Computational Study of Ion-Molecule Interactions

The interaction of ions with neutral molecules was studied by computational methods in this project. Two types of systems were studied; the first one being triethyloxonium\ud ion with water molecules and the second one is N-methylacetamide (NMA) with alkyl ammonium ions and carboxylate ions in the presence of water molecule. It was found that the interaction energy of first two water molecules to oxonium ion Et3O+ is mainly contributed by hydrophobic interactions. With the addition of more water\ud molecules, the interaction energy was found to be increasing and it comes from the stability gained due to H-bonds among water molecules. The lowest energy isomers\ud of the clusters for 3, 4 and 5 water molecules, are found to have cyclic structures of water molecules. The cyclic water tetramer also stabilizes the clusters with more\ud water molecules, indicating that it is the most stable cyclic structure of water molecules. Et3O+ ion is found to be stabilized in basic medium, and got destabilized in\ud acidic medium. When compared for the hydrophobic nature, Et3O+ is found to be slightly more hydrophobic than its analogous carbon counterpart Et3C+. NMA, being amide, has nucleophilic as well as electrophilic centre. So, it can\ud interact with charged and neutral chemical entities. Based on the interactions of NMA with protonated amines, carboxylates and water molecules, it was found that NMA\ud can interact more with electrophiles than nucleophiles. Transfer of two protons took place, when protonated amine and carboxylate interacted with NMA simultaneously.\ud One proton transfer took place from protonated amine to NMA, and second one from NMA to carboxylate ion. When these cations and anions were made to interact separately with NMA in the presence of water molecule, the cations and anions didn’t undergo proton transfer. This finding can be exploited in stabilizing the peptides as well as proteins by using suitable ion- pairs in aqueous solution