Clathrate formation

An interesting PLC variation of the nrea clathration techniqne has been proposed by Chaffee and Johns [82]. Component mixtnres are applied onto TLC plates (20 cm X 20 cm X 0.5 mm) coated with Kieselgnhr G/nrea (2 1), prepared from a slnrry in urea-saturated methanol (Iml/g powder). Spotted plates are left in methanolic atmosphere overnight to allow clathrate formation. To remove methanol, plates are air dried for 2 to 3 h at room temperatnre and then developed in -heptane. Two bands of acychc (Rf 0.9 to 1.0) and cyclic (Rf 0.0 to 0.1) hydrocarbons are distinguished, and components are recovered qnantitatively by extraction with chloroform. 

Thiourea clathrates may also be applied for geochemical separations. They are similar to the molecnlar complexes of nrea bnt owing to the larger dimensions of the thiourea crystal channels, only branched alkanes, cycloalkanes, and their derivatives participate in clathrate formation. In contrast, the unbranched molecules are too small and hence cannot be held rigidly [77]. 

This book is the second of a two-volume series that attempts to bring together in-depth investigations in currently important areas of molecular inclusion and molecular recognition mainly based on clathrate formation. The first volume of this series (Volume 140) was published in 1987. 

Nevertheless, as in many previous observations, the clathrate formation by dipolar host compounds could not have been predicted in advance. In fact, there are no channels in the crystal structures of hydrated moxnidazole hydrochloride (closely related species to furaltadone hydrochloride) and of hydrated furaltadone base (Fig. 13)37). Rather, the latter two structures are best described as solvates with the H20 molecules contained in local voids between adjacent moieties of the host. 

The difficulties encountered in predicting clathrate formation are further illustrated... 

Notwithstanding the variety of structural patterns that characterize the crystalline inclusions referred to above, it has recently been shown that the clathrate formation can be induced in a more systematic manner. The use of the clathration phenomenon to store selected guest molecules or to separate one type of guest species from another is the subject of the following sections in this account. 

On the basis of previous observations it was anticipated that the clathrate formation would be more selective in controlling guest selectivities if a functionality which could form hydrogen-bonds with guest species and add this strengthening feature to that... 

Selectivity studies with DTU indicated marked discrimination in the clathrate formation 23,45). As in other types of clathrates, the steric factor is important in differentiation between compounds of similar functionality but different shape. For example, DTU forms crystalline complexes with some alcohols (methanol, ethanol, propanol, 1-butanol) but not with others (2-butanol). It complexes the ethyl esters of N-acetyl derivatives of glycine, alanine, methionine and aspartic acid, but not of proline, serine, phenylalanine and glutamic acid. 

Functional Group Assisted Clathrate Formation — Scissor-Like and Roof-Shaped Host Molecules... 

Functional Group Assisted Clathrate Formation 2.1 An Old Matter or a Missing Event ... 

To probe the range of application of the new inclusion strategy (coordination-assisted clathrate formation) in different ways, directed structural modifications were undertaken starting from the basic constitution 1, for instance as to the molecular skeleton (basic structure) and/or the sensor section (functional groups). The formulae 7-24 show different possibilities of such variations. 

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