Department of Physics, Bryn Mawr College
In the Physics Department at Bryn Mawr College we study intramolecular rotation of methyl (CH3) and fluoromethyl (CF3) groups in organic van der Waals solids. We perform variable-temperature (77–370 K) and variable-frequency (8.50, 22.5, and 53.0 MHz) solid state 1H (proton) and 19F (fluorine) nuclear magnetic resonance (NMR) spin-lattice relaxation experiments to investigate intramolecular motion in these systems. When both F and H atoms are present in a compound, we can exploit the complex interactions between unlike spin-½ particles using both 1H and 19F NMR relaxation. We also study the complex, geared, and superimposed motions of tertiarybutyl groups [C(CH3)3] and their constituent CH3 groups. The distinguishing feature of the experiments is the very low NMR frequencies needed in the nuclear spin relaxation experiments to investigate the motions. The goal is to develop complete and consistent molecule-independent models for the intramolecular motions in these systems and determine how these motions relate to the molecular and crystal structure. Our work is complemented by the work of others doing compound synthesis and purification, electronic structure calculations, field emission scanning electron microscopy, single-crystal X-ray diffraction, powder x-ray diffraction, differential scanning calorimetry, and high-resolution 1H and 19F NMR spectroscopy.