(360c) The Role of Heteroatom Placement and Chain Branching on Quaternary Ammonium Salts for Phase Transfer Catalysis

Phase transfer catalysts (PTC) facilitate the movement of a reactant from one phase to another phase where the reaction occurs. These can reduce the need for volatile solvents such as DMSO or DMF for alkylation, cyanation, and oxidation reactions, and can also assist in the recovery of rare earth metals from electronic waste. Quaternary ammonium (quat) salts are frequently used because of their pervasiveness and the benefits that come with tuneability. Previous computational investigations on the relation between the structure and the action of quat salts as PTCs have focused on the role of alkyl chain length, anion strength, and the formation of water strands that develop during phase transfer. To date, there is still not a full molecular-level understanding of how heteroatom placement and chain branching effects the strength of ion pairing and the transfer between phases. Knowing how processes are affected by these structural properties will allow for the design of PTCs that target specific reactions and separations. In this work, molecular dynamics simulations (MD) are used in conjunction with umbrella sampling to determine the potential of mean force (PMF) associated with ion paring and the transfer between phases. Chain length, branching, and heteroatom placement are systematically varied and the PMF is determined. For several of the structures we also find the development of fine water filaments that develop as well as hysteresis that occurs during phase transfer. Further, we find that heteroatom placement and to a lesser extent branching have a significant effect on ion pairing and transfer, which are further discussed.