Molecular Rotational Potentials in Solid Halogenated Benzenes: C6Cl6, sym-C6Cl3F3, and C6F6

Abstract

Abstract In a previous paper which examined the molecular rotation in sym-C6Cl3F3 crystal by the 35Cl NQR an unrealistically long activation parameter, τ0, for the molecular rotation was obtained by assuming the three-fold molecular uniaxial reorientation. In order to examine the reason for such unacceptable result the relaxation time (T1) data were reanalyzed with the help of the computer experiment which calculates the rotational potential function for each molecule in the crystal lattice. The computation was carried out using the 6-exponential type interatomic pair potential functions. It was found that the potential for the plane molecular rotation in the hexagonal unit cell is non-sinusoidal and has, in addition to the most stable orientation, an extra minimum where the molecules can stay for a finite time at moderately high temperatures. The rotational potential thus obtained was used successfully to reanalyze our previous T1 data for this material. 35Cl NQR T1’s were measured for C6Cl6 crystal and analyzed using anisotropic rotational potentials. The analysis predicted possible disordered structure of this material. In the case of C6F6 it was confirmed that the in-plane rotation of two crystallographically inequivalent molecules can be excited with different activation energies from each other, being consistent with the previous fluorine NMR work.