A study of the nuclear quadrupole resonance of halogens in chloro-substituted pyridinium bromides and iodides

Abstract

The temperature dependences of 35Cl, 79Br and 127I NQR frequencies were observed for 2-chloropyridinium bromide and iodide, while 35Cl NQR frequencies were determined at various temperatures for 3-chloropyridinium bromide and 3,5-dichloropyridinium iodide. 3,5-Dichloropyridinium iodide yielded two 35Cl NQR lines at all temperatures studied, indicating the existence of two crystallographically nonequivalent chlorines in the crystal. The nuclear quadrupole spin-lattice relaxation time T 1Q of 35Cl as well as 127I nuclei for 2-chloropyridinium iodide and that of 35Cl for the remaining salts were determined as a function of temperature. Below 200 K, an almost linear relation with a gradient of about −2 was obtained for the 35Cl log T 1Q vs. log T plot of 2-chloropyridinium iodide, indicating that the relaxation mechanism might be governed by the librational modes. Above 200 K, the plot deviated from the linear relation suggesting the presence of considerable anharmonicity in the librational modes. Similar results of 35Cl T 1Q were obtained for the other salts studied. The 127I T 1Q of 2-chloropyridinium iodide decreased with increasing temperature as a function of T −2.2 and T −2.1 for ν 1 and ν 2 lines, respectively, in the whole temperature range (80–350 K) studied. This can be well interpreted by the ionic model proposed by Van Kranendonk.