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Spin-lattice relaxation in ammonium compounds with a complex molecular dynamics
Z T Lalowicz1, M Punkkinen, A H Vuorimãki
1Institute of Nuclear Physics, Cracow, Poland.
Solid State Nuclear Magnetic Resonance
|April 1, 1997
Summary
This study explores how ammonium (NH4) and deuterated ammonium (ND4) groups reorient in solids, revealing two relaxation time (T1) minima under distorted potentials. These findings are crucial for understanding molecular dynamics in crystalline materials.
Area of Science:
- Solid-state nuclear magnetic resonance (NMR) spectroscopy
- Materials science
- Chemical physics
Background:
- Nuclear relaxation rates (1/T1) provide insights into molecular motion in solids.
- Ammonium (NH4) and deuterated ammonium (ND4) groups exhibit complex reorientational dynamics influenced by their crystalline environment.
- Distortions in the hindering potential can significantly alter reorientation rates and lead to observable phenomena like multiple relaxation minima.
Purpose of the Study:
- To derive expressions for the relaxation rate (1/T1) of protons and deuterons in NH4 and ND4 groups undergoing reorientation.
- To investigate the effect of distorted hindering potentials (trigonal, tetragonal, monoclinic) on reorientation rates and relaxation times.
- To experimentally verify the theoretical models using various ammonium salt compounds and their deuterated analogues.
Main Methods:
- Theoretical derivation of relaxation rate expressions for reorienting NH4 and ND4 groups.
- Experimental measurements of spin-lattice relaxation times (T1) for protons and deuterons in NH4VO3, (NH4)2S2O8, and (NH4)2PtCl4, and their deuterated forms.
- Analysis of experimental data in conjunction with theoretical models, considering factors like potential distortions and proton tunneling.
Main Results:
- Two distinct spin-lattice relaxation time (T1) minima were observed in all studied compounds, consistent with modified reorientation rates.
- NH4VO3 and ND4VO3 data align well with a trigonal distortion model.
- (NH4)2S2O8 requires consideration of proton tunneling in addition to reorientation.
- Purely reorientational models failed for (NH4)2PtCl4, suggesting domain ordering and boundary effects at low temperatures, with evidence of a 32 MHz proton tunneling frequency.
Conclusions:
- The study successfully models the relaxation behavior of NH4 and ND4 groups in solids with distorted potentials, explaining the observation of two T1 minima.
- Specific compounds like NH4VO3 and (NH4)2S2O8 demonstrate the influence of potential symmetry and proton tunneling on relaxation dynamics.
- (NH4)2PtCl4 exhibits complex low-temperature behavior attributed to domain ordering, highlighting limitations of simple reorientational models and providing evidence for significant proton tunneling.