Molecular motion of n-alkane chains in urea inclusion adducts studied by analysis of Raman band profiles

Abstract

A quantitative analysis of the temperature dependence of the motional broadening of the Raman bands associated with the guest alkane molecules accommodated in the hexagonal phase of urea inclusion adducts of even-numbered n-alkanes of n–C14H30 through n–C22H46 was performed according to the site-hopping theory. The profiles of a certain polarization component of the following five bands; the CH2 antisymmetric stretch νa(CH2) (XY), the CH2 scissoring δ(CH2)(ZZ) and the CH2 twist t(CH2)(XZ), and the symmetric and antisymmetric CC stretches νs(CC)(ZZ) and νa(CC)(XZ)[Z∥c,X,Y⊥c], were found to be reproducible by a single Lorentzian function in the whole temperature range covering both the orthorhombic and hexagonal phases. Difference in the broadening behavior among the five Raman bands was interpreted quantitatively in terms of the equation derived previously by the authors. For each adduct, the value of the potential barrier height E* to the rotational motion of the alkane molecules was obtained in a good constancy from the temperature dependence of the half-width of the bands which belonged to different symmetry species and exhibited different broadening behavior. The value of E* for the C-16 adduct agreed well with the barrier height calculated on the basis of van der Waals intermolecular potential functions and the crystal structure. The E* value was found to increase monotonically with an increase in the chain length of the guest n-alkane molecule, except for an anomalous increase at C-20, as has been observed in the chain-length dependence of the transition temperature between the orthorhomic and hexagonal phases. The end-gauche content of the guest molecules in the hexagonal phase was evaluated as about 5 mol% irrespective of the chain length. For the alkane vibrations, bandshifts with variation in temperature were measured and analyzed according to the libration–torsion theory presented by Wood et al.