The pre-melt phase of n-alkanes: Crystallographic evidence for a kinked chain structure.

Electron diffraction measurements on epitaxially grown crystals of orthorhombic n-hexatriacontane, n-C(36)H(74), give evidence for the chain-defect mechanism for linear chain melting. The derived structural model is also in accord with recent spectroscopic studies of odd-chain n-alkanes, and the diffraction data specifically exclude models based on helices or rigid chain rotors. Stability of a kinked chain structure, moreover, is indicated by an observed hysteresis effect that gives different pretransition temperatures for solution-grown and annealed crystals.