The stable polyiodides: Experimental and theoretical studies of formation mechanism

Abstract

The structures of polyiodides have been extensively studied for long time, but it was rarely involved of the mechanism of polyiodide species formation in preparation. The tetraethylammonium polyiodides were chosen as model chemicals and prepared with different molar ratios of reactants in ethanol solvent, the products were analyzed by titration, Raman spectra and TGA/DTG/DSC analysis, with the increase of the molar ratios of reactants, the iodine contents of the products reached a constant value. Finally, only two stable structures, Et4NI3 (1, space group Cmca) and Et4NI7 (2, space group Cmca), were isolated by recrystallization of slow evaporating solvent(s). The thermal analysis of tetraethylammonium polyiodides showed that all tetraethylammonium polyiodides lost I2 together to become Et4NI3 firstly and then continue to decompose. The decomposition of Et4NI3 was not a simple process which Et4NI3 lost one iodine molecule and then became Et4NI. In addition, the thermodynamical properties of species (I2 and Et4NIx, x=1, 3, 5, 7) and the molar standard Gibbs free energy changes of reactions (ΔrGmθ), as well as the mechanism of tetraethylammonium polyiodides formation were interpreted by density functional theory (DFT) calculations. The calculations show that the solvent effects have impact on the reactions. Accordingly, a comparative study was carried out on the solid phase synthesis of Et4NI5 at 80°C and at molten state. Furthermore, the solid synthesis of Et4NI5 was characterized by variable temperature (−25°C∼150°C) Raman Spectrometer. The polyiodide components interconversion showed the discrete I5- could be formed in molten state. In short, what species of polyiodides crystallized from reactions in solutions are decided by the synergy between ΔrGm of reactions and Coulombic interactions etc. of polyiodide crystals.