1H NMR and 35Cl NQR Studies on the Motion of Pyridinium Ions in Crystalline Pyridinium Tetrachloro‐ and Tetrabromoaurate(III): (pyH)AuX4 (X = Cl, Br)

The temperature dependences of 1H spin-lattice relaxation times T1, at the Larmor frequencies of 10.5, 16.0, 20.0, and 45.5 MHz, and of 1H NMR second moments M2 were determined for (pyH)AuCl4 and (pyH)AuBr4, where pyH+ indicates a pyridinium ion. The small M2 data less than 1 G2 obtained above ca. 380 K indicated that the cations perform reorientational motion rapidly enough about its pseudohexad C′6 axis existing at the center of the cation and perpendicular to its plane. Below 140 K, both complexes yielded rigid lattice M2 values of the cation. It is interesting features for these complexes that motional narrowing for the NMR line occurs over an extremely wide range of temperature, the log T1, vs. T−1 plots are asymmetric about the 1H T1 minimum with a gentler gradient on the low temperature side, and the minima are extraordinarily long. These unusual results were interpreted by assuming the C′6 reorientation of the cations having electric dipoles among nonequivalent in-plane orientations. Three potential barriers to the C′6 reorientation were determined as 21.8, 17.2, and 13.5 kJ mol−1 for (pyH)AuCl4, and 22.2,17.4, and 13.7 kJ mol−1 for (pyH)AuBr4. The fade-out phenomenon of 3SCl NQR signals observed for (pyH)AuCl4 at ca. 230 K when the sample temperature was lowered is also discussed, by referring to the motion of the pyridinium cations.