François Guillaume

On the vibrational T2 processes in partially ordered systems

The stretched (‘‘extended’’ or ‘‘fractional’’) exponential, exp{−(t/T)α}, where 0<α≤1 and T (ps) a time constant, is introduced as a modulation function for a vibrational T2 process in order to describe the non‐Markovian dynamics of the bath of amorphous systems in the picosecond domain. The theory simulates well published dephasing data of probe oscillators in concentrated aqueous solutions of KSCN, molten mixtures of Li and Rb nitrates, and of polycrystalline and smectic phases of the liquid crystal 4‐n‐octyloxy‐4’‐cyanobiphenyl. The temperature and phase dependence of dispersion parameter α follows an accepted model of constrained‐sequential relaxation with a scale‐invariant (‘‘fractal’’) distribution of relaxation times.

Conformational and vibrational properties of α,ω-dihalogenoalkane/urea inclusion compounds: a Raman scattering investigation

Raman spectroscopic investigations of urea inclusion compounds containing α,ω-dihalogenoalkane [X(CH2)nX; n= 8 for X = Cl; n= 7–11 for X = Br; n= 8 for X = I] guest molecules are reported. In these inclusion compounds, the urea molecules form a tunnel structure within which the guest molecules are located. Vibrational modes due to the urea confirm the structural identity of the inclusion compounds, and lattice modes of the urea host structure are assigned tentatively. Investigations of the vibrational properties of the X(CH2)nX guest molecules included within this host structure have focused on the longitudinal acoustic mode (LAM-1) and the C—X stretching vibrations. Bands in the Raman spectrum due to the ν(C—X) mode have been studied as a function of: (i) the length (n) of the guest molecule; (ii) the identity of the terminal substituent X; (iii) temperature and (iv) pressure. From these results, trends in the relative amounts of gauche and trans end-groups for the X(CH2)nX guest molecules in their urea inclusion compounds have been assessed.

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...

Molecular dynamics simulation of n‐nonadecane in urea inclusion compound. II. Rotational distribution and elastic incoherent structure factor

The rotational distribution of n‐nonadecane molecules around the channel axis in urea inclusion compound at 180 K is examined using molecular dynamics simulation and compared with data derived from incoherent neutron scattering experiment. The potential surface for rotation and translation of a single guest n‐nonadecane molecule in a urea channel contains helical valleys. However, it is found that the incommensurate packing of the n‐alkane molecules in the inclusion compound prevents their executing helical motion. The calculated rotational potential of mean force exhibits a barrier of ∼0.3 kcal/mol for transitions of the molecules between equivalent sixfold minima. The motion of the urea host molecules is coupled to that of the guest molecules which undergo jump rotational diffusion when the urea substructure is fixed in the simulation and continuous diffusion when the urea molecules are allowed to move. The elastic incoherent structure factor (EISF), which is determined by the rotational distribution of...

Molecular dynamics simulation of n‐nonadecane in urea inclusion compound. I. Comparison with quasielastic neutron scattering experiment

The diffusive dynamics of n‐nonadecane molecules in urea inclusion compound at 180 K are examined using molecular dynamics simulations. Incoherent quasielastic neutron scattering profiles are calculated from the simulation trajectories and compared with experiment. The effect on the calculated scattering functions of varying the model system in the simulation is examined. Interactions between alkane chains within the channels are found to strongly influence their rotational and translational dynamics and associated scattering spectra. The diffusive alkane dynamics is well described in terms of the molecules moving as rigid bodies. Analytical models of restricted translational and rotational rigid‐molecule diffusion of the alkanes are fitted to the simulation‐derived scattering functions in the time and energy domains. The effects of the experimental instrumental energy resolution function on the scattering functions and on the dynamical parameters obtained by fitting to the analytical models are examined....

Study of the fast photoswitching of spin crossover nanoparticles outside and inside their thermal hysteresis loop

We have studied the low spin to high spin phase transition induced by nanosecond laser pulses outside and within the thermal hysteresis loop of the [Fe(Htrz)2 trz](BF4)2-H2O spin crossover nanoparticles. We demonstrate that, whatever the temperature of the compound, the photo-switching is achieved in less than 12.5 ns. Outside the hysteresis loop, the photo-induced high spin state remains up to 100 μs and then relaxes. Within the thermal hysteresis loop, the photo-induced high spin state remains as long as the temperature of the sample is kept within the thermal loop. A Raman study indicates that the photo-switching can be completed using single laser pulse excitation.

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.

Vibrational dephasing under fractional (“stretched”) exponential modulation

A fractional (or stretched) time exponential, F(t)= exp[−(t/F)α], where 0<α<1, T a time constant, is proposed as the modulation function for vibrational band-narrowing in locally structured, amorphous media. It leads to physically sounder modulation times than current, pure exponential models, yields the same superior data fit as recently proposed, two-process relaxation mechanisms and promises useful and interesting physical insight into structural-dyamical aspects of the systems.

Local structure and dynamics in concentrated aqueous solutions and glasses of univalent ions

We have analyzed the VV‐ and VH‐Raman band profiles of the N–O symmetric stretching fundamental of the nitrate anion of 1 mol/l  NaNO3 (or LiNO3) in Li+Cl−‐enriched aqueous solutions for the liquid phase between 263 and 353 K and for the vitreous state at 77 K. From the observed isothermal correlation time of the orientational mobility of the C3 axis of NO−3 in Li+‐rich solution and from the band asymmetry of the corresponding isotropic profile at increasing temperature, we propose an endothermic local‐structure‐reforming process that favors incorporation of a few Li+ ions into the solvation sphere of the anions. The vitreous state of 1 mol/l  LiNO3 or NaNO3 in the LiCl‐rich systems, which shows considerable local order about the nitrate anion, is conceptualized as a percolation cluster of solvated cation–anion interaction that breaks up into cluster distributions for the liquid phase.

Raman spectroscopy of white wines.

The feasibility of exploiting Raman scattering to analyze white wines has been investigated using 3 different wavelengths of the incoming laser radiation in the near-UV (325 nm), visible (532 nm) and near infrared (785 nm). To help in the interpretation of the Raman spectra, the absorption properties in the UV-visible range of two wine samples as well as their laser induced fluorescence have also been investigated. Thanks to the strong intensity enhancement of the Raman scattered light due to electronic resonance with 325 nm laser excitation, hydroxycinnamic acids may be detected and analyzed selectively. Fructose and glucose may also be easily detected below ca. 1000 cm(-1). This feasibility study demonstrates the potential of the Raman spectroscopic technique for the analysis of white wines.