The molecular and electronic structures of six ionic liquids (ILs) having the same cation (tetradecyl(trihexyl) phosphonium [P-66614]), 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 (mu), the cation-anion interaction energy (Delta 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 (Lambda) increases, whereas viscosity decreases, exponentially with mu, and the melting point increases linearly with -Delta E. (c) 2014 Elsevier B.V. All rights reserved.