Hydrogen isotope effect on the temperature dependence of the 35Cl-NQR frequency in (NH4)IrCl6

The Cl-35-NQR frequencies and spin-lattice relaxation times of (NH4)(2)IrCl6 and (ND4)(2)IrCl6 were measured in the temperature range 4.2 - 300 K. It was confirmed that no phase transition takes place down to 4.2 K. The observed frequency at 4.2 K in (ND4)(2)IrCl6 was higher by 18 kHz than that in the non-deuterated analogue. This discrepancy was attributed to a difference between the lowest rotational states of the ammonium ions in these compounds. The energy separation between the ground state and excited state of rotational motion, in which N-H of the ammonium ion rotates among the three directions slightly apart from the triad axis, was estimated by fitting analysis to be 31 cm(-1) and 93 cm(-1) for (ND4)(2)IrCl6 and (NH4)(2)IrCl6, respectively. The activation energy of the reorientation of the ammonium ion among the four directions of the triad axis was estimated to be 6.9 and 7.6 kJ mol(-1), respectively.