Structure and Stability of Quaternary Ammonium Interhalides: Experimental and Quantum-Chemical Study

The electronic structure of a series of ammonium interhalides [R1R2R3R4N]XI2, where R1 = CH3, C2H5, C3H7, F, H; R2 = R3 = R4 = CH3, H; X = Cl, Br, I, was studied by ab initio calculations (RHF/3-21G, RHF/HW, MP2/HW). The thermodynamic stability of these compounds correlates with the strength of the hydrogen bond N-H···X and three-center interhalide bond X-I-I. Calculations confirmed that, in polar solvents, these compounds preferably decompose to [R1R2R3R4N]+ and XI2- (with subsequent decomposition of the anion), and in nonpolar solvents, to the neutral species [R1R2R3R4N]X and I2. The calculation results were compared to the experimental data obtained by single crystal X-ray diffraction, 1H NMR spectroscopy, and spectrophotometry.