CF₃ Rotation in 3-(Trifluoromethyl)phenanthrene. X-ray Diffraction and ab Initio Electronic Structure Calculations

The molecular and crystal structure of 3-(trifluoromethyl)phenanthrene has been determined by X-ray diffraction. The structure of the isolated molecule has been calculated using electronic structure methods at the HF/3-21G, HF/6-31G*, MP2/6-31G* and B3LYP/6-31G* levels. The potential energy surfaces for the rotation of the CF₃ group in both the isolated molecule and cluster models for the crystal were computed using electronic structure methods. The barrier height for CF₃ rotation in the isolated molecule was calculated to be 0.40 kcal mol^-1 at B3LYP/6-311+G**//B3LYP/6-311+G**. The B3LYP/6-31G* calculated CF₃ rotational barrier in a 13-molecule cluster based on the X-ray data was found to be 2.6 kcal mol^-1. The latter is in excellent agreement with experimental results from the NMR relaxation experiments reported in the companion paper (Beckmann, P. A.; Rosenberg, J.; Nordstrom, K.; Mallory, C. W.; Mallory, F. B. J. Phys. Chem. A 2006, 110, 3947). The computational results on the models for the solid state suggest that the intermolecular interaction between nearest neighbor pairs of CF₃ groups in the crystal accounts for roughly 75% of the barrier to rotation in the solid state. This pair is found to undergo cooperative reorientation. We attribute the CF₃ reorientational disorder in the crystal as observed by X-ray diffraction to the presence of a pair of minima on the potential energy surface and the effects of librational motion.