The molecular structures and the relationships between the calculated molecular and observed bulk phase properties of phosphonium-based ionic liquids

• Molecular properties of phosphonium-based ionic liquids were calculated using DFT.
• Correlations between molecular and bulk phase properties were established.
• The molar conductivity increases linearly with the dipole moment.
• The viscosity decreases exponentially with the dipole moment.
• The melting point increases linearly with interaction energy.

The molecular and electronic structures of six ionic liquids (ILs) having the same cation (tetradecyl(trihexyl)phosphonium [P66614]), but with different anions, were obtained by quantum chemical calculations using density functional theory (DFT). Various molecular parameters were computed, including the inter-ionic H-bond length and angle, the energies of various molecular orbitals including HOMO and LUMO, the dipole moment (μ), the cation–anion interaction energy (∆E) and the electrostatic potential. The wavelengths and oscillator strengths of vibrational and electronic transition lines were also calculated and found to be in good agreement with measured IR and UV–vis absorption spectra. We have found strong correlations between the calculated quantum chemical parameters and the measured physical and chemical properties of the phosphonium ILs. In general, molar conductivity (Λ) increases, whereas viscosity decreases, exponentially with μ, and the melting point increases linearly with − ∆E.