X-ray diffraction study of some liquid alkanes

The structure of the liquids n-heptane, n-decane, n-pentadecane, and n-eicosane was investigated using x-ray diffraction. The data are analyzed using group scattering factors for -CH2 and -CH3 sites which permits extraction of the carbon–carbon structure and distribution functions. Analysis has yielded the short-range intramolecular distances and bond angles, the average torsional angles for the gauche and trans rotational isomer segments, and the gauche/trans populations. Intramolecular carbon–carbon distances at 3.11±0.03 Å and 3.94 ±0.01 Å are assigned to gauche and trans molecular segments, respectively. The average torsion angle of the gauche segments is near 64±2°, and the trans segments appear to be twisted from a planar conformation by 10±2°. The average fraction of gauche segments in the liquid phase of these alkanes is approximately 45±8%, slightly higher than in the gas phase. The conformational equilibrium of the n-alkanes is only minimally perturbed by the intermolecular interactions in the liquid phase, and the results are inconsistent with the possibility of a large fraction of extended chains (or chain segments) in the liquid.