Raman spectra of n-alkane crystals: Lattice vibrations of n-hexane, n-heptane and n-octane

Abstract

Raman spectra have been obtained for polycrystalline samples of the normal alkanes hexane, heptane and octane in the temperature range 20 K to the melting points. In the case of the isomorphous n-hexane and n-octane crystals the three theoretically predicted torsional lattice frequencies have been observed as well as several low-lying internal molecular vibrational frequencies. In both crystals the lowest lattice frequency (53 and 47 cm −1, respectively at 20 K) increases in frequency and rapidly broadens with temperature, and is unobservable at higher temperatures. For n-heptane a larger number of lattice frequencies are observed in addition to several internal molecular frequencies below 400 cm −1. Crystal structures, energies and lattice vibration frequencies for n-hexane and n-octane are calculated using an atom—atom intermolecular potential, and the mixing of internal and lattice vibrational modes due to the crystal potential is investigated. It is concluded that the atom—atom potential model is able to explain the lattice dynamic properties with a reasonable quantitative accuracy. Dispersion curves for the lattice frequencies are calculated for n-hexane.