Molecular dynamics in n-alkanes: premelting phenomena and rotator phases.

Molecular dynamics simulations of the n-alkanes C18H38, C19H40, and C20H42 are reported for temperatures just below the melting point. Besides thermodynamic and average structural data for the ordered phase, we discuss the molecular motions initiating the rotator phases observed in spontaneous phase transitions in isothermal, isostress simulations. The RI phase of C19H40 is initiated by particular cork-screw-like jumps combining a quarter turn about the long molecular axis and a half-chain-period translation along the axis. This motion occurs between the minimum-energy conformation of the ordered crystal and a secondary minimum. Transient analogs of the RI and RII phases of the odd alkanes are found on melting C18H38 and C20H42. Collective motions within lamellae of molecules are prominent in the dynamics.