META-SUBSTITUTED STYRENE MOLECULES INCLUDED IN CYCLODEXTRINS - A RAMAN-SPECTROSCOPIC STUDY

Raman spectra of styrene (S), 3-fluorostyrene (3FS), 3-chlorostyrene (3CS) and 3-methylstyrene (3MS) and their complexes in alpha-, beta- and gamma-cyclodextrins (alpha-, beta-, gammaCD) were recorded at various temperatures. The substituted styrene molecules were found to provide good group frequencies, namely, nuC-C and nuC=H, for probing the guest molecules. Generally, the C-C oscillator is shown to be a good probe for studying conformational equilibria of the guest molecule perturbed by complex formation with the various CD molecules. The CH oscillators provide information concerning plausible structures for the complexes. On the whole, all the spectral observations are consistent with different types of structures for the (S, 3MS)-alphaCD and (3FS, 3CS)-alphaCD complexes. In particular, the Raman spectroscopic results suggest that, for 3MS-alphaCD the vinyl group keeps some rotational freedom. For 3FS-alphaCD and 3CS-alphaCD, it is suggested that the most electronegative fluorine and chlorine atoms are left outside the CD cavity, with the vinyl group rotationally hindered. For the gammaCD complexes, the wider CD cavity sould be able to accommodate all the guest molecules considered keeping the conformer distributions of the pure liquids approximately. However, the inclusion process seems to hinder the internal rotation in such a way that even below 200 K the conformer distribution remains similar to that of the pure liquid at room temperature.