C. E. González

Nuclear magnetic resonance study of the internal magnetic field distribution in water base ionic and surfacted ferrofluids

This work reports a nuclear magnetic resonance (NMR) study of both hydrogen and deuterium on two different kinds of ferrofluids. One of them is a colloidal suspension of surfacted magnetic grains dispersed in a mixture of light and heavy water. The second one is an ionic ferrofluid composed by positively charged magnetic grains of Co–Fe2O4 dispersed in the same solution of water. A model of the local magnetic field distribution in the interstitial (bulk) water is worked out in order to account for the broadening of the NMR spectra observed in both samples.

Relaxation properties of proton magnetic spin quasi-invariants in liquid crystals

Abstract We present an experimental study of the thermodynamic properties of proton pairs in two thermotropic nematic liquid crystals: chain deuterated 4-n-pentyl-4′-cyanobyphenyl, 5CBd11, and normal 5CB. In the first sample, we find the existence of pure intra-pair and inter-pair magnetic quasi-invariants. In the second, the dipolar signal is more complex, due to the nonequivalence of protons pairs in the molecule, but it is still possible to prepare states for which only one kind of order strongly dominates. In both compounds, the dipolar quasi-invariants relax independently to thermal equilibrium with the lattice. Finally, we discuss the temperature dependence of the characteristic relaxation times in terms of local and long-range cooperative molecular dynamics.

On the applicability of the Redfield high-temperature theory for proton dipolar order relaxation in liquid crystals

Nuclear magnetic resonance experiments on the Larmor frequency dependence of the intrapair dipolar order relaxation time T1D showed the existence of discrepancies between the usual two-spin theory and the experimental data. Field-cycling experiments of T1D in methyl deuterated para azoxyanisole (PAA-d6) in the nematic phase [J. Chem. Phys. 110, 8155 (1999)] showed that the difference cannot be assigned to the common assumption of isolated spin pairs or to the neglecting of the alkyl chain protons in the theoretical interpretation. Though the applicability of the spin temperature and short correlation time assumptions have not been justified for nematic liquid crystals, they have been used in all the available theoretical approaches for dipolar order relaxation. In this work, we calculate T1D of PAA-d6 within the Redfield high-temperature theory of spin–lattice relaxation but avoid the usual assumptions of short correlation times and random phase. First, we find that this fact does not alter the expression...

Water dynamics in ionic magnetic colloids studied by 1H nuclear magnetic resonance

Abstract In a previous nuclear magnetic resonance (NMR) study we observed that the NMR spectra of water in both surfacted and ionic ferrofluids are asymmetric and several orders of magnitude wider than the one of pure water. It has been proposed that this effect is produced by extremely strong magnetic field gradients in the intergrain volume and/or by surface interactions between the carrier liquid molecules and the grains surface. In the case of aqueous ionic ferrofluids the latter possibility should be interpreted as electric interactions between water (polar) molecules and the charges in the grain surface. In this work we study a series of ionic and surfacted ferrofluids prepared at different magnetic grain concentrations and sizes, and with different surface charge densities. Our experiments clearly show that the sign and the density of the electric charge on the magnetic grains have no influence on NMR spectra. On the other hand, spectral widths increase with the magnetic grain concentration.

Nuclear quadrupole resonance line shape study of the orientationally disordered phase of p‐chloro nitrobenzene

An experimental study of the nuclear quadrupole resonance (NQR) line shape of p‐chloro nitrobenzene in the temperature range 77–150 K is reported. The molecules of p‐ClC6H4NO2 in the solid phase are arranged following an orientationally disordered pattern. The experimental NQR spectra at several temperatures are unusually broad for a molecular crystal and show a structure of peaks. Our measurements suggested that the line broadening is due to a static distribution of disordered molecules. In order to explain the nontypical resonance frequency dispersion, a simulation of the NQR line shapes is carried on. The model for the simulated crystal assumes that each molecule is an electric dipole and that the crystal is a disordered binary alloy of dipoles having two possible orientations. By calculating the intermolecular contribution to the electric field gradient (EFG) at the resonant nuclei sites, we get the expected EFG distribution in the sample. The approach we propose here explicitly involves the symmetry ...

Molecular motions in thermotropic liquid crystals studied by NMR spin-lattice relaxation

Nuclear magnetic resonance relaxation experiments with field cycling techniques proved to be a valuable tool for studying molecular motions in liquid crystals, allowing a very broad Larmor frequency variation, sufficient to separate the cooperative motions from the liquidlike molecular diffusion. In new experiments combining NMR field cycling with the Jeener Broekaert order-transfer pulse sequence, it is possible to measure the dipolar order relaxation time (T1D), in addition to the conventional Zeeman relaxation time (T1Z) in a frequency range of several decades. When applying this technique to nematic thermotropic liquid crystals, T1D showed to depend almost exclusively on the order fluctuation of the director mechanism in the whole frequency range. This unique characteristic of T1D makes dipolar order relaxation experiments specially useful for studying the frequency and temperature dependence of the spectral properties of the collective motions.

Mechanisms of irreversible decoherence in solids

Refocalization sequences in Nuclear Magnetic Resonance (NMR) can in principle reverse the coherent evolution under the secular dipolar Hamiltonian of a closed system. We use this experimental strategy to study the effect of irreversible decoherence on the signal amplitude attenuation in a single crystal hydrated salt where the nuclear spin system consists in the set of hydration water proton spins having a strong coupling within each pair and a much weaker coupling with other pairs. We study the experimental response of attenuation times with temperature, crystal orientation with respect to the external magnetic field and rf pulse amplitudes. We find that the observed attenuation of the refocalized signals can be explained by two independent mechanisms: (a) evolution under the non-secular terms of the reversion Hamiltonian, and (b) an intrinsic mechanism having the attributes of irreversible decoherence induced by the coupling with a quantum environment. To characterize (a) we compare the experimental data with the numerical calculation of the refocalized NMR signal of an artificial, closed spin system. To describe (b) we use a model for the irreversible adiabatic decoherence of spin-pairs coupled with a phonon bath which allows evaluating an upper bound for the decoherence times. This model accounts for both the observed dependence of the decoherence times on the eigenvalues of the spin-environment Hamiltonian, and the independence on the sample temperature. This result, then, supports the adiabatic decoherence induced by the dipole-phonon coupling as the explanation for the observed irreversible decay of reverted NMR signals in solids.

Evidence for several dipolar quasi-invariants in liquid crystals

The quasi-equilibrium states of an observed quantum system involve as many constants of motion as the dimension of the operator basis which spans the blocks of all the degenerate eigenvalues of the Hamiltonian that drives the system dynamics, however, the possibility of observing such quasi-invariants in solid-like spin systems in Nuclear Magnetic Resonance (NMR) is not a strictly exact prediction. The aim of this work is to provide experimental evidence of several quasi-invariants, in the proton NMR of small spin clusters, like nematic liquid crystal molecules, in which the use of thermodynamic arguments is not justified. We explore the spin states prepared with the Jeener-Broekaert pulse sequence by analyzing the time-domain signals yielded by this sequence as a function of the preparation times, in a variety of dipolar networks, solids, and liquid crystals. We observe that the signals can be explained with two dipolar quasi-invariants only within a range of short preparation times, however at longer times liquid crystal signals show an echo-like behaviour whose description requires assuming more quasi-invariants. We study the multiple quantum coherence content of such signals on a basis orthogonal to the z-basis and see that such states involve a significant number of correlated spins. Therefore, we show that the NMR signals within the whole preparation time-scale can only be reconstructed by assuming the occurrence of multiple quasi-invariants which we experimentally isolate.