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129Xe gas-to-solution NMR chemical shifts for xenon dissolved in pure n-alkanes, n-alkyl alcohols, n-alkyl carboxylic acids, di-n-alkyl ketones, and cycloalkanes and in solutions of lauric acid in n-heptane are reported. The medium effect corrected for solvent density is found to be linearly dependent on the number of carbon atoms except for the shortest members of the series of linear solvents. The same slope is observed for all the linear solvents; the slope for the cycloalkanes is significantly different. These results are interpreted on the basis of a group contribution analysis. The relative contribution of methyl and methylene groups in linear solvents is found to be in very good agreement with the relative XeCH3 and XeCH2 dispersive interaction energies. The 129Xe chemical shifts for solutions of lauric acid in n-heptane calculated from the group contributions are in excellent agreement with the experimental values. The deshielding effect of the methylene group in cycloalkanes and the 129Xe chemical shift measured in the shortest members of the linear series of solvent are discussed in terms of intermolecular shielding functions and distributions of groups in the solvation shell of the Xe atom., info:eu-repo/semantics/published

29Si solid-state NMR allows us to distinguish silicon atoms involved in siloxane bridges (Q4), single silanol (Q3) and geminal silanol groups (Q2) on the surface of silica. The relative proportions of each species can be obtained by Magic Angle Spinning (MAS) associated with 1H-->29Si crosspolarization (CP/MAS). This preliminary study deals with the silicon functionality of MCM-41, a purely siliceous, amorphous and mesoporous silica. From the 29Si spectra the relative proportions are estimated to be 3.7% Q2 27.3% Q3 and 69.0% Q4, while the ratio Q2/Q2 + Q3) is 0.12. These results are found to be in perfect agreement with a binomial probability distribution of sites., Journal Article, info:eu-repo/semantics/published

The possibility of using sodium-23 spin-lattice relaxation rate measurements to probe the interaction modes of Ru11 polyazaaaromatic complexes with DNA is investigated. The following complexes are considered: Ru(phen)3(2+) (phen = 1.10-phenanthroline), Ru(phen)2HAT2+ (HAT = 1,4,5,8,9,12-hexaazatriphenylene), and Ru(diMeTAP)3(2+) (diMeTAP = 2,7-dimethyl-1,4,5,8-tetraazaphenanthrene). The addition of Ru(diMeTAP)3(2+) to a solution of NaDNA leads to a decrease in the sodium-23 spin-lattice relaxation rate (R1) similar to the effect observed upon addition of Mg2+. This indicates that Ru(diMeTAP)3(2+) interacts like Mg2+ with DNA and consequently that the electrostatic interaction dominates the association with DNA, Ru(phen)3(2+) and Ru(phen)2HAT2+ diminish R1 more efficiently than Mg2+, in a manner similar to ethidium bromide, which is known for its intercalation properties. Thus interactions other than electrostatic occur between these two complexes and DNA. These results are in agreement with data obtained from other techniques, according to which Ru(phen)3(2+) and Ru(phen)2HAT2+ are located partially inside the DNA double helix, in contrast to Ru(diMeTAP)3(2+) which remains in the ionic atmosphere around the phosphate backbone., Journal Article, Research Support, Non-U.S. Gov't, info:eu-repo/semantics/published

[129]Xe NMR has been used to investigate intermolecular interactions between xenon and α-cyclodextrin in H[2]O and in DMSO. Nuclear Overhauser enhancement effects confirmed the formation of a xenon-cyclodextrin complex. Chemical shifts were measured in solutions containing different concentrations of α-cyclodextrin and in solutions containing linear oligosaccharides. The experimental data are interpreted on the basis of a three-site model for xenon. The binding characteristics of the xenon-cyclodextrin system, including the [129]Xe NMR chemical shift in the cavity and the equilibrium constants, were determined and are discussed., SCOPUS: ar.j, info:eu-repo/semantics/published

By use of molecular-dynamics simulations for xenon dissolved in liquid benzene, it is confirmed that the intermolecular dipole-dipole spin-relaxation mechanism contributes significantly to the relaxation of 129Xe. The calculated dipolar relaxation time (750 ± 160 s) lies in the range of the experimental determinations, which are 390 and 870 s. The time correlation function of the Xe-H spin dipole-dipole interaction displays a fast decay between 0 and 5 ps, decreases exponentially between 5 and 15 ps, and follows a t^(−3/2) power law at longer times. The Xe-H correlation function can be, in very good approximation, factored into distance and angular contributions; the latter is found to be the major contribution. The influence of the rotational dynamics of the solvent molecules is investigated on the basis of comparisons between the Xe-H time correlation functions and the corresponding functions for spins located at the center of mass of the benzene molecules. Finally, the relaxation of 129Xe and 131Xe are compared. It is shown that the quadrupole relaxation of 131Xe depends essentially on the rotational motions of the solvent molecules while the dipolar relaxation of 129Xe is largely dependent on the translational motions., SCOPUS: ar.j, info:eu-repo/semantics/published

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Molecular dynamics and Monte Carlo simulations were carried out for xenon atoms dissolved in liquid benzene in order to study the quadrupolar relaxation mechanism of the xenon-131 nucleus. We confirm that a proper description of the relaxation is obtained by taking into account the quadrupole moment of benzene molecules. In contrast to the Xe-water system, the cross-correlation contribution to the electric field gradient (experienced by the Xe nucleus) is negligible in benzene, and the time correlation function of the total electric field gradient shows a relatively smooth decay. Several molecular motions take part in this decay, but not with the same efficiency. The major contribution can be attributed to the rotation of the benzene molecules around their C 2 axes. The adequacy of the Sternheimer model for quadrupolar interactions is confirmed with ab initio calculations. Some methodological aspects of simulations are also discussed, such as the use of a termination function to treat the electrostatic interactions and the effect of the xenon interaction parameters on the simulated relaxation rate. © 1993 American Institute of Physics., SCOPUS: ar.j, info:eu-repo/semantics/published

Doctorat en Sciences, info:eu-repo/semantics/nonPublished

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We present a comparative analysis of the PISA and RAM models using Monte Carlo (MC) simulations as references. The PISA and RAM models have in common a factorization of the pair distribution into an angular and a radial part. For the angular part both the PISA and RAM models use a Boltzmann weighting. First, we show that this factorization itself and the Boltzmann weighting are satisfactory approximations in a wide range of density and temperature conditions. Indeed, the Boltzmann weighting yields very good average energies (angular average). The chief approximation of the PISA method consists of taking a Heaviside step function for the distribution of the centres of mass. In the RAM theory the distribution of the centres of mass in the molecular fluid is supposed to be the same as the distribution of a spherically symmetric reference fluid. The RAM hypothesis and the PISA approximation are tested and discussed for several molecular liquids (nitrogen, bromine, carbon tetrachloride, benzene, and cis- and trans-decalin). We show that the RAM theory is unable to give a reliable radial distribution function except for systems studied under favourable conditions (nitrogen and bromine at high temperature). The agreement between MC and RAM radial distribution functions is particularly poor for highly non-spherical molecules like trans-decalin. The quality of the PISA approximation of the radial distribution function is also discussed in terms of molecular shape. Finally, we show that the more sophisticated RAM theory does not necessarily yield better internal energies than the simplest but very efficient PISA model. © 1990 Taylor & Francis Group, LLC., SCOPUS: ar.j, info:eu-repo/semantics/published

The chemical shift (δ) of a xenon monoatomic molecule is very sensitive to the environment. The major contribution to this shift arises from the van der Waals interactions between the solvent and the xenon molecules. Usually, this solvent effect is interpreted via a reaction field model. This approach leads to a search for a correlation between δ and a function of the square of the refractive index of the solvent [f(n2)]. This paper illustrates the use of a theoretical model developed in our laboratory for the interpretation of the solvent effect on the xenon chemical shift. This model allowed the calculation of the xenon-solvent dispersion energy (Edis) for different solvents, and the correlation of Edis with δ. The Edis-δ correlation is clearly better than the f(n2)-δ correlation., FLWNA, info:eu-repo/semantics/published

A new theoretical model is proposed for the estimation of interaction energies in pure liquids as well as in dilute binary solutions, with particular emphasis on the London and Pauli contributions to the interaction energy. Basically, interaction energies are calculated as the sum of pair interactions between two molecules, with the average over all pair interactions being effected with the aid of a simplified pair distribution function. The method is tested on a large ensemble of pure liquids and dilute organic solutions. It is shown that this model, called PISA (pair interaction structureless approximation), is very efficient for liquids and solutions in which the intermolecular interactions are dominated by the London-Pauli contribution., SCOPUS: ar.j, info:eu-repo/semantics/published