Recent developments and applications of reference interaction site model self-consistent field with constrained spatial electron density (RISM-SCF-cSED): A hybrid model of quantum chemistry and integral equation theory of molecular liquids

The significance of solvent effects in electronic structure calculations has long been noted, and various methods have been developed to consider this effect. The reference interaction site model self-consistent field with constrained spatial electron density (RISM-SCF-cSED) is a hybrid model that combines the integral equation theory of molecular liquids with quantum chemistry. This method can consider the statistically convergent solvent distribution at a significantly lower cost than molecular dynamics simulations. Because the RISM theory explicitly considers the solvent structure, it performs well for systems where hydrogen bonds are formed between the solute and solvent molecules, which is a challenge for continuum solvent models. Taking advantage of being founded on the variational principle, theoretical developments have been made in calculating various properties and incorporating electron correlation effects. In this review, we organize the theoretical aspects of RISM-SCF-cSED and its distinctions from other hybrid methods involving integral equation theories. Furthermore, we carefully present its progress in terms of theoretical developments and recent applications.

Tackling Solvent Effects by Coupling Electronic and Molecular Density Functional Theory

Solvation effects can have a tremendous influence on chemical reactions. However, precise quantum chemistry calculations are most often done either in vacuum neglecting the role of the solvent or using continuum solvent model ignoring its molecular nature. We propose a new method coupling a quantum description of the solute using electronic density functional theory with a classical grand-canonical treatment of the solvent using molecular density functional theory. Unlike a previous work, both densities are minimized self-consistently, accounting for mutual polarization of the molecular solvent and the solute. The electrostatic interaction is accounted using the full electron density of the solute rather than fitted point charges. The introduced methodology represents a good compromise between the two main strategies to tackle solvation effects in quantum calculation. It is computationally more effective than a direct quantum mechanics/molecular mechanics coupling, requiring the exploration of many solvent configurations. Compared to continuum methods, it retains the full molecular-level description of the solvent. We validate this new framework onto two usual benchmark systems: a water solvated in water and the symmetrical nucleophilic substitution between chloromethane and chloride in water. The prediction for the free energy profiles are not yet fully quantitative compared to experimental data, but the most important features are qualitatively recovered. The method provides a detailed molecular picture of the evolution of the solvent structure along the reaction pathway.

Comparison of Free-Energy Methods to Calculate the Barriers for the Nucleophilic Substitution of Alkyl Halides by Hydroxide

Calculating the free-energy barriers of liquid-phase chemical reactions with explicit solvent is a considerable challenge. Most studies use the energy and entropy of minimized single-point geometries of the reactants and transition state in implicit solvent using normal mode analysis (NMA). Explicit-solvent methods instead make use of the potential of mean force (PMF). Here, we propose a new energy-entropy (EE) method to calculate the Gibbs free energy of reactants and transition states in explicit solvent by combining quantum mechanics/molecular mechanics (QM/MM) molecular dynamics simulations with multiscale cell correlation (MCC). We apply it to six nucleophilic substitution reactions of the hydroxide transfer to methyl and ethyl halides in water, where the halides are F, Cl, and Br. We compare EE-MCC Gibbs free energy barriers using two Hamiltonians, self-consistent charge density functional based tight-binding (SCC-DFTB) and B3LYP/6-31+G* density functional theory (DFT) with respective PMF values, EE-NMA values using B3LYP/6-31+G* and M06/6-31+G* DFT in implicit solvent and experimental values derived via transition state theory. The barriers using SCC-DFTB are found to agree well with the PMF and experiment and previous computational studies, being slightly higher but improving on the lower values obtained for the implicit solvent. Achieving convergence over many degrees of freedom remains a challenge for EE-MCC in explicit-solvent QM/MM systems, particularly for the more expensive B3LYP/6-31+G* and M06/6-31+G* DFT methods, but the insightful decomposition of entropy over all degrees of freedom should make EE-MCC a valuable tool for deepening the understanding of chemical reactions.

Solvent Effects on the Symmetric and Asymmetric SN2 Reactions in the Acetonitrile Solution: A Reaction Density Functional Theory Study

Bimolecular nucleophilic substitution (S_N2) reactions are of great importance in chemistry and biochemistry due to their capability of constructing functional groups. In this work, we investigate the solvent effect on the free energy profiles of symmetric and asymmetric S_N2 reactions in the acetonitrile solution using the proposed reaction density functional theory (RxDFT) method. This multiscale method utilizes quantum density functional theory for calculating intrinsic reaction free energy coupled with classical density functional theory for addressing solvation contribution. We find that the presence of acetonitrile brings both the polarization effect and solvation effect on the reaction pathways. For the eight selected symmetric S_N2 reactions, the predicated reaction pathways agree well with the results from the direct and thermodynamic cycle (TC) methods with the SMD-M062X solvation model. In addition, the polarization effect reduces the free energy barriers by about 6 kcal/mol, while the solvation effect increases the barriers by about 18 kcal/mol. For the four selected asymmetric S_N2 reactions, the predicted reaction pathways agree well with the results from the Monte Carlo simulations and experiments. The polarization effect and the solvation effect mutually reduce the free energy barriers, and the solvation effect plays a dominant role.

Wetting Transition of Ionic Substrate by Modulating Surface Charge Distribution

Surface wettability regulation plays a crucial role in antifouling and related applications. For regulating surface wettability, one of the effective approaches is to modulate the surface charge distribution. Herein, we report a theoretical study for unraveling the mechanistic relation between surface charge distribution and ionic substrate wettability. Specifically, acetonitrile liquids at ambient condition in contact with various ionic substrates are considered. At different surface charge distributions, the interfacial thermodynamic properties are investigated by means of molecular density functional theory. We find that the variation of the spatial interval among the discrete charges strongly alters the substrate-acetonitrile interaction and leads to an oscillation in the interfacial tension, indicating that the substrate can be tuned from a solvophobic one to a solvophilic one. This trend can be further enhanced by increasing the charge quantity. The underlying mechanisms are extensively discussed and expatiated. Our work provides theoretical guidance to engineer and regulate surface wettability.