Predicting Solvent Effects on SN2 Reaction Rates – Comparison of QM/MM, Implicit and MM Explicit Solvent Models

Solvents are one of the key variables in the optimisation of a synthesis yield or properties of a synthesis product. In this paper, contemporary solvent models are applied to predict the rates of SN2 reactions in a range of aqueous and non-aqueous solvents. High-level CCSD(T)/CBS//M06-2X/6-31+G(d) gas phase energies were combined with solvation free energies from SMD, SM12 and ADF-COSMO-RS continuum solvent models as well as molecular mechanics (MM) explicit solvent models with different atomic charge schemes to predict the rate constants of three SN2 reactions in eight protic and aprotic solvents. It is revealed that popular solvent models struggle to predict their rate constants to within 3 log units of experimental values and deviations as large as 7.6 log units were observed. Amongst the implicit solvent models, the ADF-COSMO-RS model performed the best in predicting absolute rate constants with an average accuracy of 1.5 log units while the SM12 and CGenFF/TIP3P MM explicit solvent models were most accurate in the prediction of relative rate constants in different solvents due to systematic error cancellation. Free energy barriers obtained from umbrella sampling with explicit solvent QM/MM simulations led to excellent agreement with experiment provided that a validated level of theory is used to treat the QM region.