Magnetic Resonance Studies of Ferroelectric Methylammonium Alum

O’Reilly, D. E.; Tsang, Tung

doi:10.1103/physrev.157.417

Cited by 148 publications

(26 citation statements)

References 27 publications

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“…Taking into account the chemical structure of riboflavin, it is reasonable to assume the occurrence of rotation of the methyl groups. Since the proton relaxation processes are entirely governed by the dipolar interaction, the observed spin-lattice relaxation data can be approximated by the general expression [5,6] …”

Section: Discussionmentioning

confidence: 99%

Molecular Dynamics in Solid Riboflavin as Studied by 1H NMR

Andrew

Głowinkowski²

2000

Solid State Nuclear Magnetic Resonance

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Section: Discussionmentioning

confidence: 99%

Molecular Dynamics in Solid Riboflavin as Studied by 1H NMR

Andrew

Głowinkowski²

2000

Solid State Nuclear Magnetic Resonance

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“…For ∆ = 0 and k t replaced by the inverse correlation time τ −1 of the classical jumps, it is known as the Bloembergen-Purcell-Pound (BPP) expression. 33,34 For protonated methyl groups relaxing by the DD mechanism, addressed in the DQR approach, the Kramers quantum process could contribute to relaxation in the presence of non-negligible inter-molecular DD interactions. 12 Details of a preliminary validity test of the considered spinlattice relaxation theory, relevant for the findings reported in this work are given in the Discussion subsection.…”

Section: Spin-lattice Relaxation In the Dqr Approachmentioning

confidence: 99%

A quantum mechanical alternative to the Arrhenius equation in the interpretation of proton spin–lattice relaxation data for the methyl groups in solids

Bernatowicz

Shkurenko

Osior

et al. 2015

Phys. Chem. Chem. Phys.

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CitationQuantum mechanical alternative to Arrhenius equation in the interpretation of proton spin-lattice relaxation data for the methyl groups in solids 2015 Phys. Chem. Chem. Phys. Theory of nuclear spin-lattice relaxation in methyl groups in solids has been a recurring problem in nuclear magnetic resonance (NMR) spectroscopy. The current view is that, except for extreme cases of low torsional barriers where special quantum effects are at stake, the relaxation behaviour of the nuclear spins in methyl groups is controlled by thermally activated classical jumps of the methyl group between its three orientations. The temperature effects on the relaxation rates can be modelled by Arrhenius behaviour of the correlation time of the jump process. The entire variety of relaxation effects in protonated methyl groups has recently been given a consistently quantum mechanical explanation not invoking the jump model regardless of the temperature range. It exploits the damped quantum rotation (DQR) theory originally developed to describe NMR line shape effects for hindered methyl groups. In the DQR model, the incoherent dynamics of the methyl group include two quantum rate, i.e., coherence-damping processes. For proton relaxation only one of these processes is relevant. In this paper, temperature-dependent proton spin-lattice relaxation data for the methyl groups in polycrystalline methyltriphenyl silane and methyltriphenyl germanium, both deuterated in aromatic positions, are reported and interpreted in terms of the DQR model. A comparison with the conventional approach exploiting the phenomenological Arrhenius equation is made. The present observations provide further indications that incoherent motions of molecular moieties in condensed phase can retain quantum character over much broader temperature range than is commonly thought. Eprint version

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“…Analyzing the high tempera ture relaxation behaviour of [Sb(CH3)4]I at first (see Fig. 4), there are two elementary processes, whose Neglecting the dipolar intermethyl group interac tions at first, the relaxation rate produced by intramethyl proton-proton interaction is given by [11,12] The interaction between different methyl groups within the same [Sb(CH3)4]® unit may be considered by assuming that the protons of each CH3 group are located at the center of the triangle of the (HHH) three spin system. This implies that t c<^tc1 and gives the relaxation rate caused by the overall tumbling of the cation:…”

Section: Resultsmentioning

confidence: 99%

Rotational Excitations and Tunneling of Nonequivalent Methyl Groups in Tetramethylstibonium Iodide as Studied by Nuclear Magnetic Resonance and Inelastic Neutron Scattering

Burbach

Weiß

1991

Zeitschrift Für Naturforschung A

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Nuclear magnetic resonance (NMR) and inelastic neutron scattering techniques (INS) have been applied to study the rotational motions and methyl group tunneling in tetramethylstibonium iodide, [Sb(CH3)4]I, over a wide temperature range. Parameters describing the [Sb(CH3)4]® cation tum bling and the methyl group reorientation at high temperatures and quantum mechanical tunneling of the methyl groups at low temperatures were determined.The results for INS and NMR experiments at low temperatures can be explained in terms of two crystallographically inequivalent methyl groups CH3(1) and CH3(2), which were established earlier by the crystal structure determination. In the INS spectra two tunneling lines at 22.0 peV for CH3(1) and 1.05 peV for CH3(2) with inelastic intensities in the ratio 3:1 were observed at 7 = 4 K.The activation energies derived from proton NMR spin-lattice relaxation time measurements for the thermally activated methyl group rotation are 1.50 kJ/mol for CH3(1) and 3.81 kJ/mol for CH3(2). They are in accordance with the activation energies obtained from neutron fixed-window measurements.The activation energy for [Sb(CH3)4]® cation tumbling is 50.9 kJ/mol as determined from the high temperature spin-lattice relaxation behaviour.Rotational potentials for the methyl groups are derived. For both kinds of inequivalent methyl groups the threefold potential terms dominate; three-and sixfold potential contributions are shifted by 180°. IntroductionRotational excitations of a methyl group provide a simple example of motional processes in the solid state. They have been studied both at higher tempera tures as thermally activated random reorientation and in the quantum mechanical regime at low temperatures. At very low temperatures the methyl group rotation can be described via quantum mechanical tunneling between the splitted torsional state [1], Increasing the temperature, rotation of the methyl group becomes thermally activated and can be treated classically. In the high temperature limit, additionally tumbling of the whole molecular framework can be activated in case of highly symmetric quasi spherical molecules or ions. Using nuclear magnetic resonance (NMR) meth ods, the activation energies of the thermally activated processes of the methyl group and the molecule as a whole as well as the reorientation rates can be deter mined [2].High resolution inelastic neutron spectroscopy (INS) has provided as suitable tool to determine the tunneling splitting in the quantum mechanical regime. The deter mination of this splitting is a good probe of the height and shape of the hindering potential barriers [3].Investigations of tetramethyl metal compounds with an element of the main group IV of the periodic system as central atom (e.g. tetramethyl lead, tetra methyl tin) have demonstrated that the interpretation of the NMR and INS data can not be described quan titatively without taking into account crystal structure data [4,5].The present investigation was undertaken to study the rotational behaviour of charged X(CH...

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Magnetic Resonance Studies of Ferroelectric Methylammonium Alum

Cited by 148 publications

References 27 publications

Molecular Dynamics in Solid Riboflavin as Studied by 1H NMR

Molecular Dynamics in Solid Riboflavin as Studied by 1H NMR

A quantum mechanical alternative to the Arrhenius equation in the interpretation of proton spin–lattice relaxation data for the methyl groups in solids

Rotational Excitations and Tunneling of Nonequivalent Methyl Groups in Tetramethylstibonium Iodide as Studied by Nuclear Magnetic Resonance and Inelastic Neutron Scattering

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