(2008-SSNMR) Study of molecular dynamics and cross relaxation in tetramethylammonium hexafluorophosphate (CH3)4NPF6 by 1H and 19F NMR (original) (raw)
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Solid State Nuclear Magnetic Resonance, 2017
Molecular dynamics of the solid 3-(trifluoromethyl) benzoic acid containing proton 1 H and fluorine 19 F nuclei was explored by the solid-state NMR off-resonance technique. Contrary to the previous experiments the proton nuclei system I relaxed in the off-resonance effective field B ⎯→ ⎯ e while fluorine nuclei system S was saturated for short time in comparison to the relaxation time T I 1. New cross-relaxation solid-state NMR off-resonance experiments were conducted on a homebuilt pulse spectrometer operating at the on-resonance frequency of 30.2 MHz, at the off-resonance frequency varied between 30.2 and 30.6 MHz for protons and at the frequency of 28.411 MHz for fluorines, respectively. Based on the experimental data the dispersions of the proton offresonance spin-lattice relaxation rate ρ ρ I , the fluorine off-resonance spin-lattice relaxation rate ρ ρ S and the cross-relaxation rate σ ρ in the rotating frame were determined.
Journal of Magnetic Resonance (1969), 1975
Solid solutions of (UF,),(MoF,),-, are studied by 19F NMR. Separate signals are observed for the MoF6 and UF6 fluorine, in the plastic phase as well as in the low temperature rigid phase. The good homogeneity of the solutions is checked by linewidth analysis; measurement and theoretical analysis of cross-relaxation effects are performed in the plastic phase of a (UFS)0.08(M~F6)0.92 solution. Translational motion is the same as in pure MoF6, but slightly slower. In the ordered phase, T, and TD are measured; a model for describing the molecular motion and relaxation rates in the solution from the pure compound values is given. It is shown that, more generally, for globular compounds, the translational correlation time at the melting point in the plastic phase is about 2 x lo-' set and the reorientational correlation time in the rigid phase at the solid-solid transition temperature (rotational melting) is about 5 x 10m9 set, almost independently of the compound or the phase transition temperature. It is also stated that observation of such correlation between transition temperatures and motional rates is not restricted to globular compounds. I. INTRODUCTION When studying molecular motions in a crystal as a function of temperature, one changes not only the mean energy of the molecules, but also, because of the thermal expansion of the crystal, the potential well in which they are moving. Therefore, studies as a function of other experimental parameters such as pressure or sample volume are very useful and are sometimes performed (l-4); molecular motion of a molecular species may also be studied not in the pure compound, but as a component of a solid solution (5). With such considerations in mind, and in order to complete previous NMR studies (6-15) of molecular motions in the pure hexafluorides, we undertook the study of solid solutions of uranium and molybdenum hexafluorides (UF,),(MoF,),-,. The hexafluoride molecules are of quasispherical shape, with the metal atom at the center of the molecule, surrounded by six fluorine atoms at the corners of an octahedron. Metal fluorine distances are 1.833 A and 2.05 A for MoFs and UF,, respectively. They belong to the class of globular compounds (16) and, with the exception of the actinide hexafluorides, present a rotationally disordered plastic phase between the liquid and the low temperature crystalline ordered solid (27). In MoF,, the solid-solid transition occurs at 9.6"C and melting at 17.4"C, whereas in UF,, melting occurs at 64°C without any solid-solid transition (17). The ordered solid phases of MoF, and UF, are isomorphous (18) with only slight differences betweenthe positions of the atoms.
Journal of Magnetic Resonance (1969), 1970
A magnetic nucleus located on an internal rotor is coupled not only to the rotational magnetic field generated by the end-over-end rotation of the molecule but also to the magnetic field produced by the internal rotation of the top relative to the frame. We have examined the importance of this nuclear-spin-internalrotational coupling as a nuclear spin-lattice relaxation mechanism for the fluorine spins in benzotrifluoride and hexafluorobutyne-2. In order to elucidate the nature of the stochastic processes which modulate the spin-rotation interaction and are thereby responsible for the coupling of the nuclear-spin system to the lattice in these molecules, both the temperature and the viscosity dependences of the fluorine spin-lattice relaxation times in benzotrifluoride as well as the viscosity dependence in hexafluorobutyne-2 have been investigated. It is concluded that the rotational magnetic fields generated by overall and internal rotation are fluctuating independently and that each field is described by a distinct correlation time. It is also concluded that, contrary to previous proposals, the nuclearspin-internal-rotation interaction contributes significantly to the spin-lattice relaxation of the methyl group protons in molecules such as toluene at room temperature.
Proton-lattice relaxation in NH 4 MF 3
Journal of Physics: Condensed Matter, 1989
Nuclear magnetic resonance measurements of the proton-lattice relaxation times T , and TI, in NH4MgF3 and NH,ZnF3 between 40 and 150 K and in NH4CdF3 between 70 and 300 K are presented. The relaxation was found to be non-exponential, but fitted very well to a sum of two exponentials. Data were analysed in a three-bath model. Activation energies E, of 760 It: 50 K, 825 2 20 K and 1950 t 50 Kin theirlow-temperature phases were found for the three compounds, respectively. The trend to an increase of E, with increasing unit-cell size is in contradiction with the expected trend from calculations using atomatom electrostatic potentials. The average tunnel frequencies are consistent with activation energies. Short-range forces acting between H+ and F-are assumed in order to explain these results.
Inorganic Chemistry, 2008
The structure of ammonium gallium cryolite (NH 4) 3 GaF 6 was investigated by 19 F and 69,71 Ga magic-angle spinning (MAS) NMR in comparison with X-ray powder diffraction followed by Rietveld refinement. In agreement with previous thermodynamic measurements, NMR experiments on (NH 4) 3 GaF 6 support the model of rigid GaF 6 octahedra. At high spinning speeds (30 kHz), the scalar coupling between the six equivalent 19 F nuclei and 69,71 Ga can be directly observed in the powder spectra. The coupling constants are J 19 F 69 Ga) 197 Hz and J 19 F 71 Ga) 264 Hz. To explain the 71 Ga spectra recorded at 3 kHz a small distribution of quadrupolar frequencies has to be included. The spread of the spinning sidebands hints to a largest ν Q value of 28 kHz for 71 Ga. This can be explained by the occurrence of highly symmetric GaF 6 octahedra, which are tilted against the surrounding atoms. In addition, the incomplete motional excitation does not average out the quadrupolar effects. NMR findings are in discrepancy to those of Rietveld refinement. As result it appears that X-ray diffraction is not sensitive enough to deliver proper results.
Fluorine-19 solid state NMR study of vinylidenefluoride polymers using selective relaxation filters
Solid State Nuclear Magnetic Resonance, 2006
Two fluoropolymers, poly(vinylidenefluoride) (PVDF) and a vinylidenefluoride telomer (VDFT), with molecular weights of 1 Â 10 6 and 2 Â 10 3 Da by GPC, respectively, have been analysed by 19 F solid-state nuclear magnetic resonance (NMR) spectroscopy. Relaxation-filtered proton-decoupled magic-angle spinning (MAS) experiments, namely T 1r filter, dipolar filter (DF), direct-polarisation delayed acquisition (DPDA) and discrimination induced by variable-amplitude minipulses (DIVAM), allowed signals in the direct polarisation (DP) spectra of PVDF and the VDFT to be discussed in terms of rigid and mobile domains. Both samples showed signals, which were multi-componential, but they differ in the nature of the crystalline form present. Thus, the Vinylidenefluoride (VDF) telomer exhibited a crystalline component corresponding to b PVDF, whereas the PVDF contained crystallites of the a form. Signals relating to end groups and reverse units, plus an anomalous signal displaying long-time transverse relaxation in the DPDA spectrum, were found for both polymers, though they showed diversity in chemical shift and content. Signals related to reverse units and/or end groups were seen between approximately À115 and approximately À117 ppm for both samples. High-speed MAS at higher magnetic field resulted in an increase in resolution so that signals previously attributed to single-phase characteristics are shown to indicate the possibility of several different mobilities. The results are debated with respect to molecular weight and relaxation parameters. r
Anomalous proton spin‐lattice relaxation in organic compounds containing methyl groups
Concepts in Magnetic Resonance Part A, 2019
Temperature measurements of proton spin‐lattice relaxation time performed for acetates ((CH3COO)2Ba, (CH3COO)2Cd, and (CH3COO)2Ca) and acetyl halides ((CH3CO)2O, CH3COBr and CH3COCl) are fitted to a Haupt equation. It is impossible to fit the temperature dependence of T1 protons using the BPP equation. Of importance is the assumption that complex C3 molecular motion of methyl protons takes place. An understanding of the correlation functions of complex C3 reorientation allow for the calculation of the relaxation time, T1, and the second moment of the NMR resonance. The spectral densities are calculated applying Woessner theory of complex motion and assuming a tunneling correlation time implemented from solving the Schrödinger equation. The acceptance of the tunneling correlation time resulting from the Schrödinger equation elucidates the reduction in the second moment at 0 K. The fitting leads to an excellent agreement between the experimental results of T1 temperature dependences a...