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.

Faculty of Science Charles University Prague Hlavova 2030 12840 Prague Czech Republic

Institute of Chemical Process Fundamentals Czech Academy of Sciences Rozvojová 135 16502 Prague Czech Republic

Institute of Organic Chemistry and Biochemistry Czech Academy of Sciences Flemingovo 2 16610 Prague Czech Republic

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