Structure and lattice dynamics of dipolarly disordered 2,3-dimethylanthracene crystals

2,3-dimethylanthracene (2,3-DMA, C16H14) crystallizes in a pseudocentrosymmetric triclinic lattice with two molecules per unit cell showing dipolar structural disorder of the acentric molecules. Using powder samples the amplitude weighted phonon density of states was measured by inelastic incoherent neutron scattering for different degrees of deuteration. The differences in the resulting spectra can be explained by changes of the dynamics of the molecular bodies or the methyl side groups. By the method of inelastic coherent neutron scattering phonon dispersion curves were determined. The phonon lines are broad even at low temperatures; this fact is supposed to be related to dipolar disorder. Both the incoherent and coherent scattering results can be described satisfactorily by lattice dynamical calculations using a “6-exp” potential for the atom–atom interactions of different molecules, including internal degrees of freedom.