Claude Chachaty

Alkyl chain conformations in a micellar system from the nuclear spin relaxation enhanced by paramagnetic ions

Abstract The internal structure of sodium octanoate micelles has been investigated by 13C paramagnetic longitudinal relaxation induced by Mn2+ and Ni2+ bounds to the surface. The decrease of the relaxation rates from the polar head through the methyl end of alkyl chains is consistent with a preferred trans conformation resulting in part from their confinement in the non-polar core.

Hyperfine couplings and 13C relaxation in alkylamines coordinated to Ni(II) acetylacetonate

The conformational dependence of the proton and carbon-13 hyperfine coupling constants in aliphatic amines coordinated to Ni(AA)2 has been investigated by N.M.R. contact shift measurements. It is shown that the hyperfine coupling constants a C β and a H β are given by with B 1 H=10·5 G, B 2 H=10·6 G, B 1 C=-4·9 G, B 2 C=13·1 G where ϑ is the torsion angle about the C1-N bond. The experimental values of a H β and a C β in the ethylamine-Ni(AA)2 complex have been compared with those calculated from the above expression and the potential function computed by INDO for the rotation of C2H5 about the N-C1 bond in EtNH2 +. Good agreement is obtained for a potential barrier V 0 = 15·5 kJ mol-1. The 13C paramagnetic relaxation rates T 1M -1 and T 2M -1 in the complexes have also been determined. For n-butyl and n-hexylamine complexes, the dipolar longitudinal relaxation rates have been calculated from Monte Carlo simulations of random fluctuations of the magnetic field due to internal and overall motions and to el...

Nuclear spin dipolar relaxation in paramagnetic systems undergoing multiple internal motions

A theoretical treatment is proposed for the relaxation induced by the dipolar interaction between two spins separated by several bonds about which jump motions occur among three sites, two of them at least being equivalent. The influence of the internal and overall molecular motions upon the carbon-13 longitudinal relaxation is discussed for the model of n-butylamine coordinated to a paramagnetic ion having a comparatively long T 1e like Mn2+. The 13C paramagnetic dipolar relaxation appears to be mainly dependent upon the ratios of the transition rates among the different conformers. A computer program has been written to analyse the 13C longitudinal dipolar relaxation in terms of internal motions and transient conformations for flexible and partially rigid molecules such as butylamine, allylamine and benzylamine coordinated to Mn(AA)2 and Gd(DPM)3. The extension of these calculations to complexes where the nuclear relaxation is governed by the electron spin relaxation times is examined for the example of...

Spin densities, ligand exchange and 13C relaxation in aniline-Ni II complexes

Abstract The electron spin distribution in aniline and alkylaniline-Ni(AA) 2 complexes is deduced from 1 H, 13 C and 14 N contact shifts. The nitrogen hybridization state is given by the experimental values of a NH H / a N compared to the results of INDO calculations. The 13 C relaxation times in complexed 4-ethyl aniline indicates an N-Ni distance of 2 A and an electron relaxation time T 1 of the order of 10 −10 s.

Monte Carlo study of 13C nuclear spin—lattice relaxation in aliphatic chains bound to a macromolecule

Abstract The random motions of an n -hexyl chain bound to a macromolecule have been simulated with several models by Monte Carlo techniques. The 13 C T 1 relaxation times were calculated from the spectral density functions of the 13 C 1 H dipolar obtained with poly(N-hexyl-4-vinylpyridinium) bromide in methanol solution.

Conformational study of long alkyl chains by carbon-13 pseudo-contact shifts

Abstract The geometric factor of the 13 C pseudo-contact in several n -alkyl amines and alcohol has been calculated by taking the statistical average over all possible conformations of these chain molecules. A close agreement with experimental values has been obtained for adducts to lanthanide shift reagents. The 13 C contact and pseudo-contact shifts in Co(AA) 2 — amine and alcohol complexes were separated by means of optimized values of the calculated geometric factors.

Simulation of EPR spectra with automatic fitting of the spectroscopic parameters and of the reorientational correlation time

The simulation of EPR spectra is often necessary for the accurate determination of the principal values of the electronic Zeeman and hyperfine tensors. The automatic adjustment of these principal values and of linewidths enables us rapidly to achieve a satisfactory agreement between the observed and the calculated spectra. The usual criterion of least squares may be applied to the deviations to perform this adjustment. In two or more dimensions, a minimization algorithm derived from that of Levenberg–Marquardt is applied. In one dimension, the algorithm of the Fibonacci series has been used.This approach lends itself to the determination of parameters, such as the reorientational correlation time, which may not be deduced by a casual inspection of spectra. It is illustrated by the example of a poly(4-vinylpyridine) gel, crosslinked by vanadyl ions. Whereas the gel is rigid at low temperatures, the motion of the paramagnetic probe modifies the spectral shapes for this gel above 260 K.The first stage involves only the spectral parameters which are fitted from the low-temperature spectrum, using second-order perturbation formulae. The second stage requires a model of reorientation; simulations have clearly shown that a Brownian diffusion model must be preferred to a random-jump model. By keeping the spectral parameters as constants, the reorientational correlation time, as well as linewidths, can be determined at several temperatures by the above minimization.

NMR contact shifts and local conformation in poly(4-vinylpyridine N-oxide)-Ni(II) complexes

The 1H and 13C NMR contact shifts induced by Ni2+ ions in aqueous solutions were used to elucidate the conformation of poly[1-(1-oxo-4-pyridyl)ethylene] (1) and of the model compound 1,3-bis(1-oxo-4-pyridyl)propane (2). The equilibrium position of the pyridine N-oxide ring relative to the adjacent methine or methylene group was deduced from the angular dependence of the hyperfine coupling of β-protons and -carbons, assuming a twofold potential energy for the rotation about the C4N axis. In the case of 1 it could be shown that the dihedral angle between the plane HβCαC4 and the plane of the ring is 15±5°.