Effects of interlamellar forces on longitudinal acoustic modes of n-alkanes

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Abstract
Effects of the interlamellar interactions in n‐alkane crystals on the longitudinal acoustic modes (LAM), including both the fundamental and the higher overtones, were investigated on the basis of the polarized Raman spectra of n‐C2 8H5 8, n‐C3 0H6 2, n‐C3 4H7 0, and n‐C3 6H7 4 crystallized in the orthorhombic II form (a double layer polytype of the monoclinic form). The LAM‐1 bands of this crystal modification split into a doublet consisting of the (cc) and (bc) polarization components separated by about 6 cm− 1. The frequency gaps of the LAM‐m bands were found to decrease with m (approximately proportional to m − 1). Theoretical considerations on the correlation field splitting and the scattering intensities of the LAM bands led to the conclusion that the observed band splitting was caused by the intermolecular force acting between the successive monomolecular layers. The intrinsic value of the Young modulus of the n‐alkane molecules free from the external forces in the crystalline state and the strength of the interlayer force were evaluated from the observed frequencies of the two components of LAM using a simple dynamical model. By comparing the Raman spectra of n‐C2 8H5 8 and n‐C3 0H6 2 between the triclinic and orthohombic II modifications, it was concluded that the effect of the lateral force on the LAM was not so significant as that of the longitudinal force.