Ionic Motion of Phenethylammonium Ion in [C6H5CH2CH2NH3]2 PbX4 (X = Cl, Br, I) as Studied by 1 H NMR

The temperature dependences at 110 to 400 K of the : 1H spin-lattice relaxation time (7\) of the phenethylammonium ion in phenethylammonium lead(II) halides, [C6H5CH2CH2NH3]2PbX4 (X = Cl, Br, I), revealed that this ion shows reorientation of the NH3 moiety around the three-fold axis and torsional motion of the alkyl chain (CH2CH2). Below room temperature, the chloride and the bromide yielded two minima of 1H T1 originating from NH3 reorientation, whereas the iodide yielded only one minimum. These findings indicate that there are two kinds of NH3 sites in the chloride and bromide but only one in the iodide. The T1 minimum observed below room temperature gave similar activation energies of the NH3 reorientation, Ea = 15.7,15.1 and 15.5 kJ mol 1 for the chloride, bromide and iodide, respectively, suggesting that the corresponding NH3 groups are located at similar environments. Above room temperature, the T1 minimum in the chloride and bromide gave larger Ea values of the NH3 reorientation: Ea - 23.6 and 20.2 kJ mol-1 for the chloride and bromide, respectively. These findings suggest that the NH3 groups form stronger hydrogen bonding with halogen atoms (N-H ... X). Furthermore, the amplitude of the CH2CH2 motion is discussed, using the two sites jump model. The activation energies for the CH2CH2 motion in these compounds are almost equal (Ea = 29.1, 30.0 and 28.2 kJ mol -1 for the chloride, bromide and iodide, respectively), but that the torsional angles become larger in the order iodide ⪡ bromide<chloride.