Silvina C. Pérez

Nuclear quadrupole spin–lattice relaxation due to molecular reorientations in crystals with orientational disorder

p-chloronitrobenzene (PCNB) and p-chlorobromobenzene (PCBB) crystallize in the centrosymmetric space group P21/c with two molecules per unit cell. The space lattice will have an equal number of points with molecules facing in opposite directions. As a consequence, these compounds exhibit an orientational rigid disorder. In this work, we have measured the temperature dependence of the chlorine nuclear quadrupole spin–lattice relaxation time (T1), linewidth, and resonance frequency for both compounds for temperatures higher than 80 K. Both compounds exhibit an inhomogeneously broadened line shape and a “normal” Bayer-type temperature dependence of the resonance frequency. The analysis focuses on the identification of the dominant relaxation process at high temperatures (T>240 K in PCNB and T>260 K in PCBB). It is shown that T1(T) reflects the existence of 180° molecular reorientations through a modulation of the crystalline contribution to the electric field gradient.

SHORT-RANGE ORIENTATIONAL CORRELATION IN THE DISORDERED CRYSTAL OF 1-CHLORO-4-IODOBENZENE

p-chlorobromobenzene (PCBB) and p-chloroiodobenzene (PCIB) crystallize in the centrosymmetric space group P21/c, Z=2. Since the molecules do not possess an inversion center, the crystals exhibit orientational disorder, in other words, the electrical dipoles of regularly distributed molecules irregularly point in opposite directions. In this study, the temperature dependence of the nuclear quadrupole resonance (NQR) line shape for PCBB and PCIB and the spin-lattice relaxation time (T1) for PCIB have been measured. Both compounds exhibit an inhomogeneously broadened spectrum, as is expected for this kind of disorder. However, the measured NQR profile strongly depends on temperature, particularly that of PCIB. To explain this behavior, each orientation of the molecular dipoles has been considered as a component of what we call a binary orientational alloy. Within this framework it is shown that the minimum free energy of the system always corresponds to a centrosymmetric array, but that short-range orientati...

Characterization of solid phases and study of transformation kinetics in m-chlorofluorobenzene by 35Cl nuclear quadrupole resonance

Polymorphism is of widespread occurrence in the world of molecular crystals. In this work we present experimental results showing the existence of four solid phases in m-chlorofluorobenzene. A glass structure is achieved by quenching the liquid phase at 77 K. This glassy state crystallizes in a disordered phase at T~143 K, which in turn transforms to the high-temperature stable phase (phase I) at T~153 K. Depending on the thermal history of the sample, a different ordered phase (phase III) can be obtained. The disorder is attributed to a molecular orientational disorder. There is no evidence of molecular reorientation in any phase. A study of the disorder-order phase transformation kinetics, using nuclear quadrupole resonance, is presented. The results are analyzed following Cahns theory. Nucleation seems to take place at grain boundaries. Growth rates for different temperatures have been determined.

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.

A new ordered phase in p-chloronitrobenzene detected by NQR studies

A study of NQR frequency and line width as a function of temperature has revealed the existence of a new ordered phase in pure p-chloronitrobenzene (PCNB). This phase is stable below K. For temperatures above the disordered phase is the stable one.

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.

Proton resonance for oscillating field of the order of static magnetic field

A spin decoupling method in nuclear quadrupole resonance spin echo experiment is used to detect the proton magnetic resonance absorption spectrum. The behavior of proton resonance in alpha phase of polycrystalline p-dichlorobenzene as a function of the intensity of the proton decoupling oscillating field (H(2)) is measured. Good agreement between the experimental resonance frequency and Shirleys theory for a non-interacting 1/2 spin system is observed. To our knowledge this is the first time the NMR proton frequency dependence on linear polarized excitation field intensity for H(2)/H(o) as high as 1.8 is measured.