TG-FTIR, DSC, and quantum-chemical studies on the thermal decomposition of quaternary ethylammonium halides

The thermal decomposition of quaternary ethylammonium chloride, bromide, and iodide has been studied using the experimental techniques of thermal gravimetry coupled to Fourier transform infrared spectroscopy (TG-FTIR) and differential scanning calorimetry (DSC) as well as the density functional theory (DFT) and MP2 quantum-chemical methods. These compounds decompose in a one-step process, and the almost perfect agreement between the experimental IR spectra and those predicted at the B3LYP/6-311G(d,p) level demonstrates for the first time that decomposition produces an equimolar mixture of triethylamine and a haloethane. The respective experimental enthalpies of dissociation of the chloride, bromide, and iodide are 158, 181, and 195 kJ/mol. These values correlate well with the calculated enthalpies of dissociation based on crystal lattice energies and quantum-chemical thermodynamic barriers. The experimental activation barriers were derived from the least-squares fit of the F1 kinetic model (first-order process) to thermogravimetric traces. These estimates are 184, 286, and 387 kJ/mol for chloride, bromide, and iodide, respectively, and agree well with the theoretical calculations. It has been demonstrated that the theoretical approach assumed in this work is capable of predicting the relevant characteristics of the thermal decomposition of solids with experimental accuracy. DFT methodology is recommended for the quantum-chemical part of the model: B3LYP for evaluating the thermodynamic barriers and MPW1K for assessing the activation characteristics. These quantum-chemical data then have to be combined with crystal lattice energies. The latter should be calculated using both electrostatic and repulsion-dispersion terms.