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Behavior of H₂O surrounding NH₄⁺ and Al³⁺ in NH₄Al(SO₄)₂·12H₂O by ¹H MAS NMR, ¹⁴N NMR, and ²⁷Al NMR

Author:
Lim, Ae Ran
Source:
RSC advances 2017 v.7 no.87 pp. 55276-55281
ISSN:
2046-2069
Subject:
aluminum, ammonium, geometry, hydrogen bonding, nitrogen, nuclear magnetic resonance spectroscopy, phase transition, stable isotopes, temperature, thermal degradation, thermodynamics
Abstract:
In order to understand the thermodynamic properties and structural geometry of NH₄Al(SO₄)₂·12H₂O, we studied the magic angle spinning nuclear magnetic resonance (MAS NMR) and static NMR as a function of temperature. The changes of the chemical shifts, linewidths, resonance frequencies, and spin–lattice relaxation times for ¹H, ¹⁴N, and ²⁷Al nuclei near Td (=335 K), which is interpreted as the onset of partial thermal decomposition, are due to the structural phase transition. The changes near Td are related to the loss of H₂O, which probably disrupts the forms of the octahedra of water molecules surrounding NH₄⁺ and Al³⁺ ions. This mechanism above Td is related to hydrogen-bond transfer involving the breakage of the weak part of the hydrogen bond.
Agid:
6421788