A study of the disordered low-temperature structure of acenaphthylene, C12H8, using semi-empirical potential-energy calculations

Abstract

Semi-empirical potential-energy calculations have been used to investigate the relative stability of twelve different possible ordered structures for acenaphthylene, C₁₂H₈, at low temperatures. The twelve structures considered represent all the different combinations of the various molecular orientations, in three crystallographically inequivalent sites, revealed by the neutron diffraction study of the structure at 80 K. Of the twelve structures investigated, ten were found to have total potential energies within ca. 2.5 kJ mol^–1 of each other, thus providing strong support for the belief that the room-temperature structure is very disordered with many different molecular orientations present. The probability of the occurrence of each of the twelve structures, calculated according to a Boltzmann partition of the energy at 80 K, yields values for the site occupancies of individual molecular sites in good agreement with values obtained from the neutron diffraction experiment. Substantially the same results are obtained with the two different potentials used for comparison.