Changes in the Electronic States of Low-Temperature Solid n-Tetradecane: Decrease in the HOMO-LUMO Gap

Intermolecular interactions between alkyl chains such as CH···HC should be reflected in the phase behavior of organic compounds. We measured the attenuated total reflectance spectra in the far-UV region (145-300 nm) of n-tetradecane (T_m = 5.9 °C), through both cooling and heating, from 15 to -38 °C, to determine its temperature dependency. The chosen temperature range, with the minimum temperature much lower than the melting point of n-tetradecane, allowed for a detailed examination of the liquid-solid phase transition. With decreasing temperature, the absorption band at 153 nm in the liquid phase becomes weaker, and new bands appear at around 200 and 230 nm. Because the observed variations in the band intensities are reversible during both cooling and heating processes and the conversion from the 153 nm band to 200 and 230 nm bands takes place at the melting temperature, we concluded that these spectral changes are caused by changes in the electronic states because of the phase change. To understand the origin of this drastic change observed experimentally, we carried out time-dependent density functional theory calculations using a model dimer structure of n-pentane and periodic density functional theory calculations of the polyethylene crystal structure. Although the simulated absorption spectra of both the optimized and crystal structures did not explain the changes observed experimentally in the absorption spectra, compressed structures were found to be a potential explanation for the experimental findings. These results suggest that an unusually compressed structure might be generated on the surface at low temperatures, and this phase change, which is reversible, is responsible for the unusual absorption observed in the attenuated total reflection-far ultraviolet spectra.