Shapeshifting Nucleophiles HO^-(NH₃) React with Methyl Chloride.

Area of Science:

• Computational Chemistry
• Physical Chemistry
• Reaction Dynamics

Background:

• Microsolvation significantly influences chemical reactivity, particularly for nucleophilic substitution (SN2) reactions.
• Understanding how solvent molecules interact with nucleophiles is crucial for predicting reaction outcomes and kinetics.
• Previous studies often focused on bulk water solvation, leaving microsolvation effects less explored.

Purpose of the Study:

• To investigate the impact of microsolvation on SN2 reactions involving shapeshifting nucleophiles.
• To elucidate the mechanisms by which solvation affects reaction barriers and nucleophile stability.
• To establish correlations between nucleophile properties (HOMO level, binding energy) and SN2 reaction barriers.

Main Methods:

• High-level ab initio calculations, specifically CCSD(T), were employed to map potential energy surfaces.
• The study examined the ion-molecule SN2 reaction between chloromethane (CH3Cl) and microsolvated hydroxide (HO-) and amide (NH2-) nucleophiles.
• Incremental solvation models were used to systematically assess the effect of ammonia (NH3) and water (H2O) molecules.

Main Results:

• Microsolvated HO-(NH3)n can transform into NH2-(H2O)(NH3)m via intramolecular proton transfer, leading to dual reaction pathways.
• Calculations revealed that HO- species are energetically favored over NH2- species, with lower SN2 reaction barriers.
• Increasing solvation generally raised SN2 reaction barriers for both pathways, with water exhibiting stronger stabilization than ammonia.

Conclusions:

• The study confirms the existence of the α-effect under microsolvation conditions.
• Nucleophile stabilization by solvent molecules, reflected in HOMO levels and binding energies, directly correlates with SN2 reaction barrier heights.
• This research extends the understanding of microsolvation effects on SN2 reactions beyond aqueous environments.