Characteristics of methylammonium ion (CH₃NH₃⁺) in aqueous electrolyte solution: An ONIOM-XS MD simulation study

An ONIOM-XS MD simulation has been performed to characterize the CH₃NH₃⁺-water hydrogen bonds (HBs) in aqueous solution. The sphere which includes the CH₃NH₃⁺ ion and its surrounding waters was treated by the HF/DZP method, while the rest was described by classical pair potentials. The ONIOM-XS MD results clearly reveal a flexible CH₃NH₃⁺ solvation, showing various numbers of water molecules, ranging from 3 to 8 and from 12 to 19, cooperatively involved in the primary region of the –NH₃⁺ and –CH₃ species, respectively. The –NH₃⁺ group participates in about 3.6 HBs with its nearest-neighbor waters, and the HBs between the –NH₃⁺ hydrogens and their nearest-neighbor waters are relatively weaker than the HBs of bulk water. It is evident that the “hydrophobic effect” of the –CH₃ species results in slightly more attractive water-water HB interactions in this region. Such phenomenon corresponds to a clear “structure-breaking” ability of CH₃NH₃⁺ in aqueous solution.