Dielectric behaviour of alpha-cyclodextrin, heptakis (2,3,6-tri-O-methyl)-beta-cyclodextrin, randomly methylated beta-cyclodextrin and low frequency Raman spectra of alpha- and beta-cyclodextrins

The frequency and temperature dependence of the real (epsilon') and imagina ry (epsilon") parts of the dielectric constant of alpha -cyclodrextrin (for m 1; alpha -CD.5.9H(2)O) and alpha -cyclodextrin (form III; alpha -CD.7.6H( 2)O) and of the corresponding dried forms (alpha -CD.1.1H(2)O, alpha -CD.2. 9H(2)O, respectively) has been investigated over the frequency range 0-100 k Hz and temperature range 130-350 K. In addition the dielectric behaviour has been investigated of heptakis- 2,3,6-tri-O-methyl)-beta -cyclodextrin ( beta -CD.TRIME.0.3H(2)O) and randomly methylated beta -cyclodextrin (beta - CD.RAME.0.8H(2)O). The dielectric behaviour is described well by Debye-type relaxation (alpha -disperson). All alpha -CD systems exhibit an additional Omega -dispersion at low frequencies, which is attributed to proton transp ort. The fact that the epsilon' values of alpha -CD form III are larger tha n those of alpha -CD form I is attributed to the different numbers and diff erent strengths of the intramolecular hydrogen bonds. Form III has a stable conformation which is shown by the constant values of epsilon' in the temp erature range 125-250 K. By contrast, in form I the epsilon' values increas ed linearly with temperature, indicating that the system passed through suc cesive conformations. The temperature dependence of epsilon" and phase shif t in all the specimens of alpha -CD (forms I, III) and fully methylated bet a -CD do not reveal any order-disorder transition, because the developed hy drogen bonds ae not of the flip-flop type according to their crystal struct ures. The partially methylated beta -CD reveals the characteristics of the order-disorder transition, which was observed before in the systems dried b eta -CD.2H(2)O and non-dried beta -CD.9.8H(2)O. There is a direct relation between the hydroxyl groups of beta -CD and the order-disorder transition. The order-disorder transition could also be shown in dried beta -CD.2.4H(2) O and non-dried beta -CD.9.8H(2)O samples but not in dried alpha -CD.1.1H(2 )O and non-dried alpha -CD.5.9H(2)O samples, by low frequency Raman spectro scopy (< 180 cm(-1)). The step-like temperature dependence of the band at 3 3.7 cm(-1) reveals a transition at about 223 K for both <beta>-CD samples. In the case of alpha -CD samples the almost linear temperature dependence o f the band at 49.1 cm(-1) does not reveal any transition.