Proton spin-lattice relaxation times T , and T , as well as the proton second moment have been measured for polycrystalline methyl a-D-glucoside, methyl p-D-glucoside and methyl !3-D-galactoside over a wide temperature range. Analyses of the experimental data have yielded information about intramolecular reorientations in the solid state. C, reorientations of methyl groups take place in all the samples. Potential energy calculations have shown that the CH,OH group can execute trans-gauche jumps between two potential minima separated by 120". Analyses of the T , and second moment data have confirmed the presence of this conformational motion. A weak relaxation mechanism, related to the dynamics of an OH group, has been revealed in methyl p-D-galactoside. lntroductioaThe relationship between molecular structure and the sweetness of carbohydrates has been studied extensively. The accepted model is based on the hypothesis that the functional groups responsible for the sweet taste are two vicinal OH groups, one acting as a proton donor and the other as a proton acceptor in hydrogen bonding with the receptor site [l .-31. The crystal structures of solid methylpyranosides (C7HI4O6) [4-61 reveal a network of hydrogen bonds which range from weak to medium in strength. A broad infrared absorption band in the range 3000 to 4000 cm-', characteristic of the stretching vibration frequency of a hydrogen-bonded hydroxyl group, was observed in methylpyranosides. On the other hand, a sharp IR band was assigned to nonhydrogen bonded hydroxyl groups, which are most probably involved in weak hydrogen bonding [!, 2 , 7 -101. Another characteristic feature of methylpyranosides, which are derivatives of simple carbohydrates, is the presence of a methyl group at the site of the anomeric hydroxyl proton. The molecular structure of the a and p forms of methylpyranoside are shown in Fig. 1 . Neutron diffraction, X-ray and IR studies have

No abstract

Abstraet. Molecular dynamics of three derivatives of 1,2,4-benzothiadiazine-l,l-dioxide, hydrochlorothiazide (HCTZ), althiazide (ATZ) and chlorotbiazide (CTZ), was studied by 35C1 nuclear quadrupole resonance (NQR) spectroscopy. The temperature dependence of the resonance frequency was analyzed within the 6 known standard models. The activation energies estimated from the temperature dependence of the 3~C1 NQR frequency assuming the Bayer model were 1.07, 2.35 and 2.76 kJ/mot for HCTZ, ATZ and CTZ respectivety, wbich confirms that HCTZ is Iess rigid than CTZ and ATZ is much more rigid than HCTZ, and suggests that the mechanism of relaxation is based on small amplitude librations. The characteristic temperatures estimated from the Bayer model, with that for CTZ (332.5 K) being much higher than for HCTZ (132.1 K), mean that the intermolecular interactions in CTZ are much stronger than in HCTZ, as suggested by the melting point of CTZ being higher than that for HCTZ. For ATZ the characteristic temperature (288 K) takes an intermediate value, which suggests that the intermolecular interactions in this compound are stronger than in HCTZ and weaker than in CTZ. A significant narrowing of the resonance 3~C1 NQR line observed for all these compounds at room temperature, relative to that at the liquid nitrogen temperature, suggests an averaging of dipolar interactions asa result of fast rotation of nonquadrupole nuclei in the vicinity of the quadrupote nuclei, when 2nv~r~ 87 1 (a rotation of the -NH z group in the direct neighborhood of the chlorine nuclei) or a change in the gradient o¡ with its value preserved (which is equivalent to rotation of the quadrupole nucleus CI). The influence of the rotations of the -NH~ and -CH2SCH2CH=CH z groups (ATZ) or -CHCI z group (TCTZ) on the 3~C1 NQR frequency was modelled by the B3LYP/6-31G" method. The frequencies of the libration vibrations calculated from the temperature dependence of the NQR resonance frequency were compared with experimental ones and those implied by the density functional theory, infrared and Raman spectra. For HCTZ the anomalies in the temperature dependence of the 35C1 NQR frequency, the lack of hysteresis and small but notable changes in the slope and the jump in the frequency observed at 253 K which does not exceed 0.05 MHz suggest a second-order phase transition at 253 K.