Quasi-elastic neutron scattering of cyanobiphenyl compounds with different terminal chains

Quasi-elastic neutron scattering measurements were performed for five cyanobiphenyl compounds with different terminal chains (5CB, 5*CB, 8OCB, 8*OCB and 6O2OCB) in their liquid and liquid-crystalline phases to investigate the molecular dynamics. The spectra were successfully decomposed into two components, indicating that two types of molecular motions are involved. For the broad (or fast) component, the temperature dependence as well as the momentum transfer dependence did not differ very much among those compounds. For the narrow (or slow) component, on the other hand, the compounds with a branched alkyl-chain (5*CB and 8*OCB) gave significantly smaller diffusion constants than the others. A molecular dynamics simulation for 5*CB and 5CB suggests that the reorientational motion of the whole molecule around the long molecular axis is responsible for the broad component, whereas a diffusive motion of the terminal chain is responsible for the narrow one.