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

- An ONIOM-XS MD simulation of CH3NH3+ in water has been performed.
- The NH3+ group participates in about 3.6 HBs with its nearest-neighbor waters.
- The HBs between the NH3+ hydrogens and their nearest-neighbor waters are relatively weaker than those of bulk water.
- The “hydrophobic effect” of the CH3 species results in slightly more attractive water-water HBs in this region.
- The ONIOM-XS MD results reveal a clear “structure-breaking” ability of CH3NH3+ in aqueous solution.

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.

Selected geometrical arrangements of the CH3NH3+-water complexes, showing the HB formations between the NH3+ hydrogens and their nearest-neighbor waters, as well as the HBs among the first-shell waters and between the first-shell waters and the bulk.76