Density Functional Theory Study of Degradation of Tetraalkylammonium Hydroxides

We report density functional theory (DFT) studies of the degradation mechanism of tetraalkylammonium cations which are of interest for anion exchange membrane fuel cells. Three mechanisms of attack by hydroxide anions are explored: an SN2 pathway leading to alcohol formation, an ylide pathway that gives rise to unstable intermediates, and Hofmann elimination. Tetramethylammonium, ethyltrimethylammonium, and benzyltrimethylammonium are the model cations studied here. SN2 attack on tetramethylammonium was found to have a free energy barrier of 17.0 kcal/mol at 298 K. In the case of ethyltrimethylammonium, the overall barrier for the SN2 pathway was found to be 23.0 kcal/mol while Hofmann elimination was 12.8 kcal/mol. The ylide and SN2 attacks on benzyltrimethylammonium show similar energy changes as in the case of tetramethylammonium. In the case of benzyltrimethylammonium, additional side reactions starting from the ylide intermediate are also shown to be feasible. We also discuss the influence of the immediate solvation shell on the reaction mechanism. A refined model in which the immediate solvation shell of hydroxide is modeled explicitly is found to have better experimental agreement than a model in which solvation is modeled implicitly.