Crystal structure, chemical bonds nature and thermodynamic functions for the new ionic liquid pyridinium bis(dihydrogen phosphate)

Abstract A crystal structure (confirmed via single X-ray diffraction analysis) of the new ionic liquid pyridinium bis(dihydrogen phosphate) ([H–Pyr]+([H2PO4]–)2) is presented in this work for the first time. Optimized molecular geometry of [H–Pyr]+([H2PO4]–)2 is determined by means of density functional theory (DFT) at B3LYP/6-311++G(2d,2p) level and a good correlation between experimental and calculated structural parameters (bond lengths and angles) is revealed. Different types of intramolecular hydrogen bonds between [H–Pyr]+ and [H2PO4]– fragments are found to stabilize the title compound structure. Moreover, a hydrogen bond between the dihydrogen phosphate anions is also detected. The Quantum theory of atoms in molecules and natural bond orbital analysis have been performed to elucidate the chemical bonds nature and electron density delocalization, respectively in [H–Pyr]+([H2PO4]–)2. The standard thermodynamic functions specific heat capacity ( C p , m ∘ ), enthalpy and entropy for the title compound are calculated. Polynomial equations for solid and liquid phases describe the temperature dependence of C p , m ∘ for [H–Pyr]+([H2PO4]–)2 sample. A relationship between the internal hydrogen bonding, chemical bonds nature and the standard thermodynamic functions is presented.