Studies of molecular dynamics of thiazides by35Cl NQR spectroscopy and DFT calculation

Abstract

Molecular dynamics of three derivatives of 1,2,4-benzothiadiazine-1,1-dioxide, hydrochlorothiazide (HCTZ), althiazide (ATZ) and chlorothiazide (CTZ), was studied by35Cl 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 the35Cl NQR frequency assuming the Bayer model were 1.07, 2.35 and 2.76 kJ/mol for HCTZ, ATZ and CTZ respectively, which confirms that HCTZ is less 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 resonance35Cl NQR line observed for all these compounds at room temperature, relative to that at the liquid nitrogen temperature, suggests an averaging of dipolar interactions as a result of fast rotation of nonquadrupole nuclei in the vicinity of the quadrupole nuclei, when 2πνQτc ≫ 1 (a rotation of the −NH2 group in the direct neighborhood of the chlorine nuclei) or a change in the gradient orientation with its value preserved (which is equivalent to rotation of the quadrupole nucleus Cl). The influence of the rotations of the −NH2 and −CH2SCH2CH=CH2 groups (ATZ) or −CHCl2 group (TCTZ) on the35Cl 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 the35Cl 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.