The vibrational dynamics of n-alkanes in the terahertz region
Abstract
Much attention has been paid to the studies on the dynamics of n-alkanes, which is also the key physical problems exist in the rapid identification of crude oil or the database establishment for hundreds of petroleum products. In this paper, the optical properties of C5-C10, C18, C22 and C32 n-alkanes were investigated using terahertz time-domain spectroscopy. The refractive index of C5-C10 n-alkanes is found to vary linearly with chain length. The refractive index of C18 n-alkane encountered inflection point, and then vary linearly with chain length in the range of C18-C32. The absorption coefficients of n-alkanes has no significant direct relationship with carbon numbers and surface topography. In order to further understand the absorption spectra, combined with density functional theory, the vibrational spectroscopy of C5-C10 and C18 n-alkanes were calculated in the terahertz region using GAUSSIAN 03 software package. The B3LYP method was used in conjunction with the 6-31 G basis set. The vibration of C-C chain skeleton is the main absorption model of n-alkanes in the terahertz range. The vibrational frequency of C-C chain skeleton is moved toward the low frequency when the carbon numbers increased. The results provided a theoretical basis for terahertz analysis of petroleum and organic compounds, and indicated that terahertz technology can be used for the physical properties prediction of n-alkanes.
