Investigation of gas chromatographic interaction mechanism on permethylated cyclodextrins by molecular modelling

The capillary gas chromatographic retention behavior of alpha-pinene and tricyclene has been investigated on stationary phases of different polarities. On all bur one of the columns employed, tricyclene eluted before alpha-pinene; only permethylated beta-cyclodextrins dissolved in moderately polar polisiloxanes gave a reversed elution order. The intermolecular interactions which caused the unexpected retention behavior were investigated in detail, applying methods of computer simulation. To achieve this, we have developed a calculation algorithm on the basis of molecular mechanical optimizations and programmed it in a macro. This makes it possible to systematically investigate a given configuration space in which all the possible interactions can take place. It was shown that permethylated beta-cyclodextrin as host molecule for both guest molecules offers an optimum cavity size. As a result the number of energetically favorable contacts between host and guest molecules as well as the strength of the interactions in this stationary phase were larger. As a consequence the elution order, normally only influenced by the vapor pressure of the compounds at a given temperature, was changed. Non-specific interactions played an especially important role for these kinds of substances.