Phase transitions and NH₃ motions in [Zn(NH₃)₄](ClO₄)₂ studied by incoherent neutron scattering and ¹H NMR methods

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• 作者：Anna Migdał ; -Mikuli ; Krystyna Hoł ; derna-Natkaniec ; Edward Mikuli ; Ł ; ukasz Hetmań ; czyk ; Ireneusz Natkaniec
• 关键词：Tetraamminezinc chlorate (VII) ; Phase transitions ; Neutron scattering methods ; ¹H NMR
• 刊名：Chemical Physics
• 出版年：2007
• 出版时间：20 June 2007
• 年：2007
• 卷：335
• 期：2-3
• 页码：187-193
• 全文大小：328 K

文摘

Quasielastic and inelastic incoherent neutron scattering (QENS and IINS) and the analysis of the second moment (M₂) of proton magnetic resonance (¹H NMR) line for [Zn(NH₃)₄](ClO₄)₂ crystals in the function of temperature are reported. The QENS peak registered at 75 K and also at higher temperatures shows distinct broadening, which is typical for dynamically, orientationally disordered crystals (ODIC). Reorientational motion of NH₃ ligands can be well described by a model of 120° proton jumps around a 3-fold axis of Zn–N bonds on a picoseconds time scale. The NH₃ do not drastically change either the jump rate or the character of their reorientational motion at the phase transitions at: T_C1 ≈ 271 K and T_C2 ≈ 164 K. However, it undergoes a very small change of the activation energy E_a (120°) value for such a motion at T_C2. The estimated E_a value equals 2.5 kJ mol⁻¹ for high and intermediate temperature phases and 1.7 kJ mol⁻¹ for the low temperature phase. ¹H NMR studies revealed that NH₃ ligands perform reorientation around the 3-fold symmetry axis close to 90 K when the second moment of the H NMR line exhibits the plateau of M₂ (reorientational correlation time is less than reciprocal of the line width). The estimated E_a(120°) ≈ 3 kJ mol⁻¹ is in quite good agreement with that calculated from the QENS measurements. Above the phase transition at T_C2, the anisotropic reorientation of the whole [Zn(NH₃)₄]²⁺ cation around the 3-fold symmetry axis is set in motion with a frequency in the order of several kHz (the ¹H NMR line width).