Mariano Zuriaga

Dynamic heterogeneity in the glass-like monoclinic phases of CBrnCl4−n, n = 0,1,2

Glassy dynamics of rigid molecules is still a matter of controversy: the physics behind the relaxation process at time scales faster than that ruled by the viscosity, the so called Johari-Goldstein process, is not known. In this work we unravel the mechanism of such a process by using a simple molecular model in which the centers of mass of the molecules are forming an ordered lattice, and molecular reorientation is performed by jumps between equilibrium orientations. We have studied the dynamics of simple quasi-tetrahedral molecules CBr(n)Cl(4-n), n = 0, 1, 2, in their monoclinic phases by means of dielectric spectroscopy and nuclear quadrupole resonance: the first technique allows to measure in a broad time scale but it is insensitive to molecular particularities, while the second has a restricted time window but senses the movement of each chlorine atom separately. The dynamic picture emerging from these techniques is that the secondary relaxation process is related to the different molecular surroundings around each nonequivalent atom of the molecule. Dynamical heterogeneities thus seem to be the cause of the secondary relaxation in this simple model of glass.

Molecular kinetics of solid and liquid CHCl3

Abstract We present a detailed analysis of the molecular kinetics of CHCl 3 in a range of temperatures covering the solid and liquid phases. Using nuclear quadrupolar resonance we determine the relaxation times for the molecular rotations in solid at pre-melting conditions. Molecular dynamics simulations are used to characterize the rotational dynamics in the solid and liquid phases and to study the local structure of the liquid in terms of the molecular relative orientations. We find that in the pre-melting regime the molecules rotate about the C–H bond, but the rotations are isotropic in the liquid, even at supercooled conditions.

Orientational relaxations in solid (1,1,2,2)tetrachloroethane

We employ dielectric spectroscopy and molecular dynamic simulations to investigate the dipolar dynamics in the orientationally disordered solid phase of (1,1,2,2)tetrachloroethane. Three distinct orientational dynamics are observed as separate dielectric loss features, all characterized by a simply activated temperature dependence. The slower process, associated to a glassy transition at 156 ± 1 K, corresponds to a cooperative motion by which each molecule rotates by 180° around the molecular symmetry axis through an intermediate state in which the symmetry axis is oriented roughly orthogonally to the initial and final states. Of the other two dipolar relaxations, the intermediate one is the Johari-Goldstein precursor relaxation of the cooperative dynamics, while the fastest process corresponds to an orientational fluctuation of single molecules into a higher-energy orientation. The Kirkwood correlation factor of the cooperative relaxation is of the order of one tenth, indicating that the molecular dipoles maintain on average a strong antiparallel alignment during their collective motion. These findings show that the combination of dielectric spectroscopy and molecular simulations allows studying in great detail the orientational dynamics in molecular solids.

Isomorphism and disorder in o-chlorohalobenzenes studied by NQR.

In this work we present experimental results that allow to characterize different solid modifications found in o-chlorohalobenzenes. Three disordered phases have been found in o-chlorobromobenzene. The stable phase at high temperature (phase I) is also obtained by quenching the sample at 77 K. Slow cooling allow to obtain the low temperature phase III which, on heating, transforms to phase II at 183 K and this, in turns, transforms to phase I at T~210 K. The disorder evidenced through the Nuclear Quadrupole Resonance spectra, is attributed to a random occupation of chlorine and bromine sites. In all phases there is evidence of molecular reorientations out of the benzene ring plane around the pseudo-symmetry axis between the atoms of Cl and Br. In o-chlorofluorobenzene two phases have been found depending on the cooling rate. One phase is disordered due to the random exchange of the occupation of Cl and F sites. In this case, there is also evidence of molecular reorientations out of the benzene ring plane, but in this case the reorientation is around the pseudo-symmetry axis that pass through the C-Cl bonds. Comparisons with the behavior of o-dichlorobenzene are also made.

Chlorine nuclear quadrupole resonance transition frequency in p-chloroaniline

Abstract The Nuclear Quadrupole Resonance transition frequency of Cl in p-chloroaniline has been measured as a function of temperature in the range 80–300 K. Its temperature dependence is analyzed using Bayers theory in the quasi-harmonic approximation and including the internal modes contribution. Excellent agreement is found with Raman and IR data. Also, the importance of the internal modes is clearly shown. The N transition frequency data are analyzed, and in order to produce good agreement with previous work a simple model to account for the electric field gradient at the N nuclear site had to be proposed.

A model to describe the inhomogeneous broadening of NQR lines in chlorohalobenzenes with orientational or substitutional disorder

A simple model to explain the NQR lineshape in solids with orientational disorder or substitutional disorder is presented. The particular case of m-chlorobromobenzene is studied. It is based on the assumption that Bromine atoms, of m-chlorobromobenzene molecules, behave as point defects in the m-dichlorobenzene lattice that modify the crystalline Electric Field Gradient. The model is also tested successfully in solid solutions of p-dichlorobenzene-p-dibromobenzene, where Bromine atoms of p-dibromobenzene molecules are assumed to be homogeneously distributed in the p-dichlorobenzene lattice. The lineshape, of others disordered chlorohalobenzenes, are also analyzed. Also, a characterization of m-chlorobromobenzene dynamics is included. In particular, there is no evidence of molecular reorientations as it is observed in the disordered phases of o-chlorobromobenzene.

Dynamic heterogeneity in an orientational glass

The family of compounds CBrnCl4-n has been proven helpful in unraveling microscopic mechanisms responsible for glassy behavior. Some of the family members show translational ordered phases with minimal disorder which appears to reveal glassy features, thus deserving special attention in the search for universal glass anomalies. In this work, we studied CBrCl3 dynamics by performing extensive molecular dynamics simulations. Molecules of this compound perform reorientational discrete jumps, where the atoms exchange equivalent positions among each other revealing a cage-orientational jump motion fully comparable to the cage-rototranslational jump motion in supercooled liquids. Correlation times were calculated from rotational autocorrelation functions showing good agreement with previous reported dielectric results. From mean waiting and persistence times calculated directly from trajectory results, we are able to explain which microscopic mechanisms lead to characteristic times associated with α- and β-relaxation times measured experimentally. We found that two nonequivalent groups of molecules have a longer characteristic time than the other two nonequivalent groups, both of them belonging to the asymmetric unit of the monoclinic (C2/c) lattice.

Dynamic heterogeneity in the glass-like monoclinic phases of some halogen methane compounds

In this work we study the heterogeneity of the dynamics on the low-temperature monoclinic phases of the simple molecular glassy systems CBrnCl4−n, n = 0, 1, 2. In these systems the disorder comes exclusively from reorientational jumps mainly around the C3 molecular axes. The different time scales are determined by means of the analysis of the spin-lattice relaxation time obtained through Nuclear Quadrupole Resonance (NQR) technique. Results are compared with those obtained from dielectric spectroscopy, from which two α- and β-relaxation times appear. NQR results enable us to ascribe with no doubt that the existence of two relaxations is due to dynamical heterogeneities which are the consequence of the different molecular surroundings of the molecules in the asymmetric unit cell of systems here studied.

Dynamic and static effects on 35Cl nuclear quadrupole resonance due to substitutional impurities in crystalline p-dichlorobenzene

The 35Cl nuclear quadrupole resonance (NQR) spectra of p-dichlorobenzene doped with p-bromochlorobenzene and p-dibromobenzene have been studied. The measurements were carried out as a function of the concentration up to 10% of impurities and at four temperatures in the 77 K-295 K range. The impurities diminish the integrated intensity, shift the peak of the line, and increase the linewidth. The frequency of the peak varies linearly with impurity concentration, with a slope depending upon the solute molecule and the temperature. The increment of the linewidth depends on the solute molecule, but not on the temperature. The observed changes of the 35Cl NQR signal are accounted for by considering the perturbation of the electric field gradient produced by elastic distortion of the lattice, and by variations in the electric charge distribution and phonon spectra.

Electronic population effect in halogen nuclear spin - lattice relaxation in praseodymium trihalide compounds

Nuclear quadrupole resonance determinations of the spin - lattice relaxation rates of the , and nuclei in the praseodymium trihalide crystals and are reported. Data are presented in the temperature range 7 - 297 K. They are shown to be dominated by magnetic dipole interactions between the halogen nuclear spins and the paramagnetic ions. The relaxation rates display unusual temperature dependences. This behaviour is a consequence of the crystalline electric field splitting of the ground-state multiplet of the configuration into six states, and the resultant depopulation of the ground paramagnetic state as the temperature is raised. The qualitatively different temperature variation for the two compounds is a result of the different energy splittings of the multiplet levels in the two instances. The analysis utilizes an electron-spin correlation time described by a temperature-independent term, the characteristic time for spin exchange between neighbouring ions, plus a temperature-dependent exponential term.