Molecular interaction and optical properties of the acetic acid–benzene system: dielectric spectroscopy and first-principles study

Dielectric relaxation spectroscopy is an effective approach for investigating molecular dynamics and intermolecular interactions in hydrogen-bonded liquids. The present work aims to elucidate the mechanisms of dielectric relaxation and microstructural organization in acetic acid–benzene solutions through a combined experimental and theoretical approach. The dielectric properties were investigated by dielectric spectroscopy over the wavelength range of 0.75–40 cm at temperatures from +20 to +50 °C, while temperature-dependent measurements were performed at a frequency of 1.6 MHz over the range from −80 to +50 °C. To interpret the experimental observations, first-principles density functional theory (DFT) calculations were carried out to investigate the structural, electronic, and optical properties of the acetic acid–benzene complex. The results show that dielectric dispersion begins at wavelengths below 50 cm and that the relaxation behavior is most accurately described by a three-process relaxation model associated with different types of molecular associates. Dilution with benzene disrupts extended hydrogen-bonded structures and promotes the formation of specific O–H···π intermolecular complexes, leading to a redistribution of the individual relaxation contributions and corresponding changes in the characteristic relaxation times. Unlike previously reported binary organic systems, the present study demonstrates that the dielectric relaxation behavior of the acetic acid–benzene system is governed by the competition among acetic acid dimerization, hydrogen-bonded associates, and specific O–H···π intermolecular interactions, giving rise to three distinct relaxation processes. DFT calculations confirmed the weak noncovalent nature of the intermolecular interaction, and revealed pronounced anisotropy in the electronic and optical response of the complex. The calculated HOMO–LUMO energy gap, derived from the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), is approximately 4.96 eV, indicating that the system is a wide-gap molecular system with limited low-energy electronic excitations. The combined experimental and first-principles approach establishes direct correlations among molecular structure, dielectric relaxation processes, and optical properties, thereby providing a molecular-level interpretation of the observed relaxation behavior and extending the current understanding of intermolecular interaction mechanisms in associated liquid systems.