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

Area of Science:

• Physical Chemistry
• Spectroscopy
• Materials Science

Background:

• Intermolecular interactions, specifically CH···HC bonds in alkyl chains, are expected to influence the phase behavior of organic compounds.
• Understanding phase transitions in organic materials is crucial for various applications, including materials science and chemical engineering.

Purpose of the Study:

• To investigate the temperature-dependent spectral changes of n-tetradecane in the far-UV region.
• To elucidate the relationship between spectral variations and the liquid-solid phase transition of n-tetradecane.
• To explore the underlying molecular structures responsible for the observed spectroscopic phenomena.

Main Methods:

• Attenuated total reflectance (ATR) spectroscopy was employed to measure far-UV (145-300 nm) spectra of n-tetradecane.
• Spectra were recorded during both cooling and heating cycles across a temperature range that included the melting point (5.9 °C).
• Time-dependent density functional theory (TD-DFT) and periodic DFT calculations were performed on model systems (n-pentane dimer, polyethylene crystal) to simulate and interpret spectral data.

Main Results:

• A distinct absorption band at 153 nm in the liquid phase diminished with decreasing temperature.
• New absorption bands emerged around 200 and 230 nm during the transition to the solid phase.
• These spectral changes were fully reversible upon heating and cooling, correlating with the melting point.

Conclusions:

• The observed reversible spectral changes are attributed to alterations in electronic states associated with the liquid-solid phase transition of n-tetradecane.
• Standard DFT calculations on optimized and crystal structures did not fully explain the experimental findings.
• The results suggest the formation of unusually compressed molecular structures on the surface at low temperatures, driving the observed spectroscopic behavior.