Molecular polar,differing strengths

In some molecules the molecular dipole is simply dominated by a single polar bond. In sufficiently complex molecules there may be several polar bonds of differing strength. In this case the molecular dipole is determined by their relative orientation. Ab initio studies of 2-2 -difluoro biphenyl have revealed a strong shape dependence of the molecular dipole as a function of inter-ring angle. This is illustrated in Fig. 12. 

Liquid crystals have found widespread use as stationary phases in gas chromatographic applications due to the benefits of coupling the usual analytical strengths of gas chromatography with the unique structure and shape selective properties of the liquid crystalline phase. Interaction of solutes with the orientational order provided by the anisotropy of the liquid crystal stationary phase allows for the effective and selective separation of positional and geometric isomers. This remarkable solute structural discrimination is especially important for the separation of isomers that have similar physical properties and thus cannot be conveniently separated on conventional capillary columns that mainly differentiate on the basis of boiling point/molecular weight or polarity differences. The mechanism of separation in liquid crystalline stationary phases is based on specific intermolecular inter-... 

The physical properties of the haloalkanes are quite distinct from those of the corresponding alkanes. To understand these differences, we must consider the size of the halogen substituent and the polarity of the carbon-halogen bond. Let us see how these factors affect bond strength, bond length, molecular polarity, and boiling point. 

Molecular interactions are the result of intermolecular forces which are all electrical in nature. It is possible that other forces may be present, such as gravitational and magnetic forces, but these are many orders of magnitude weaker than the electrical forces and play little or no part in solute retention. It must be emphasized that there are three, and only three, different basic types of intermolecular forces, dispersion forces, polar forces and ionic forces. All molecular interactions must be composites of these three basic molecular forces although, individually, they can vary widely in strength. In some instances, different terms have been introduced to describe one particular force which is based not on the type of force but on the strength of the force. Fundamentally, however, there are only three basic types of molecular force. 

Interactive LC systems are those where solute retention is predominantly controlled by the relative strengths of the molecular interactions between solute molecules with those of the two phases. In such systems, exclusion and entropically driven interactions will be minor contributions to retention. The three basically different types of molecular interaction, dispersive, polar and ionic give rise to three subgroups, each subgroup representing a separation where one specific type of interaction dominates in the stationary phase and thus governs solute retention. The subgroups are as follows ... 

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