Raman spectra of n-pentane, n-hexane, and n-octadecane: experimental and density functional theory (DFT) study

We carried out a density functional theory (DFT) modeling of the stable conformations of n-pentane (C5H12), n-hexane (C6H14), and n-octadecane (C18H38) as well as their Raman spectra. The functional/basis combination OLYP/4z provides the best agreement between the calculated and experimental characteristics (the angles and bond lengths, Raman shifts, depolarization ratios, and relative Raman intensities) among the twelve approximations considered here. Using the DFT calculations, we obtained the Raman spectra of individual conformations of n-pentane and n-hexane molecules. We modeled the n-pentane and n-hexane Raman spectra by summing the spectra of individual conformations, taking into account the conformation energies and the Boltzmann distribution. Such spectra turned out to describe the experimental spectra quite well. This fact indirectly confirms that the conformational composition of n-pentane and n-hexane molecules is well described by the Boltzmann distribution. The calculated contents of n-pentane and n-hexane molecules in the all-trans-conformation were in a good agreement with the experimental values calculated by using the integral intensities of the Raman lines. We specified the experimental Raman bands belonging to the symmetric C-C stretching mode of the n-alkane molecules in accordance to our DFT calculations. In the cases of both n-pentane and n-hexane, mostly the vibrations of molecules in the all-trans-conformation contribute to this band intensity, while the contributions of molecule vibrations in all other conformations are negligible. We also defined the assignment of the Raman bands in the range 700-900 cm(-1) to the vibrations of molecules in different conformations.