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.     

Structural links between zeolite-type and clathrate hydrate-type materials: Synthesis and crystal Structure of [N(CH3)4]6[AlxSi8−xO18−x(OH)2+x] · 44H2O (x = 3–4)

A novel tetramethylammonium aluminosilicate hydrate with the approximate composition [NMe₄]₆[Al_xSi_8?xO_18?x(OH)_2+x] · 44H₂O (x = 3–4) has been identified by powder X-ray diffraction as a component in a polyphasic solid mixture which crystallized at room temperature from an aqueous NMe₄OH? Al₂O₃? SiO₂ solution. Large crystals of the novel hydrate phase could be mechanically selected from that mixture. The crystal structure has been determined from 1 196 unique MoKα diffraction data measured at 180 K: Tetragonal crystal system, cell constants a = 16.181(4) and c = 17.450(4) Å, space group P4/mnc with Z = 2 formula units per unit cell, R = 0.072. The host-guest compound is of polyhedral clathrate type with a mixed three-dimensional, (mainly) four-connected network composed of oligomeric aluminosilicate anions [Al_xSi_8?xO_18?x(OH)_2+x]^6? and H₂O molecules linked via hydrogen bonds O? H …? O. The aluminosilicate anions possess a cube-shaped (double four-ring) structure. Orientationally disordered cationic guest species NMe₄⁺ are enclosed in the large [4⁶6⁸] and [4¹5¹⁰6⁷] polyhedral voids of the host framework; the small [4⁶] cages (i.e. the double four-ring anions) and [4³5⁶] cages are empty. The hydrate is a further member in a recently discovered series of clathrates with mixed tetrahedral networks, which provides a structure-chemical link between zeolite- and clathrate hydrate-type host-guest compounds.     

Spectroscopic Observation of the Hydroxy Position in Butanol Hydrates and Its Effect on Hydrate Stability

In this study, we investigate the crystal structures and phase equilibria of butanols+CH₄+H₂O systems to reveal the hydroxy group positioning and its effects on hydrate stability. Four clathrate hydrates formed by structural butanol isomers are identified with powder X‐ray diffraction (PXRD). In addition, Raman spectroscopy is used to analyze the guest distributions and inclusion behaviors of large alcohol molecules in these hydrate systems. The existence of a free OH indicates that guest molecules can be captured in the large cages of structure II hydrates without any hydrogen‐bonding interactions between the hydroxy group of the guests and the water‐host framework. However, Raman spectra of the binary (1‐butanol+CH₄) hydrate do not show the free OH signal, indicating that there could be possible hydrogen‐bonding interactions between the guests and hosts. We also measure the four‐phase equilibrium conditions of the butanols+CH₄+H₂O systems.     

Phase relations and binary clathrate hydrate formation in the system H2-THF-H2O

Experimentally determined equilibrium phase relations are reported for the system H2-THF-H2O as a function of aqueous tetrahydrofuran (THF) concentration from 260 to 290 K at pressures up to 45 MPa. Data are consistent with the formation of cubic structure-II (CS-II) binary H2-THF clathrate hydrates with a stoichiometric THF-to-water ratio of 1:17, which can incorporate modest volumes of molecular hydrogen at elevated pressures. Direct compositional analyses of the clathrate phase, at both low (0.20 mol %) and stoichiometric (5.56 mol %) initial THF aqueous concentrations, are consistent with observed phase behavior, suggesting full occupancy of large hexakaidecahedral (51264) clathrate cavities by THF, coupled with largely complete (80-90%) filling of small dodecahedral (512) cages by single H2 molecules at pressures of >30 MPa, giving a clathrate formula of (H2) < or =2.THF.17H2O. Results should help to resolve the current controversy over binary H2-THF hydrate hydrogen contents; data confirm recent reports that suggest a maximum of approximately 1 mass % H2, this contradicting values of up to 4 mass % previously claimed for comparable conditions.     

Determination of NMR Lineshape Anisotropy of Guest Molecules within Inclusion Complexes from Molecular Dynamics Simulations

Nonspherical cages in inclusion compounds can result in non‐uniform motion of guest species in these cages and anisotropic lineshapes in NMR spectra of the guest. Herein, we develop a methodology to calculate lineshape anisotropy of guest species in cages based on molecular dynamics simulations of the inclusion compound. The methodology is valid for guest atoms with spin 1/2 nuclei and does not depend on the temperature and type of inclusion compound or guest species studied. As an example, the nonspherical shape of the structure I (sI) clathrate hydrate large cages leads to preferential alignment of linear CO₂ molecules in directions parallel to the two hexagonal faces of the cages. The angular distribution of the CO₂ guests in terms of a polar angle θ and azimuth angle ? and small amplitude vibrational motions in the large cage are characterized by molecular dynamics simulations at different temperatures in the stability range of the CO₂ sI clathrate. The experimental ¹³C NMR lineshapes of CO₂ guests in the large cages show a reversal of the skew between the low temperature (77 K) and the high temperature (238 K) limits of the stability of the clathrate. We determine the angular distributions of the guests in the cages by classical MD simulations of the sI clathrate and calculate the ¹³C NMR lineshapes over a range of temperatures. Good agreement between experimental lineshapes and calculated lineshapes is obtained. No assumptions regarding the nature of the guest motions in the cages are required.     

Micro-Raman investigations of mixed gas hydrates

We report laser Raman spectroscopic measurements on mixed hydrates (clathrates), with guest molecules tetrahydrofuran (THF) and methane (CH₄), at ambient pressure and at temperatures from 175 to 280 K. Gas hydrates were synthesized with different concentrations of THF ranging from 5.88 to 1.46 mol%. In all cases THF molecules occupied the large cages of sII hydrate. The present studies demonstrate formation of sII clathrates with CH₄ molecules occupying unfilled cages for concentrations of THF ranging from 5.88 to 2.95 mol%. The Raman spectral signature of hydrates with 1.46 mol% THF are distinctly different; hydrate growth was non-uniform and structural transformation occurred from sII to sI prior to clathrate melting.     

Computational Study on the Antifreeze Glycoproteins as Inhibitors of Clathrate‐Hydrate Formation

The ability of antifreeze glycoproteins to inhibit clathrate‐hydrate formation is studied using DFT. A 5¹² cavity, dodecahedral (H₂O)₂₀, and the AATA peptide are used to model the inhibitor–clathrate interaction. The presence of AATA in the vicinity of the water cavities not only leads to the formation of complexes, with different peptide/cavity ratios, but also to the deformation of the cavity and to the elongation of several of the hydrogen bonds responsible for keeping the dodecahedral (H₂O)₂₀ together. The complexes are formed through hydrogen bonding between the peptides and the water cavities. The glycoproteins are expected to anchor onto the clathrate surface, blocking the access of new water molecules and preventing the incipient crystals from growing. They are also expected to weaken the clathrate structure. Amide IR bands are associated with the complexes’ formation. They are significantly red‐shifted in the hydrogen‐bonded systems compared to isolated AATA. The amide A band is the most sensitive to hydrogen bonding. In addition a distinctive band around 3100 cm^?1 is proposed for the identification of clathrate–peptide hydrogen‐bonded complexes.     

Molecular hydrogen occupancy in binary THF-H2 clathrate hydrates by high resolution neutron diffraction

We have determined the time-space average filling of hydrogen molecules in a binary tetrahydrofuran (THF)-d(8) + D(2) sII clathrate hydrate using high resolution neutron diffraction. The filling of hydrogen in the lattice of a THF-d(8) clathrate hydrate occurred upon pressurization. The hydrogen molecules were localized in the small dodecahedral cavities at 20 K, with nuclear density from the hydrogen approximately spherically distributed and centered in the small cavity. With a formation pressure of 70 MPa, molecular hydrogen was found to only singly occupy the sII small cavity. This result helps explain discrepancies about the hydrogen occupancy in the THF binary hydrate system.     

Heat capacity and transition phenomena of structure I and II trimethylene oxide clathrate hydrates

Heat capacities of structure I and II trimethylene oxide (TMO) clathrate hydrates doped with small amount of potassium hydroxide (x=1.8×10^–4 to water) were measured by an adiabatic calorimeter in the temperature range 11–300 K. In the str. I hydrate (TMO·7.67H₂O), a glass transition and a higher order phase transition were observed at 60 K and 107.9 K, respectively. The glass transition was considered to be due to the freezing of the reorientation of the host water molecules, which occurred around 85 K in the pure sample and was lowered owing to the acceleration effect of KOH. The relaxation time of the water reorientation and its distribution were estimated and compared with those of other clathrate hydrates. The phase transition was due to the orientational ordering of the guest TMO molecules accommodated in the cages formed by water molecules. The transition was of the higher order and the transition entropy was 1.88 J·K^–1(TMO-mol)^–1, which indicated that at least 75% of orientational disorder was remaining in the low temperature phase. In the str. II hydrates (TMO·17H₂O), only one first-order phase transition appeared at 34.5 K. This transition was considered to be related to the orientational ordering of the water molecules as in the case of the KOH-doped acetone and tetrahydrofuran (THF) hydrates. The transition entropy was 2.36 JK^–1(H₂O-mol)^–1, which is similar to those observed in the acetone and THF hydrates. The relations of the transition temperature and entropy to the guest properties (size and dipole moment) were discussed.Contribution No 57 from the Microcalorimetry Research CenterThe authors would like to express their sincere thanks to the Nissan Science Foundation for their financial support.     

Hydrogen bonding between a water molecule and electronegative additives (O or Cl) on a Pt(111) surface

Water adsorption on Pt(111) surfaces treated with oxygen or hydrogen chloride at 20 K has been studied by Fourier transform infrared spectroscopy and scanning tunneling microscopy. Water molecules chemisorb predominantly on the sites of the electronegative additives (O or Cl^-), forming hydrogen bonds of O-HO or O-HCl^-. On a Pt(111)-2×2-O surface, water adsorption produces species (O(D₂O)), monomeric water (D₂O), (O(D₂O)₂) and ring tetramer-like cluster (O(D₂O)₃) on a Pt(111) surface. On a Pt(111)-3×3-Cl^- (θ=0.44) surface, water adsorption gives rise to a Pt(111)-(4×2)-(H₃O⁺+Cl^-) co-adsorption structure to form a hydrogen-bonding network between Cl^- and H₃O⁺ ions.     

The influence of pressure on the stability of clathrate hydrates of hydrogen and tetrahydrofuran

Changes in the Gibbs energy of hydration of molecular hydrogen and tetrahydrofuran (THF) at pressures of 0.1, 6.0, and 12.0 MPa over the temperature range 230–300 K were studied by the molecular dynamics method. The Gibbs energy of hydrogen in water-tetrahydrofuran-hydrogen solutions passed minima over the temperature range 235–265 K, which were indicative of a comparatively stable clathrate hydrate state. The Gibbs energy of the hydrogen molecule at the local minimum at 262 K was ∼4.5 kJ/mol; at atmospheric pressure and room temperature, it was ∼2 kJ/mol. An analysis of the radial distribution function and the coordination number of the THF molecule showed that, at 240–257 K, a clathrate hydrate of THF with the structure close to clathrate sII was predominantly formed.     

Abnormal Thermal Expansion of Clathrate Hydrates Induced by Asymmetric Guest Molecules

We investigated for the first time the abnormal thermal expansion induced by an asymmetric guest structure using high‐resolution neutron powder diffraction. Three dihydrogen molecules (H₂, D₂, and HD) were tested to explore the guest dynamics and thermal behavior of hydrogen‐doped clathrate hydrates. We confirmed the restricted spatial distribution and doughnut‐like motion of the HD guest in the center of anisotropic sII‐S (sII‐S=small cages of structure II hydrates). However, we failed to observe a mass‐dependent relationship when comparing D₂ with HD. The use of asymmetric guest molecules can significantly contribute to tuning the cage dimension and thus can improve the stable inclusion of small gaseous molecules in confined cages.     

A Novel Bismuth‐Based Metal–Organic Framework for High Volumetric Methane and Carbon Dioxide Adsorption

Solvothermal reaction of H₄L (L=biphenyl‐3,3′,5,5′‐tetracarboxylate) and Bi(NO₃)₃ ? (H₂O)₅ in a mixture of DMF/MeCN/H₂O in the presence of piperazine and nitric acid at 100 °C for 10 h affords the solvated metal–organic polymer [Bi₂(L)_1.5(H₂O)₂] ? (DMF)_3.5 ? (H₂O)₃ (NOTT‐220‐solv). A single crystal X‐ray structure determination confirms that it crystallises in space group P2/c and has a neutral and non‐interpenetrated structure comprising binuclear {Bi₂} centres bridged by tetracarboxylate ligands. NOTT‐220‐solv shows a 3,6‐connected network having a framework topology with a {4 ? 6²}₂{4² ? 6⁵ ? 8⁸}{6² ? 8} point symbol. The desolvated material NOTT‐220a shows exceptionally high adsorption uptakes for CH₄ and CO₂ on a volumetric basis at moderate pressures and temperatures with a CO₂ uptake of 553 g L^?1 (20 bar, 293 K) with a saturation uptake of 688 g L^?1 (1 bar, 195 K). The corresponding CH₄ uptake was measured as 165 V(STP)/V (20 bar, 293 K) and 189 V(STP/V) (35 bar, 293 K) with a maximum CH₄ uptake for NOTT‐220a recorded at 20 bar and 195 K to be 287 V(STP)/V, while H₂ uptake of NOTT‐220a at 20 bar, 77 K is 42 g L^?1. These gas uptakes have been modelled by grand canonical Monte Carlo (GCMC) and density functional theory (DFT) calculations, which confirm the experimental data and give insights into the nature of the binding sites of CH₄ and CO₂ in this porous hybrid material.     

A New Silicate Clathrate Hydrate: An X-Ray Diffraction and Nuclear Magnetic Resonance Study of a System with Octameric Silicate Anions and Trivalent Cations

The first crystal structure is reported for a silicate clathrate hydrate involving a triply charged cation [C₁₈H₃₀N₃]³⁺ and an octameric cubic silicate cage. The structure is essentially a host/guest system, with the silicate cages linked into a framework by hydrogen bonding to water molecules. The space group is P with Z = 2, and the asymmetric unit includes a complete cation and half the anion, plus 21 water molecules (4 of which are in disordered positions). Solid-state (CPMAS) ²⁹Si and ¹³CNMR spectra are consistent with the diffraction-determined structure and indicate substantial distortion of the anion from cubic symmetry. Solution-state spectra of precursor solutions and of melted material are also presented and discussed.     

Conversion of Nitric Oxide into Nitrous Oxide as Triggered by the Polarization of Coordinated NO by Hydrogen Bonding

Reduction of the {Co(NO)}⁸ cobalt–nitrosyl N‐confused porphyrin (NCP) [Co(CTPPMe)(NO)] ( 1 ) produced electron‐rich {Co(NO)}⁹ [Co(CTPPMe)(NO)][Co(Cp*)₂] ( 2 ), which was necessary for NO‐to‐N₂O conversion. Complex 2 was NO‐reduction‐silent in neat THF, but was partially activated to a hydrogen‐bonded species 2 ??? MeOH in THF/MeOH (1:1, v/v). This species coupling with 2 transformed NO into N₂O, which was fragmented from an [N₂O₂]‐bridging intermediate. An intense IR peak at 1622 cm^?1 was ascribed to ν(NO) in an [N₂O₂]‐containing intermediate. Time–course ESI(?) mass spectra supported the presence of the dimeric [Co(NCP)]₂(N₂O₂) intermediate. Five complete NO‐to‐N₂O conversion cycles were possible without significant decay in the amount of N₂O produced.     

Hydration of choline salts studied by NMR relaxation

The concentration dependence of T₁ for protons and deuterons in H₂O and D₂O solutions of three choline salts is anomalously high in dilute solutions. We suggest that this is due to formation of clathrate hydrate shells. Acetylcholine chloride does not show anomalous behaviour.     

Ordering of guest-molecule dipoles in the structure I clathrate hydrate of trimethylene oxides

The rotational mobility of encaged trimethylene oxide (TMO) molecules was studied down to 1.8°K by sub-MHz dielectric measurements of the structure I H₂O clathrate and by proton magnetic resonance measurements of the corresponding D₂O clathrate. The results indicate that below a transitional temperature range about 105°K most TMO dipoles assume parallel alignment along the $\text{4}$ axes of the cages. Below the transition the proton second moment suggests the presence of hindered rotation of TMO about its polar axis until the rigid-lattice condition is reached below 5°K. Some residual very broad dielectric absorption (activation energy 2.1 kcal/mole) persists to very low temperatures. Guest-guest and guest-host interaction energies are calculated for simple models     

Ordering and Clathrate Hydrate Formation in Co‐deposits of Xenon and Water at Low Temperatures

Local ordering in co‐deposits of water and xenon atoms produced at low temperatures can be followed uniquely by ¹²⁹Xe NMR spectroscopy. In water‐rich samples deposited at 10 K and observed at 77 K, xenon NMR results show that there is a wide distribution of arrangements of water molecules around xenon atoms. This starts to order into the definite coordination for the structure I, large and small cages, when samples are annealed at ～140 K, although the process is not complete until a temperature of 180 K is reached, as shown by powder Xray diffraction. There is evidence that Xe ? 20 H₂O clusters are prominent in the early stages of crystallization. In xenon‐rich deposits at 77 K there is evidence of xenon atoms trapped in Xe ? 20 H₂O clusters, which are similar to the small hydration shells or cages observed in hydrate structures, but not in the larger water clusters consisting of 24 or 28 water molecules. These observations are in agreement with results obtained on the formation of Xe hydrate on the surface of ice surfaces by using hyperpolarized Xe NMR spectroscopy. The results indicate that for the various different modes of hydrate formation, both from Xe reacting with amorphous water and with crystalline ice surfaces, versions of the small cage are important structures in the early stages of crystallization.     

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.