For the faster 2-fold motion, its rotational frequency (krot1) increases from 0.8 MHz at 233K to 8 MHz at 333K. At the same time, the frequency of the slower 4-fold motion (krot2) is only 70 kHz at 333K, while it falls below 1 kHz at 273K. The rotation frequencies of both types of motions in 1-R-d20 fit well on Arrhenius and Eyring plots, allowing for the evaluation of their thermodynamic parameters (Figures ). The activation energy (Ea) of the fast 2-fold flipping was found to be 3.5 kcal•mol -1 , while the pre-exponential factor (A) was 1.27 × 10 9 s -1 . Meanwhile, the Ea of the slower 4-fold motion was 10.8 kcal•mol -1 , with the A being 9.81 × 10 11 s -1 . Utilizing the Eyring plot allowed us to determine the entropy of activation (ΔS ‡ ) of rotational motions, which are negative in both cases: -18.7 cal•mol -1 •K -1 (rotation 1) and -5.7 cal•mol -1 •K -1 (rotation 2). The latter indicates the possibility of correlated rotational motion of the phenyl moieties in the crystal structure, which is also highly probable due to their "shaft" arrangement in the crystal (Figure ).