Influence of Hydrogen/Fluorine Substitution on Structure, Thermal Phase Transitions, and Internal Molecular Motion of Aromatic Residues in the Crystal Lattice of Steroidal Rotors

Abstract

Two, acyclic (1) and cyclic (2), steroidal molecular rotors containing 1,4-diethynyl-2,3-difluoro-phenylene units as rotators were investigated by means of single crystal X-ray diffraction, high resolution solid state NMR spectroscopy, and computer methods. The aim of this study was to understand and search for a correlation between the size of difluoro-phenylene units and free space in the crystal lattice required for molecular reorientation as well as the topology and time scale of dynamic processes. As a primary tool for analysis of molecular motions in the solid state, 1H–13C PISEMA, a technique which allows following the dynamics in the range of 10–3–10–6 s, was employed. The PISEMA data defining the 1H–13C dipolar couplings, whose values are sensitive to local motion, were confronted with 13C CSA parameters. Our studies revealed that replacing hydrogen by fluorine in acyclic rotors has significant consequences for dynamic processes. In the case of hydrogen-substituted species, free rotation around the 1–4 axis of the benzene ring was proven. For fluorine derivatives 1, only small amplitude wobbling of aromatic residues was observed. The only large amplitude reorientation, a so-called π-jump around the 1–4 axis, was observed during the phase transition related with solvent migration from the crystal lattice. For cyclic rotors (2) two crystallographic forms 2A (triclinic, P1 space group) and 2B (monoclinic, P21 space group) are established. The form 2B containing a heptane molecule in the crystal lattice undergoes a thermal transition with large amplitude motion of building units of the steroidal frame. The high dynamics of the fluorinated rotator for 2A is proven.