A 35CL NQR Spin-Lattice Relaxation Study on the Motion of D4h Anions in [C(NH2)3]2PdCl4, [C(NH2)3]2PtCl4, and [C(NH2)3]AuCl4
The temperature dependence of 35Cl NQR spin-lattice relaxation times T1ClQ was observed for the crystal of the title complexes. For the Pd(II) and Pt(II) complexes, the log T1ClQ vs. 103 T-1 curves having gentle positive gradients at lower temperatures decreased sharply with increasing temperature from ca. 150 and ca. 130 K, respectively. This sharp decrease of T1ClQ can be explained by the C4 reorientation of the D4h complex anions with the activation energy Ea of 34 kJ mol-1 for the former and 29 kJ mol-1 for the latter complex. These values agree well with those estimated from 1H T1 showing temperature dependent dipolar-quadrupolar cross relaxation. For the Au(III) salt, two of four 35Cl NQR lines showed a sharp decrease in T1ClQ from ca. 270 K, suggesting the onset of the C4 reorientation of the one kind crystallographically equivalent anions with Ea of 67 kJ mol-1.