The low temperature phase transition in octane and its possible generalisation to other n-alkanes

A neutron powder diffraction study of three n-alkanes, octane, nonane and pentadecane, down to 2 K is presented. The temperature dependence of the octane diffraction pattern reveals a solid state phase transition between 40 and 55 K, which involves a doubling of the unit cell in the b direction, the space group remaining P1̄. Confirmation of the phase transition, which results in a doubling of the number of crystallographically inequivalent methyl groups, is sought in the published NMR, tunnelling data and neutron scattering, vibrational data. Density functional theory and force field techniques are used to simulate spectroscopic data based on the measured structures. While no unequivocal evidence is found in spectroscopic data, the published data does not rule out the existence of energetically inequivalent methyl groups. Indeed close inspection of the spectroscopic data for other n-alkanes suggests that the phase transition may be common to many alkanes. © 2001 Elsevier Science B.V