Simultaneous Orientational and Conformational Molecular Dynamics in Solid 1,1,2-Trichloroethane

Abstract

The molecular dynamics in the ambient-pressure solid phase of (1,1,2)trichloroethane is studied by means of broadband dielectric spectroscopy and molecular dynamics simulations. The dielectric spectra of polycrystalline samples obtained by crystallization from the liquid phase exhibit, besides a space-charge relaxation associated with accumulation of charges at crystalline domain boundaries, two loss features arising from dipolar 2 molecular relaxations. The most prominent and slower of the two loss features is identified as a configurational leap of the molecules which involves a simultaneous change in spatial orientation and structural conformation, namely between two isomeric forms (gauche + and gauche -) of opposite chirality. In this peculiar dynamic process, the positions of the three chlorine atoms in the crystal lattice remain unchanged, while those of the carbon and hydrogen atoms are modified.This dynamic process is responsible for the disorder observed in an earlier x-ray diffraction study and confirmed by our simulation, which is present only at temperatures relatively close to the melting point, starting 40 K below. The onset of the disorder is visible as an anomaly in the temperature dependence of the dc conductivity of the sample at exactly the same temperature.While the slower relaxation dynamics (combined isomerization/reorientation) becomes increasingly more intense on approaching the melting point, the faster dynamics exhibits significantly lower but constant dielectric strength. Based on our molecular dynamics simulations, we assign the faster relaxation to large fluctuations of the molecular dipole moments, partly due to large-angle librations of the chloroethane species.

Structure and Dynamics of the Crystalline Stable Phase of 2-Chlorothiophene

“…We demonstrate that the out-of-plane rotations are accompanied by a small change of the molecular plane, in particular, by a change of the orientation of the C–Cl bond. The in-plane rotations proposed early by Fujimori and Oguni can only appear as short-lived molecular fluctuations undetectable by X-ray diffraction; they must therefore involve at least one nonequilibrium configuration as it was recently found in some haloethane compounds. , These results allow a microscopic identification of both the primary and secondary dielectric relaxation processes that we observe experimentally.…”

Single-Crystal X-ray Diffraction Studies of Solvated Crystals of C60 Reveal the Intermolecular Interactions between the Component Molecules

“…Disorder in the solid-state structures of various chlorocarbons (1,1,2-trichloroethane, , 1,1,2,2-tetrachloroethane, 1,2-dichloroethane) has been a topic of interest. The positioning of the relatively large chlorine atoms appears to dominate how these various molecules crystallize.…”

Polymorphism with Conformational Isomerism and Incomplete Crystallization in Solid Ethanolamine

“…The smooth temperature dependence of τ α , which contrasts with the abrupt changes displayed by both σ dc and τ BNN , rules out a possible space-charge origin and points to a dipolar origin of the α relaxation (in fact, the increment of τ α has the opposite sign than σ dc at both phase changes). Structural relaxations corresponding to molecular reorientation dynamic process are observed in liquids but also in some disordered solids, such as plastic crystals or rotator phases, orientationally disordered crystals, and Condis crystals with dynamic conformational disorder . Since the α relaxation is visible in both the metastable phase and the fully ordered stable one, it cannot be ascribed to the dynamic hydrogen-bond disorder that we have observed in the metastable phase II.…”

“…Structural relaxations corresponding to molecular reorientation dynamic process are observed in liquids but also in some disordered solids, such as plastic crystals 33 or rotator phases, 61 orientationally disordered crystals, 62 and Condis crystals with dynamic conformational disorder. 63 Since the α relaxation is visible in both the metastable phase and the fully ordered stable one, it cannot be ascribed to the dynamic H-bond disorder that we have observed in the metastable phase II. We can also rule out an origin related to the migration of H-bond defects (missing H-bonds) such as that reported in liquid water and ice, 46,51 as in the case of H2O it is observed that the relaxation frequency for this process increases by six orders of magnitude upon crossing the melting transition, 50 while in the EAM case there is barely any change in relaxation frequency upon melting (see inset to Figure 6(a)).…”