Challenges in predicting Δ_rxnG in solution: The chelate effect.

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Physical Chemistry

Background:

• Predicting Gibbs energies for reactions with aqueous ions is challenging due to significant solvation energies.
• The aqueous-phase chelate effect, an entropic phenomenon in reactions with minimal enthalpy changes, serves as a rigorous test for computational models.

Purpose of the Study:

• To investigate the requirements for accurately reproducing the aqueous-phase chelate effect using ab initio methods.
• To evaluate and improve continuum solvation models (CSMs) for coordination-complex ions.

Main Methods:

• Examined the paradigmatic reaction M(NH3)4^2+ + 2 en → M(en)2^2+ + 4 NH3.
• Utilized continuum solvation models (CSMs) from Gaussian, ADF, and JDFTx software, avoiding explicit solvation simulations.
• Tested cavity size reoptimization and semicontinuum modeling approaches.

Main Results:

• Achieved a uniform accuracy of 2.2 kcal mol^-1 in the reaction Gibbs energy (ΔrxnG*) for M = {Zn, Cd, Hg}.
• This accuracy was not attainable with existing CSMs, cavity reoptimization, or semicontinuum methods.
• Post-hoc solvation energy corrections for each solute type were necessary to reach the desired accuracy.

Conclusions:

• Accurate prediction of the aqueous-phase chelate effect requires more than standard CSMs.
• Post-hoc solvation energy corrections are crucial for improving the accuracy of reaction energy predictions for coordination-complex ions.
• The findings encourage further development of advanced CSMs for complex ionic systems.