F. Guillaume

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...

Rotational molecular dynamics in the RI phase of n-nonadecane

Abstract Reorientational dynamics of n-nonadecane molecules in the R I phase have been investigated by means of molecular dynamics computer simulations (MD) and incoherent quasielastic neutron scattering (IQNS) techniques. Models based on jump process and rotational diffusion are compared with the calculated intermediate scattering functions and with the experimental spectra. A four-site jump model must be ruled out while a model based on a fourfold potential with two different barriers could be used to interpret the data successfully.

Reorientational motions of the alkyl chains in C10H21ND3Cl bidimensional crystal

Abstract INS experiments were performed at the Institut Laue-Langevin on the bidimensional C 10 H 21 ND 3 Cl crystal to investigate the hydrocarbon chain motions through the various structural phases of the compound. The quasielastic profiles obtained on high and medium resolution spectrometers were refined simultaneously to get a reliable estimate of the elastic incoherent structure factors. In the high temperature phase, there is a gradient of disorder in the -CH 2 -units on going from the polar head to the methyl ends in agreement with the “tree-like” structure suggested from previous Raman and infrared investigations.

Reorientations of tetramethylammonium ions in TMMC studied by incoherent quasielastic neutron scattering (IQNS)

Abstract INS experiments were performed at the Institut Laue-Langevin (Grenoble, France) on crystals with formula (CH3)4NMnCl4 (TMMC) in order to analyze the reorientations of the tetramethylammonium cations (TMA) in the dynamically disordered phase of the crystal. From the analysis of the elastic incoherent structure factors (EISF), extracted from the data recorded on a very large scale of instrumental resolutions, we were able to propose a model of jumps of TMA over 20 sites distributed among three semicircular arcs joining together on a threefold C3 axis. Then, a preliminary quantitative description of the motions involved in the high temperature phase is proposed.

Molecular motions in perovskite type layered compounds (NH3(CH2)nNH3)MnCl4, with n = 3, 4, 5; An incoherent neutron scattering study

Abstract Dynamics of the alkylene chains and of the NH+3 polar heads in (NH3(CH2)nNH3)MnCl4 compounds (n =3, 4, 5) were studied by means of incoherent neutron scattering techniques. In the disordered phases, the propylene chains (n = 3) keep an “all trans” structure while a conformational equilibrium between “trans” and “twisted” states is evidenced for longer chain compounds; in addition, the NH3 groups perform independent oscillations. The phase sequences in all these compounds are then discussed.

Molecular Motions of Hydrocarbon Chains (Decylammonium, n-Nonadecane) in Their Dynamically Disordered Phases: An Incoherent Neutron Scattering Study

It is well known that many compounds containing hydrocarbon chains such as liquid crystals or lipid membranes undergo solid state phase transitions prior to melting. In these intermediate phases, between the completely ordered crystal and the isotropic liquid, large amplitude motions of the hydrocarbon chains are effective and give rise to a strong dynamical disorder.