Clathrate lability

Despite the formation of clathrate-like clusters and complete 512 cages during these simulations, the increased ordering observed from the radial distribution functions and local phase assignments resulted in the authors concluding that their simulation results are consistent with a local order model of nucleation, and therefore do not support the labile cluster model. 

We find that the nucleation proceeds via "the local structuring mechanism/ " i.e., a thermal fluctuation causing the local ordering of CO2 molecules leads to the nucleation of the clathrate, and not by the labile cluster hypothesis, one current conceptual picture. The local ordering of the guest molecules induces ordering of the host molecules at the nearest- and next-to-nearest-neighbor shells, which are captured by a three-body host-host order parameter, f these thermodynamic fluctuations lead to the formation of the critical nucleus. 

Experimental results [9,11,14,16], obtained after article [3] had appeared, confirm the conclusion about hydrate structures stability only in the case, where the big (T,P,H) cavities are completely occupied. This all the more concerns the combined cavities. Thus, we can draw a conclusion of the clathrate frameworks lability (dynamic instability) rather than of their metastability in the absence of the guest. 

ABSTRACT. Studies of chiroptical properties need separation or at least enrichment of enantiomers. The separation is difficult with chiral con-formers of molecules with very low barriers of internal rotation. However, in association with cyclodextrin, these labile molecules can exhibit a strong Cotton effect in solution one chiral conformer is favoured by complexation. The cyclodextrins have the advantage to yield inclusion complex in solution as well as crystalline clathrates. Therefore, the absolute configuration of the guest can be obtained by determining the structure of the cyclodextrin. In this work, the first CD spectrum of 4-helicene is recorded And crystal structures of several clathrates of labile molecules are studied. 

At present, clathrate compounds are considered as both ideal [l] and nonideal solid solutions [2-3] of the guest component in a crystalline framework of the host component. In the absence of the guest component, clathrate 3-modification is considered to be metastable with respect to the stable a-modification under ordinary conditions, i.e. Ap.= i - i <0, where [x, is the chemical potential of the corresponding modification. A number of experimental data on water, urea and thiourea clathrates, hardly consistent with the classical theory, made us study the formation of clathrates with unstable (labile) host frameworks. A comprehensive motivation of such possibility is given in [4]. 

In our model we shall assume the framework, to become at least metastable when the cavities of a labile type are completely filled by the guest molecules. For simplicity, clathrates with two types of cavities (labile and stable in the absence of the guest) and one type of non-dipolar guest molecules will be considered. 

Let us assume that when the guest molecules fill the labile cavities a metastable framework ) forms, whose free energy F can be determined using a number of suppositions accepted for classical clathrates. 

If one knows the expression for the free energy of a metastable structure consisting of a framework whose labile cavities are filled with guest molecules, the further theory is created just as the traditional clathrate solution theory [1-3]. The expression for the clathrate free energy will look as follows ... 

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