DYNAMICS OF ACETONITRILE CRYSTALS AND CLUSTERS

Three crystalline phases of acetonitrile have been investigated by high-resolution neutron inelastic scattering spectroscopy for 20 < T/K < 300 over the energy-transfer range 1 < DELTA-E/cm-1 < 4000. Clear differences are seen between the phonon densities of states for the alpha and beta phase crystals at 20 K as well as between each of these and the approximately five unit cell clusters of acetonitrile encaged in lanthanum-exchanged zeolite Y. For the intramolecular region corresponding frequencies are found in all phases. Using the excitation energies and the intensities of scattering the most consistent intramolecular force field is defined and the coupling between internal and lattice modes, as represented by combination bands, studied over a wide frequency domain for the alpha and beta phases. Experimental and calculated neutron scattering intensities agree well for the two bulk solid phases and are consistent with strongly bound acetonitrile dimers as a persistent dynamical unit, an aspect which also strongly determines the rotational dynamics. The low-energy maximum in the density of vibrational states for clusters is strongly shifted to low energies, indicating an appreciable effect of the cluster size on the rotational dynamics.