Can We Unambiguously Define the Dipole Moment of Molecules in the Condensed Phase?

Various theoretical methods were applied and evaluated to determine the dipole moment of polar protic (methanol, ethanol) and aprotic (acetonitrile, pyridine, acetone) dipoles in the crystal phase. In mono-alcohols, the dipole moment is influenced by the hydrogen bonding (H-bonding) environment, similarly to earlier findings with liquid water. Using localization techniques without considering the effect of neighboring molecules gives similar results for the dipole moment of mono-alcohols than those obtained from the polarized continuum model (PCM). However, the PCM for polar aprotic molecules provides significantly different dipole moment values compared to localization methods. Our results clearly show that the magnitude of the dipole moment in the condensed phase cannot be unambiguously determined.