Structural and harmonic force fields studies of anhydrous, mono and dihydrated tetraphenylphosphonium chloride salts for ionic liquids

Structures of [PPh4]+ cation and anhydrous, mono and dihydrated tetraphenylphosphonium (TPP) chloride salts were optimized by using the B3LYP/6–311++G** calculations in gas and aqueous media to investigate their structural and vibrational properties. These species are useful as reagents in many organic synthesis reactions to obtain ionic liquids, including organometallic chemistry. Structures and the predicted IR, Raman, 1H and 13C NMR spectra are in agreement with the corresponding experimental ones. The effects of the medium on the properties have been analysed by using atomic charges, molecular electrostatic potential (MEP), natural bond orbital (NBO), atoms in molecules (AIM), and frontier orbitals. Of three different Merz-Kollman (MK), natural population atomic (NPA) and Mulliken charges analysed, only the NPA charges predict positive charges on P, as expected due to presence of cations in three salts. Higher solvation energies are predicted for the cation and the dihydrated salt while the NBO studies reveal strong hyperconjugations in the three salts supporting the higher stability of the anhydrous salt in both media than the hydrated ones. Complete assignments of 129, 132, 141 and 150 vibration modes expected respectively for cation and anhydrous, mono and dihydrated salts are reported together with the scaled force constants. Strong couplings of vibration modes of water molecules in the mono and dihydrated salts are associated to H bonds interactions, as predicted by AIM analyses. The effect of Cl and the two water molecules supports the lower reactivity of dihydrated salt and the higher scaled C-C and P-C force constants values.