‐diad content of 65.1% were 0.99, 0.97, 279 °C, 187 Å, and 4.66 N/tex, respectively. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1263–1271, 2001     

Crystal structure of the Hofmann-dma type clathrate

Seven new Hofmann-dma type clathrates Cd(dma)₂Ni(CN)₄-xG (x = 1, G = aniline, 2,3-xylidine, 2,4-xylidine, 2,5-xylidine, 2,6-xylidine, 3,5-xylidine and x = 2, G = 2,4,6-trim ethylaniline) were prepared by replacing the amine in a Hofmann type clathrate Cd(NH₃)₂Ni(CN)₄-2G by dimethylamine (dma). The structure of the Hofmann-dma type clathrate is formed with stacked host two-dimensional metal complexes of Cd(dma)₂ Ni(CN)₄ and guest molecules accommodated in the space between the stacked host complexes. This basic structure scheme is the same as that of the Hofmann type clathrate. However, the guest species accommodated in the Hofmann-dma type clathrate are more various than those of the Hofmann type clathrate, and their crystal structures are classified into four types depending on the geometry of the guest species. In order to clarify the structure of the Hofmann-dma type clathrate, single crystal X-ray diffraction experiments were canied out on the seven new clathrates, and the crystal structures of the o-, m- and p- toluidine clathrates were refined. The X-ray structure analyses showed that the host two-dimensional metal complex of the Hofmann-dma tvpe clathrate has stmctural flexibility to form a puckered structure, which results from the angular distortion of the bond between Cd and N of the cyanide bridge in the host two-dimensional complex. This stmctural flexibility of the host complex leads to the diversity of crystal structures and guest species in Hofmann-dma type clathrates. Translated fromZhurnal Strukturnmoi Khimii, Vol. 40, No. 5, pp. 898–926, September–October, 1999.     

Thermal Behavior of Syndiotactic Polystyrene/1,2‐Dichloroethane Gels and Stoichiometry of Polymer‐Solvent Compounds

Abstract

The phase behavior of syndiotactic polystyrene (sPS)/1,2‐dichloroethane (DCE) gels has been investigated by differential scanning calorimetry (DSC) and wide angle x‐ray diffraction (WAXD) for different gel formation conditions. DSC data show that the gel melting temperatures are independent of the cooling conditions leading to gel formation, whereas the melting enthalpies of the gels depend on the cooling history. WAXD measurements show that for low concentration gels the sPS/DCE clathrate structure is obtained, whereas for high concentration gels both the clathrate structure and the solvent‐free β phase can be obtained. Furthermore, the onset of formation of the β phase is dependent on the cooling rate. In light of combined DSC and WAXD data, the reliability of the DSC in evaluating the absolute stoichiometry of polymer‐solvent compounds formed in sPS/DCE gels is discussed.     

Desorption kinetics of a xanthenol–dioxane clathrate

The host xanthenol compound forms a 1:1 clathrate with dioxane, namely 9‐(1‐naphthyl)‐9H‐xanthen‐9‐ol–1,4‐dioxane, C₂₃H₁₆O₂·C₄H₈O₂. The structure of this clathrate is reported, along with a study of the kinetics of desolvation and the determination of an activation energy. The guest mol­ecules are stabilized by O_host—H⋯O_guest hydrogen bonds [O—H = 0.968 (2) Å, O⋯O = 2.7532 (13) Å and O—H⋯O = 151.9 (4)°].     

Phase Transition of a Structure II Cubic Clathrate Hydrate to a Tetragonal Form

The crystal structure and phase transition of cubic structure II (sII) binary clathrate hydrates of methane (CH₄) and propanol are reported from powder X‐ray diffraction measurements. The deformation of host water cages at the cubic–tetragonal phase transition of 2‐propanol+CH₄ hydrate, but not 1‐propanol+CH₄ hydrate, was observed below about 110 K. It is shown that the deformation of the host water cages of 2‐propanol+CH₄ hydrate can be explained by the restriction of the motion of 2‐propanol within the 5¹²6⁴ host water cages. This result provides a low‐temperature structure due to a temperature‐induced symmetry‐lowering transition of clathrate hydrate. This is the first example of a cubic structure of the common clathrate hydrate families at a fixed composition.     

Building an Organic Zeolite from a Macrocyclic TADDOL Derivative or How to Teach an Old Dog New Tricks

Crystal structures of the macrocyclic TADDOL (α,α,α′,α′‐tetraaryl‐1,3‐dioxolane‐4,5‐dimethanol) derivative A and its clathrate with diethyl ether are presented. Both the host and clathrate crystallize in the same space group (P2₁2₁2₁) with very similar cell dimensions, i.e., they are isomorphous. At ambient temperature, diethyl ether escapes from A ⋅Et₂O, and clathrate single crystals are transformed into the guest‐free form without breakdown of the crystal lattice. Conversely, crystalline A , when exposed to Et₂O vapor, incorporates guest molecules in a ratio of up to ca. 1 : 1. This behavior, usually the hallmark of inorganic nanoporous materials, allows to describe this compound as an ‘organic zeolite'.     

DUAL BEHAVIOR OF ACETONITRILE IN A NEW CADMIUM CYANIDE CLATHRATE: CRYSTAL STRUCTURE OF Cd(CH3CN)2Cd(CN)4·2CH3CN

Abstract

We report a new crystal structure of the title clathrate containing tetrahedral and octahedral Cd atoms in a ratio of 1:1. The preparation of the compound is similar to that of the cristobalite-like clathrate Cd(CN)₂·G, where all Cd atoms are tetrahedral. The new inclusion compound crystallizes in the monoclinic space group C2/c, a = 12.337(4), b = 11.964(3), c = 13.594(3) Å, β = 108.60(2)°, Z = 4, R = 0.034 for 1631 reflections. The three-dimensional host framework is built of alternate linkages between the tetrahedral Cd atom of the tetracyanocadmate and the octahedral Cd atom similar to that of the Hofmann-Td and the en-Td types. In the new clathrate dual behavior of acetonitrile, one as a unidentate ligand in the three-dimensional host framework and the other as the guest in the cage-like cavity, has been demonstrated.     

Alkyl‐chain disorder in tetraisohexylammonium bromide

Tetraisohexylammonium bromide [systematic name: tetrakis(4‐methylpentyl)azanium bromide], C₂₄H₅₂N⁺·Br⁻, is a powerful structure II clathrate hydrate crystal‐growth inhibitor. The crystal structure, in the space group P3₂21, contains one ammonium cation and one bromide anion in the asymmetric unit, both on general positions. At 100 K, the ammonium cation exhibits one ordered isohexyl chain and three disordered isohexyl chains. At 250 K, all four isohexyl chains are disordered. In an effort to reduce the disorder in the alkyl chains, the crystal was thermally cycled, but the disorder remained, indicating that it is dynamic in nature.     

25‐Allyloxy‐5,11,17,23‐tetra‐tert‐butyl‐26,27,28‐trihydroxycalix[4]arene chloroform disolvate

In the title solvated calixarene, C₄₇H₆₀O₄·2CHCl₃, the host chalice displays an almost undistorted cone conformation, stabilized by three strong O—H...O hydrogen bonds at the calixarene's lower rim. One chloroform solvent molecule is fixed in the calixarene cavity by C—H...π interactions, while the second is accommodated in a clathrate‐like mode in elliptical packing voids. These voids are spanned by six host molecules connected via C—H...π contacts and van der Waals interactions. Within the crystal structure, one tert‐butyl group of the calixarene host is disordered over two orientations, with occupancies of 0.884 (4) and 0.116 (4). Furthermore, both solvent molecules show disorder, with occupancies of 0.857 (2) and 0.143 (2) for the cavitate‐type, and 0.9359 (17) and 0.0641 (17) for the clathrate‐type chloroform solvent molecules.