Internal dynamics in helical molecules studied by X-ray diffraction, NMR spectroscopy and DFT calculations

The conformational behaviour of a prototype helical molecule, [6] helicene, and its derivatives was studied in solution and the solid state. Available crystal structures of [6] helicene revealed surprisingly large flexibility of this molecule and variable-temperature NMR experiments provided unusual temperature dependence of chemical shifts of hydrogen, carbon and fluorine atoms in the peripheral aromatic rings of [6] helicene and tetrafluoro[6] helicene. These chemical shift changes were interpreted as a consequence of the helicene 'pitch' opening with elevated temperature, and the experimental data were corroborated by DFT calculations of the chemical shift dependence on the helicene conformation and by variable-temperature DFT molecular dynamics simulations.