Rotational isomerism and crystal structures of 9,9′-dicyano-9,9′-bifluorenyl and 9,9′-dinitro-9,9′-bifluorenyl

Dipole moments of the compounds 9,9'-dicyano-9,9'-bifluorenyl 1 and 9,9'-dinitro-9,9'-bifluorenyl 2 in 1,4-dioxane and benzene have been measured over a range of temperatures. The crystal and molecular structures of the compounds were determined by single-crystal X-ray diffraction methods. Analyses of the crystal structures and relative permittivity data show that the compounds exist in the polar gauche conformation in the, solid state and as rotameric mixtures in solution with a predominantly high gauche population of 94% and 99% respectively at 25 degrees C. The experimentally derived values of the energy difference between the gauche and trans rotamers and the gauche/trans population quotients were compared with the values predicted by the semiempirical MO programme AMPAC using AM1 parametrization. Dynamic H-1 NMR was employed to determine the activation parameters for the barriers to rotation in solution.