Microhydration effects on a model S(N)2 reaction in a nonpolar solvent

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Chemical Kinetics

Background:

• Nucleophilic substitution reactions (S(N)2) are fundamental in chemistry.
• Solvent effects play a crucial role in reaction mechanisms and rates.
• Understanding microhydration is key to accurately modeling solution-phase reactions.

Purpose of the Study:

• To investigate the impact of microhydration on the S(N)2 reaction Cl(-) + CH(3)Cl in chloroform.
• To quantify the effect of 1-5 water molecules on the reaction-free energy profile and rate constant.
• To analyze the influence of hydration on electronic structure and transition state stabilization.

Main Methods:

• Utilized a recently developed empirical valence bond (EVB) model.
• Simulated the S(N)2 reaction in liquid chloroform.
• Examined systems with varying numbers of hydrating water molecules (1-5).

Main Results:

• Activation-free energy increased significantly with the addition of water molecules.
• The first water molecule raised the barrier by ~4 kcal/mol.
• Five water molecules resulted in a barrier height comparable to that in bulk water, indicating strong hydration effects.

Conclusions:

• Microhydration has a substantial and sensitive effect on S(N)2 reaction barriers.
• The transition state stabilization and electronic structure changes are strongly dependent on the number of hydrating water molecules.
• Deviation from transition state theory due to barrier recrossing was intermediate between bulk water and bulk chloroform behavior.