Are Explicit Solvent Models More Accurate than Implicit Solvent Models? A Case Study on the Menschutkin Reaction

Area of Science:

• Computational chemistry
• Physical chemistry

Background:

• Accurate prediction of reaction free energy barriers in solution is crucial for understanding chemical reactivity.
• Both quantum mechanical (QM) implicit solvent models and molecular mechanical (MM) explicit solvent models are employed for such predictions.

Purpose of the Study:

• To compare the performance of contemporary QM implicit solvent models (SMD, SM12, COSMO-RS) against an MM explicit solvent model in predicting the aqueous free energy barrier of a Menschutkin reaction.
• To identify the sources of error in the explicit solvent model and explore methods for accuracy improvement.

Main Methods:

• Application of QM implicit solvent models (SMD, SM12, COSMO-RS) and an MM explicit solvent model.
• Calculation of the aqueous free energy barrier for the Menschutkin reaction (NH3 + CH3Cl).
• Analysis of errors in the explicit solvent model, particularly related to Lennard-Jones parameters in free energy perturbation (FEP) calculations.
• Validation of QM interaction energies against high-level benchmark calculations (DLPNO-CCSD(T)/CBS).
• Implementation of end-state QM corrections (M06-2X/6-31+G(d,p)) to the MM explicit solvent results using FEP.

Main Results:

• Implicit solvent models (SMD, SM12, COSMO-RS) provided reasonably accurate aqueous free energy barriers, consistent with experimental data.
• The MM explicit solvent model performed poorly due to the use of fixed Lennard-Jones parameters in FEP.
• QM calculations (M06-2X/6-31+G(d,p)) showed good agreement with benchmark solute-solvent interaction energies.
• Incorporating end-state QM corrections significantly enhanced the accuracy of the explicit solvent MM results.

Conclusions:

• QM implicit solvent models are reliable for predicting aqueous reaction free energy barriers.
• Explicit solvent models can be improved by incorporating QM-based end-state corrections.
• Systematic improvement of explicit solvent model accuracy is achievable through validated QM approaches.