A.J. Dianoux

Inelastic incoherent neutron scattering study of molecular motions of n‐nonadecane in urea chlatrate

Inelastic incoherent neutron scattering experiments were performed on semi‐oriented samples of C19H40 –urea chlatrates over the 1.5–306 K temperature range. Using appropriate sample orientations and different instrumental resolutions, all the motions parallel or perpendicular to the main chain axes were analyzed in the 0–400 cm−1 frequency range. In the quasielastic region and for the Q momentum transfer vector parallel to the chain axes, a side peak attributed to a longitudinal acoustic mode was evidenced at 12 cm−1 at 1.5 K (overdamped at higher temperatures) in addition to large quasielastic broadenings due to restricted translational motions. For Q perpendicular to the chains, a diffusive rotational process was also evidenced in the disordered solid phase. All these results are discussed and compared to literature data.

Molecular motions in n‐nonadecane C19H40: An incoherent neutron scattering study

Incoherent neutron scattering (INS) experiments were performed on semioriented samples of n‐nonadecane C19H40 and of the selectivity deuterated derivatives CD3–C17H34–CD3 and C2D5–C15H30–C2D5 over a wide temperature range, in particular, over the domain of existence (295–305 K) of the disordered ‘‘rotator phase’’ RI. From analyses of the inelastic and quasielastic parts of the spectra, we were able to discriminate between motions parallel or perpendicular to the main long‐chain axis. In the low frequency region (0–300 cm−1), numerous lattice and internal vibrational modes, showing pronounced polarization effects, were assigned. In the quasielastic regions, large broadenings were observed in the RI phase, providing clear evidences of both fast restricted translational motions along the chain axis and uniaxial rotational diffusions. For the rotational process, we have elaborated and made use of a model of diffusion for a particle in a twofold potential. Finally, the INS spectra of C19H40 in the RI phase and...

Dynamic properties of dioctanoyl peroxide guest molecules constrained within the urea tunnel structure: A combined incoherent quasielastic neutron scattering and solid state 2H nuclear magnetic resonance investigation

The dynamic properties of dioctanoyl peroxide guest molecules within the urea host tunnel structure in the dioctanoyl peroxide/urea inclusion compound have been investigated by incoherent quasielastic neutron scattering (IQNS) and solid state 2H nuclear magnetic resonance (NMR) techniques. The IQNS investigations were carried out on samples of urea inclusion compounds containing perdeuterated urea to ensure that the incoherent scattering is dominated by the dioctanoyl peroxide guest molecules. Using semioriented polycrystalline samples, translational motions of the guest molecules along the tunnel were investigated separately from reorientational motions of the guest molecules about the tunnel axis. The 2H NMR experiments used dioctanoyl peroxide deuterated selectively in both the α CD2 groups and urea with natural isotopic abundance. The dynamic models that have been found to describe the translational and reorientational motions of the guest molecules from the IQNS and 2H NMR data are discussed in detai...

Neutron scattering study of the reorientational motions in Ni(C5H5)2

Incoherent quasi-elastic neutron scattering experiments, using different resolutions, have been performed on polycrystalline samples of nickelocene, Ni(C5H5)2, in the 4–300 K temperature range. It is shown that a unique 2π/5 reorientational jump process of C 5H5 rings is taking place below and above the diffuse phase transition at about 200 K. A proton correlation time equal to 4 × 10-12 s at 300 K, associated with an activation energy of 6·3 kJ mol-1, is estimated in agreement with literature data. Finally, it is concluded that the asymmetry of the double-well potential function governing the reorientations of C5H5 rings, as deduced from heart capacity and Raman results, is too small to be evidenced in neutron scattering experiments.

Inelastic and quasielastic neutron scattering study of the reorientational motions in the layer-type compound {NH3(CH2)3NH3}MnCl4

Incoherent inelastic scattering (INS) and quasielastic neutron scattering (QNS) experiments on polycrystalline derivatives of the propylenediammonium manganese tetrachloride compound, namely |NH3(CH2)3NH3| MnCl4, |NH3(CD2)3NH3|MnCl4 and |ND3(CH2)3ND3|MnCl4, have been performed over the 100–500 K temperature range, i.e. under three different structural phases. A detailed assignment of the INS spectra is proposed and we observe in particular a pronounced frequency softening of the torsional τ(NH3) vibration in conjunction with an intensity enhancement when the temperature increases; this suggests quite different dynamical regimes for the NH3 groups in the disordered phases. In fact, QNS spectra of the three derivatives under study have been satisfactorily fitted using the following models: ((i) 90° jumps of only the N-H protons over two equivalent positions in the room temperature phase ( = 2·7 × 10-12 s). (ii) Two kinds of jumps for the N-H (∼ 100°) and C-H protons (∼ 110°) in the intermediate phase II, th...

Dynamic properties of α,ω-dibromoalkane guest molecules constrained within urea inclusion compounds: a neutron scattering study

Abstract Incoherent quasielastic neutron scattering from semi-oriented polycrystalline samples of α,ω-dibromoalkane/urea-d4 inclusion compounds has been studied as a function of sample orientation, length of guest (α,ω-dibromoalkane) molecule, and temperature. In the high-temperature phase of these materials, the guest molecules, constrained within the urea tunnel structure, undergo translational (and oscillatory) motions along the tunnel axis and uniaxial rotational diffusion about the tunnel axis. Quantitative dynamics information relating to these motional models is presented.

Molecular dynamics of cyclohexane guest molecules in the cyclohexane/thiourea inclusion compound: an incoherent quasielastic neutron scattering investigation

Incoherent quasielastic neutron scattering (IQNS) has been used to investigate the dynamic properties of cyclohexane guest molecules within the tunnel/cage host structure in the cyclohexane/thiourea inclusion compound. These investigations have been carried out on samples containing perdeuterated thiourea to ensure that the incoherent scattering arises from the cyclohexane molecules. IQNS spectra have been recorded for two experimental geometries of semi-oriented polycrystalline samples (with the tunnel axes of all crystals either parallel or perpendicular to the neutron momentum transfer vector), allowing components of the motion parallel to the tunnel axis to be investigated separately from components of the motion perpendicular to the tunnel axis. In the high-temperature phase (above 149 K; phase I), the dynamics of the cyclohexane molecules can be described as 120° jumps of the molecule about the molecular 3-fold symmetry axis, combined with 120° jumps of this axis about the tunnel axis of the thioure...

Reorientational motions of dioctanoyl peroxide guest molecules within the urea tunnel structure: Assessment of two-site jump models

Abstract Dynamic properties of dioctanoyl peroxide guest molecules within the urea host tunnel structure have been investigated by incoherent quasielastic neutron scattering and solid-state 2 H-NMR spectroscopy. The dynamic properties differ substantially from those established previously for alkane and α,ω-dibromoalkane guest molecules within the urea tunnel structure. Although a model of two-site jumps gives a reasonable fit to the 2 H-NMR data, this model can be ruled out on the basis of the IQNS results.

Neutron scattering study of the reorientational motions of NH+4 ions in NH4B(C6H5)4 and NH4B(C6D5)4 crystals

Abstract Incoherent neutron scattering experiments have been performed on polycrystalline samples of NH4B(C6H5)4 and NH4B(C6D5)4 in the 1.5 to 350 K temperature range. From the variations of the variations of the incoherent structure factor, it is shown that a 2π 2 reorientational jump process of NH+4 ions takes place in the 30–350 K temperature range. The reorientational correlation time τc varies from 131 ps at 40 K to 2.6 ps at 350 K. The Arrhenius log plot of τc between 350 and 120 K gives an activation energy of 2.9 kJ mol−1 for both samples. Such a low barrier is quite exceptional and within the Arrhenius theory it can be related to a librational attempt frequency of about 34 cm−1, while using the librator model one derives a libration frequency of about 73 cm−1. Both fall in a frequency range where inelastic features are observed as well in neutron- as in Raman scattering. A discontinuity in the Arrhenius plot could indicate that the high temperature atomic organization is frozen at low temperature at least for the compounds investigated in the present study. This property could be related to the degree of crystallization.

Molecular Dynamics of Tetrakis(Trimethylsilyl)Silane in the Solid State: An Incoherent Quasi-Elastic Neutron Scattering Investigation

Incoherent quasi-elastic neutron scattering (IQNS) spectroscopy has been used to investigate the dynamic properties of solid tetrakis(trimethylsilyl)silane (Si[Si(CH3)3]4). This material is known to undergo a phase transition to a plastic crystal phase at ca. 240 K. The evolution of the elastic intensity as a function of temperature between 4.5 K and 300 K indicates three distinct dynamic regimes, with changes in dynamic behaviour at 120 K and 240 K. Detailed consideration of quasi-elastic broadening in the IQNS spectra (including consideration of the effects of multiple scattering) has established the following information on the dynamic processes occurring on the experimental timescale within each of these regimes. Below 120 K, there are only molecular vibrational motions with no quasi-elastic broadening in the IQNS spectra. Between 120 K and 240 K, the dynamic properties are interpreted as three-fold jumps of the CH3 group about the Si—CH3 bond. Above 240 K, two different dynamic models have been considered: (a) combined three-fold jumps of the CH3 group about the Si—CH3 bond and three-fold jumps of the Si(CH3)3 group about the Si—Si(CH3)3 bond; and (b) isotropic reorientational diffusion. Model (a) fails to fit the IQNS spectra, whereas model (b) provides an acceptable fit to the IQNS spectra in the temperature range investigated (245-300 K) for the high-temperature phase. The isotropic reorientational diffusion is considerably slower than the threefold jumps of the CH3 group about the Si—CH3 bond.