MOLECULAR-DYNAMICS SIMULATIONS OF NEAT ALKANES - THE VISCOSITY DEPENDENCE OF ROTATIONAL RELAXATION

Molecular dynamics simulations of neat octane, dodecane, hexadecane, a nd eicosane at 323 K are compared with experiment and analyzed with re spect to terms that contribute to reorientational relaxation of CH vec tors. Agreement with experiment for translational diffusion constants and rotational relaxational times is excellent. It is found that gauch e-trans isomerization rates, while weakly dependent on chain position, are essentially independent of chain length and, hence, shear viscosi ty. In contrast, rotational correlation times of the symmetry. (or ''l ong'') axis of the instantaneous moment of inertia tensor increase mon otonically with chain length, and, taking the shape dependence into ac count with a hydrodynamic cylinder model, are proportional to viscosit y. Rotation about the long axis is strongly coupled with isomerization . Using an approximate separation of internal and overall motion, reor ientation of octane is shown to be largely ''rigid body''. For the cen tral carbons in eicosane flexibility contributes at most 40% to the to tal relaxation time that would be observed in, for example, NMR T-1 me asurements.