Dipolar interaction and molecular ordering in liquid propylene carbonate: Anomalous dielectric susceptibility and Raman non-coincidence effect

Abstract

Dielectric constant of non-associating fluids normally exhibits a 1/ T temperature dependence according to Curie's law. Kirkwood correlation factor in such cases is close to unity and almost independent of the temperature. Recent broadband dielectric studies of a well-known solvent propylene carbonate (PC) revealed anomalous behavior of its loss spectrum, whose amplitude somewhat increases at high temperatures contrary to Curie's law, while it is known from the literature that its Kirkwood factor is close to unity. We analyze the origin of this discrepancy and find, based on recent literature data, that the Kirkwood factor of PC is strongly temperature dependent and significantly differs from unity, which signals the presence of temperature dependent molecular ordering. We find other signatures of this ordering in vibrational spectra of PC, where it leads to splitting of molecular vibrations due to interaction between the transition dipoles of neighboring molecules (the so-called non-coincidence effect). Spectroscopic and dielectric results agree well with each other and reveal the presence of significant temperature-dependent anti-parallel dipolar arrangement. Based on these results, it is likely that other polar liquids with large and localized molecular dipoles, albeit traditionally classified as non-associating, in fact do possess significant local molecular order due to dipolar interactions.