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Abstract: 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 77K. Slow cooling allow to obtain the low temperature phase III which, on heating, transforms to phase II at 183K and this, in turns, transforms to phase I at T~210K. 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. [Copyright &y& Elsevier]

Abstract: 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 77K. This glassy state crystallizes in a disordered phase at T∼143K, which in turn transforms to the high-temperature stable phase (phase I) at T∼153K. 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 Cahn''s theory. Nucleation seems to take place at grain boundaries. Growth rates for different temperatures have been determined. [Copyright &y& Elsevier]

Abstract: 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 α 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 Shirley’s 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. [Copyright &y& Elsevier]