Comparison of the Molecular Dynamics of C70 in the Solid and Liquid Phases

R M Hughes,P Mutzenhardt,A A Rodriguez

doi:10.1155/2009/953198

Abstract

A previous study of C70 in deuterated benzenes generated evidence suggesting C70 exhibited unique reorientational behavior depending on its environment. We present a comparison of the dynamic behavior of this fullerene, in the solid and solution phases, to explore any unique features between these two phases. The effective correlation times, τCeff, of C70 in the solid state are 2 to 3 times longer than in solution. In the solid state, a noticeable decrease in all the carbons' correlation times is seen between 293 K to 303 K; suggesting a transition from isotropic to anisotropic reorientational behavior at this temperature change. Although C70 in solution experiences van der Waals type interactions, these interactions are not strong enough to slow the solution-state motion below what is observed in the solid state. All observed differences in the diffusion constants, DX and DZ, in solution are smaller than in the solid state suggesting a lower energy of activation between these two modes of reorientation in the liquid phase. A small-step diffusion “like” condition appears to be thermally generated in the solid phase at 323 K.

Highlights

Molecular reorientational dynamics are sensitive to several factors including temperature, pressure, free volumes, solute-solvent interactions, and, are commonly employed to probe a solute’s rotational energetics or the effects of the immediate environment on the rotational motion
Tycko and coworkers found that solid C70 molecules reorientational motion was anisotropic between 223–330 K but became isotropic at temperatures beyond 330 K [10]
In order to explore any unique features of the dynamic behavior of C70 in the solid and liquid phase, we present a comparison of our solution measurements to values obtained in the solid state

Summary

Introduction

Molecular reorientational dynamics are sensitive to several factors including temperature, pressure, free volumes, solute-solvent interactions, and, are commonly employed to probe a solute’s rotational energetics or the effects of the immediate environment on the rotational motion. Solid-state NMR measurements indicated that C70 ’s reorientational motion was anisotropic near room temperature [9]. Later differential scanning calorimetry measurements on solid C70 indicated that C70 ’s dynamic behavior dependent on the thermally stable solid-phase structure (e.g., fcc, rhomobohedral, or monoclinic phase) at a given temperature while zero-field measurements suggested C70 molecules rotated isotropically [11]. Tycko and coworkers found that solid C70 molecules reorientational motion was anisotropic between 223–330 K (monoclinic and rhombohedral phases) but became isotropic at temperatures beyond 330 K (facecentered phase) [10]. In terms of molecular dynamics in solution, our recent investigation into the molecular dynamics of C70 in several deuterated benzenes revealed evidence suggesting C70 ’s reorientational motion oscillated between anisotropic and isotropic behavior depending on the solvent and the temperature [12]. In order to explore any unique features of the dynamic behavior of C70 in the solid and liquid phase, we present a comparison of our solution measurements to values obtained in the solid state [10]

Methods

Discussion

Conclusion