Transformation of the Hexagonal‐Structure Clathrate Hydrate of Cyclooctane to a Low‐Symmetry Form Below 167 K

Udachin, Konstantin A.; Ratcliffe, Christopher I.; Enright, Gary D.; Ripmeester, John A.

doi:10.1002/ange.200801694

Cited by 3 publications

(2 citation statements)

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“…The details of how molecular factors affect the formation conditions are not clearly understood despite that the recent advances on understanding guest–host interactions on the lattice structure, molecular conformation of LMGSs in the lattice, and the formation conditions. The previous studies related to these points are briefly reviewed below. − Udachin et al revealed that lowering the temperature below 167 K induced the distortion of the sH hydrate lattice containing cyclooctane . They speculated that the distortion can due to the change of cyclooctane orientation at the low temperature.…”

Section: Introductionmentioning

confidence: 99%

“…9−12 Udachin et al revealed that lowering the temperature below 167 K induced the distortion of the sH hydrate lattice containing cyclooctane. 10 They speculated that the distortion can due to the change of cyclooctane orientation at the low temperature. Lee et al observed that 13 hydrates move upfield in the spectrum compared to the same peaks of the molecule in gas phase.…”

Section: ■ Introductionmentioning

confidence: 99%

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Effect of Guest Size and Conformation on Crystal Structure and Stability of Structure H Clathrate Hydrates: Experimental and Molecular Dynamics Simulation Studies

et al. 2013

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To better understand the effect of size and flexibility of large molecule guest substances (LMGSs) on crystal lattice structure and thermodynamic stability of structure H (sH) clathrate hydrates, we performed powder X-ray diffraction (PXRD) measurements and Parrinello–Rahman molecular dynamics (MD) simulations on six alkane and cycloalkane LMGS in the presence of methane help gas. We first quantitatively analyze the dependence of the experimental lattice constants and formation pressure on the average maximum length of the LMGS guests as determined by MD simulation. The PXRD results show that within a family of LMGSs with similar molecular structures molecules of optimum size give better stability of sH hydrate phase. The most stable hydrate in each class has larger lattice constants along the a-axis and smaller lattice constant along the c-axis. The lattice expansion/shrinkage is well reproduced in the MD simulations. From the MD simulations, we determine the changes in the conformation as a result of encapsulation and the tilt angle with respect to the long axis of the sH large cages of the LMGSs. The results indicate that the molecular shapes inside the sH large cages can significantly differ from those of the most stable molecular structure in the gas phase. In the case of flexible molecules, such as 2-methylbutane, the 1–4 dihedral angles and effective molecular sizes change upon encapsulation. Molecules with shorter length generally have larger tilt angles in the large cages; however, the effective width dimension of the LMGS also affects the tilt angle. Understanding the stability of sH hydrates of various LMGSs requires a consideration of guest molecule size, structural flexibility, and tilt angle in the cages. None of these quantities alone explain trends in the stability. During simulation trajectories, we observe changes in conformation in the LMGS molecules in the large cages. The effects of these different factors make a priori structural determinations of the large cages guests extremely complex.

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Section: Introductionmentioning

confidence: 99%

Section: ■ Introductionmentioning

confidence: 99%

Effect of Guest Size and Conformation on Crystal Structure and Stability of Structure H Clathrate Hydrates: Experimental and Molecular Dynamics Simulation Studies

et al. 2013

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

et al. 2016

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The crystal structure and phase transition of cubic structure II (sII) binary clathrate hydrates of methane (CH 4 ) and propanol are reported from powder X-ray diffraction measurements.T he deformation of host water cages at the cubic-tetragonal phase transition of 2-propanol + CH 4 hydrate,b ut not 1-propanol + CH 4 hydrate,w as observed belowabout 110 K. It is shown that the deformation of the host water cages of 2-propanol + CH 4 hydrate can be explained by the restriction of the motion of 2-propanol within the 5 12 6 4 host water cages.T his result provides al ow-temperature structure due to atemperature-induced symmetry-lowering transition of clathrate hydrate.This is the first example of acubic structure of the common clathrate hydrate families at af ixedc omposition.Clathrate hydrates,a lso known as gas hydrates,a re hostguest compounds that are crystalline materials consisting of water molecule frameworks.The host water cages of clathrate hydrate are hydrogen-bonded and encage guest molecules inside via van der Waals interactions. [1] Each guest molecule is encaged into different types of cages ordered in threedimensional structures according to the size of the guest molecule.T hree different crystal structure of clathrate hydrate are well known depending on type of guest molecules;c ubic structure I( sI) with space group Pm3 n,c ubic structure II (sII) with space group Fd3 m, and hexagonal structure H( sH) with space group P6/mmm. [2] These host structures are analogues of the semiconducting Group 14 clathrates [3] and silica clathrates (so-called clathrasils). [4] Natural-gas or methane hydrate is now seen as apossible global source of methane, [5,6] and the capability of gas hydrates to store large amounts of gas has opened up possibilities for potential industrial applications such as hydrogen [7,8] and ozone [9,10] storage.P hysical properties of gas hydrates are attributed to host-guest interactions such as rattling motions [11] or hydrogen bonding [12,13] of guest mole-cules.H ost-guest interactions also play ac rucial role,a sg as hydrate structures are thermodynamically stable only when am inimum number of cages are occupied by the guest molecules,depending on their nature.Phase changes of clathrate hydrates are known to take place with pressure,and cubic clathrate structures,sIand sII, at ambient pressure transform to hexagonal sH or astructure closely related to it at pressure above 0.5 GPa. [14] Up to now, however, few examples of the phase change with temperature have been reported in the common clathrate hydrate structural families sI, sII, and sH at constant composition. [15,16] On the other hand, clathrasils have av ariety of polyhedral cages and show temperature-induced phase changes. [17] Given the analogy between oxygen linking tetrahedral atoms in silicate framework structure in clathrasils and hydrogen bonds linking oxygen atoms in clathrate hydrates, [18,19] there is ag reat interest in determining whether lower symmetry clathrate hydrates are indeed realized from cubic clathrate st...

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