Arnaud Desmedt

Low-energy spin excitations in the molecular magnetic cluster V15

We report an Inelastic Neutron Scattering (INS) study of the fully deuterated molecular compound K6[V15IVAs6O42] · 9D2O (V15). Due to geometrical frustration, the essential physics at low temperatures of the V15 cluster containing 15 coupled V4+ (S = 1/2) is determined by three weakly coupled spin-(1/2) on a triangle. The INS spectra at low energy allow us to directly determine the effective exchange coupling J0 = 0.211(2) meV within the triangle and the gap 2Δ = 0.035(2) meV between the two spin-(1/2) doublets of the ground state. Results are discussed in terms of deviations from trigonal symmetry and Dzyaloshinskii-Moriya (DM) interactions.

Water diffusion in fully hydrated porcine stratum corneum

Abstract The microscopic mechanisms of water diffusion in fully hydrated porcine stratum corneum (SC) have been studied by a combination of incoherent quasielastic neutron scattering (QENS) and pulsed field gradient-nuclear magnetic resonance (PFG-NMR) for two sample orientations. The presence of three types of water in fully hydrated SC is inferred on the basis of water sorption isotherm data, i.e., (a) bound and (b) weakly bound hydration water forming layers between adjacent lipid bilayers of SC, as well as (c) bulk water probably located in the corneocytes and in intercellular regions. Water self-diffusion coefficients for motions parallel and perpendicular to the membrane plane of D ∥ =3.30×10 −10 m 2 /s and D ⊥ =1.56×10 −10 m 2 /s, respectively, were determined by PFG-NMR and assigned to the translational diffusion of weakly bound water. QENS measurements have been carried out using different samples hydrated with H 2 O and D 2 O, respectively, in order to separate the contribution of SC from that of the water. The QENS data for both sample orientations and two different energy resolutions can be fitted by a model which accounts for the microscopic dynamics of all three aforementioned types of water. This analysis establishes rotational diffusion coefficients for bound and weakly bound hydration water of 0.025 and 0.030 meV, respectively. Furthermore, the QENS data prove the presence of bulk water in fully hydrated SC samples.

Molecular dynamics simulation study of cyclohexane guest molecules in the cyclohexane/thiourea inclusion compound

Molecular dynamics simulations of the cyclohexane/thiourea inclusion compound within the high temperature rhombohedral phase (at 173 and 273 K) are presented. The simulated systems consist of 96/288 cyclohexane/thiourea molecules, corresponding to 12 contiguous host thiourea channels of approximately 50 A length. The atomic trajectories were obtained over 600 ps assuming rigid molecules. The orientational distribution of cyclohexane guest molecules is described through the polar angles between the guest molecular C m symmetry axis and the thiourea host reference frame. From the molecular dynamics simulations we obtain the three-dimensional orientational distribution functions POh; uU. It is shown that the cyclohexane guest molecules reorient about the C c and the C2 symmetry axes of the crystallographic 32 site. The incoherent intermediate scattering functions have been computed from molecular dynamics trajectories, allowing the development of theoretical models that could be used to interpret the dynamics of cyclohexane guest molecules from experiments. ” 2000 Elsevier Science B.V. All rights reserved.

Probing adsorption sites in a cubic II water clathrate cage by methyl group rotation of CH3I guest molecules

Methyl iodide enclosed in the 51264 cage of cubic II water clathrate is investigated by neutron spectroscopy in the temperature regime from 1.8 to 145?K. The three tunnelling bands observed at the lowest temperature at 497, 372 and 243??eV of relative intensities 2 : 3 : 5 represent three distinct adsorption sites of CH3I. Sites of occurrence probabilities corresponding closely to the observed line intensities are related to the three types of hydrogen bonds of the cage. The band at 372??eV shows fast spin conversion compared to the two others. The corresponding site is facing the shortest O?H..O hydrogen bond with the fastest proton dynamics. Spin conversion may thus reflect hydrogen jumps within the H-bond network of host molecules. Jump rates of the order of 10?4?s?1 can be estimated. Above a temperature T ~ 45?K the methyl dynamics become classical and can be described by rotational diffusion on a circle. A second quasielastic process becomes visible at a higher temperature and is identified as head-to-tail flips of the whole molecule with a characteristic jump distance of about 5 ?. Above T ~ 90?K translational freedom of the CH3I leads to a rattling mode appearing at ~0.95?meV.

MD simulation of the dynamics of chlorocyclohexane guest molecules in the thiourea inclusion compound

Abstract We present the results of molecular dynamics simulations of the chlorocyclohexane/thiourea inclusion compound within the high temperature rhombohedral and low temperature monoclinic phases of the composite crystal. By defining a molecular reference frame by means of two vectors n → and p → , we have been able to analyze separately the reorientations of the guest molecules as a whole in the laboratory reference frame and the reorientations of the guest molecules about an intramolecular reference axis. In the rhombohedral phase, we found that the chlorocyclohexane guest molecules reorient as a whole over six favored sites around the C 3 and the C 2 symmetry axes of the crystallographic site (D 3 point group). In the low temperature phase, the symmetry elements of phase I are lost and the guest molecules sit in general positions with a dynamic disorder among two orientations. Orientational distribution functions and space–time correlation functions (intermediate incoherent neutron scattering laws) for the dynamics of n → and p → have been analyzed in both structural phases. These results are discussed in relation to incoherent quasi-elastic neutron scattering (QENS) experiments and compared to those previously obtained for the cyclohexane/thiourea inclusion compound. Marked differences between both systems have been found for the reorientations of guest molecules. These simulations will be exploited to interpret QENS experiments in view of elucidating the phase transition mechanisms in these compounds.

In-situ monitoring of alkane-alkane guest exchange in urea inclusion compounds using confocal raman microspectrometry

The occurrence of alkane-alkane guest exchange within the one-dimensional channels of the urea host structure in alkane/urea inclusion compounds is demonstrated using confocal Raman microspectrometry. With appropriate deuteration of one of the alkane guest components, this technique represents a powerful strategy for in situ characterization of the alkane-alkane guest exchange process.

Methyl group rotation in trimethylaluminium

The stable molecular unit of trimethylaluminium is the dimer linked via bridging methyl groups. Neutron spectroscopy is used to determine internal and external modes from µeV (rotational tunnelling transitions of methyl groups, stochastic rotational jumps) to meV (deformations, librations, phonons etc). The dimer appears to be non-rigid with a very soft bend mode about an axis connecting the two bridging methyl groups. Rotational potentials are derived from tunnelling modes and barrier heights deduced from quasielastic spectra. Internal modes obtained from ab initio calculations for the isolated dimer are consistent with measured vibrations with respect to eigenenergies and band intensities calculated by the CLIMAX program. The bridging CH3 group shows the strongest and almost purely intramolecular potential due to a loss of a single bond axis of its pentacoordinated carbon atom.

Methyl rotational potentials of trimethyl metal compounds studied by inelastic and quasielastic neutron scattering

Abstract Neutron scattering data on classical and quantum rotation of methyl groups in the organometallic molecular crystal In(CH 3 ) 3 are presented. The tunneling spectra are analyzed using the mean field single particle model of rotational tunneling. The presence of six inequivalent rotors of equal occurrence probabilities in In(CH 3 ) 3 is reflected in the number and intensities of the transitions. In a similar way classical spectra are interpreted as superposition of quasielastic Lorentzians. A consistent combination of tunneling and quasielastic spectroscopies is based on equal relative intensities of the respective spectral components and allows a determination of the coefficients of a Fourier expansion of the rotational potentials up to second order with only a few percent of uncertainty. The results are compared to published data on the homologues Al and Ga compounds on the basis of the molecular and crystal structures. Intra- and intermolecular interactions are found to be of similar importance.

The methyl rotational potentials of Ga(CH3)3 derived by neutron spectroscopy

High resolution neutron spectra of Ga(CH3)3 show tunnelling transitions between 4.5 and 19 μ eV. The spectrum can be explained within the single-particle model on the basis of the monoclinic C2/c (Z = 16) low temperature crystal structure of Ga(CH3)3 with six inequivalent methyl groups in the unit cell. The overlapping tunnelling lines prevent the extraction of temperature dependent linewidths which would allow us to assign the librational energies measured in the phonon density of states. Classical rotational motion is studied by quasielastic neutron scattering. Three activation energies could be extracted. Methyl librations, tunnelling energies and barrier heights are combined with consistent intensities into rotational potentials. Only the concerted application of all spectroscopic techniques yields a conclusive description.