Influence of methyl-substitution on the dynamics of the C–H⋯F–C interaction in binary adducts

Abstract

Variable-temperature single-crystal and powder X-ray diffraction (PXRD) has been used to probe the structure and dynamics of two solid adducts, viz. toluene : hexafluorobenzene (C₆H₅CH₃ : C₆F₆) and p-xylene (1,4-dimethyl benzene) : hexafluorobenzene (p-C₆H₄(CH₃)₂ : C₆F₆). A combination of PXRD patterns and differential scanning calorimetry (DSC) traces of the solid adducts reveal two solid-state phase-transitions in each adduct: at 200 K and 246 K in C₆H₅CH₃ : C₆F₆ and at 230 K and 297 K in p-C₆H₄(CH₃)₂ : C₆F₆. The crystal structures of all three solid phases in each adduct have been solved by single-crystal X-ray diffraction (SXD). The structures of both adducts are based on close-packed parallel columns of alternating methyl benzenes and hexafluorobenzene molecules packed face to face. The fundamental difference between the two adducts involves the presence or absence of rotational disorder of the methyl-substituted benzene rings. Whereas in p-xylene the presence of two methyl groups locks the orientation of the molecule with respect to the column axis in all phases, the single methyl group of toluene fails to define a unique orientation except in the lowest temperature phase III. At higher temperatures, C₆H₅CH₃ : C₆F₆ transforms to an adduct with, initially, two-fold disorder (in phase II) and then dynamic six-fold disorder that dominates the highest temperature phase (phase I). Differences in structure between the phases as a function of temperature illustrate the subtle interplay of quadrupole versus bond–dipole electrostatic interactions.