The Diels-Alder reaction of cyclopentadinene (CP) with methyl acrylate (MA) in room-temperature ionic liquids (RTILs) is theoretically examined. In the present study, quantum molecular orbital theory is combined with a multicomponent reference interaction site model (RISM). Because RISM is free from statistical error, it is possible to overcome the serious difficulty in the description of the strong Coulombic interaction in RTILs. We focused on the origin of the relatively moderate solvation effects of RTILs and the mechanism of endo-exo selectivity.

A S(N)2 reaction of methyl p-nitrobenzenesulfonate (p-NBS) and chloride anion in ionic liquid ([mmim][PF(6)]) was studied using RISM-SCF-SEDD method coupled with a highly sophisticated ab initio electronic structure theory (CCSD). The solvation structure as well as the energy profile along the reaction were discussed through comparison with an ordinary solvent system, dichloromethane.

Integral equation theory for molecular liquids is one of the powerful frameworks to evaluate solvation free energy (SFE). Different from molecular simulation methods, the theory computes SFE in an analytical manner. In particular, the correction method proposed by Kovalenko and Hirata [Chem. Phys. Lett. 290, 237 (1998); and J. Chem. Phys. 113, 2793 (2000)] is quite efficient in the accurate evaluation of SFE. However, the application has been limited to aqueous solution systems. In the present study, an improved method is proposed that is applicable to a wide range of solution systems. The SFE of a variety of solute molecules in chloroform and benzene solvents is evaluated. A key is the adequate treatment of excluded volume in SFE calculation. By utilizing the information of chemical bonds in the solvent molecule, the accurate computation of SFE is achieved.

In this study, we reported the development of a new quantum mechanics/molecular mechanics (QM/MM)-type framework to describe chemical processes in solution by combining standard molecular-orbital calculations with a three-dimensional formalism of integral equation theory for molecular liquids (multi-center molecular Ornstein-Zernike (MC-MOZ) method). The theoretical procedure is very similar to the 3D-reference interaction site model self-consistent field (RISM-SCF) approach. Since the MC-MOZ method is highly parallelized for computation, the present approach has the potential to be one of the most efficient procedures to treat chemical processes in solution. Benchmark tests to check the validity of this approach were performed for two solute (solute water and formaldehyde) systems and a simple SN2 reaction (Cl(-) + CH3Cl → ClCH3 + Cl(-)) in aqueous solution. The results for solute molecular properties and solvation structures obtained by the present approach were in reasonable agreement with those obtained by other hybrid frameworks and experiments. In particular, the results of the proposed approach are in excellent agreements with those of 3D-RISM-SCF.

Mean‐field treatment of solvent provides an efficient technique to investigate chemical processes in solution in quantum mechanics/molecular mechanics (QM/MM) framework. In the algorithm, an iterative calculation is required to obtain the self‐consistency between QM and MM regions, which is a time‐consuming step. In the present study, we have proposed a noniterative approach by introducing a linear response approximation (LRA) into the solvation term in the one‐electron part of Fock matrix in a hybrid approach between molecular‐orbital calculations and a three‐dimensional (3D) integral equation theory for molecular liquids (multicenter molecular Ornstein–Zernike self‐consistent field [MC‐MOZ‐SCF]; Kido et al., J. Chem. Phys. 2015, 143, 014103). To save the computational time, we have also developed a fast method to generate electrostatic potential map near solute and the solvation term in Fock matrix, using Fourier transformation (FT) and real spherical harmonics expansion (RSHE). To numerically validate the LRA and FT‐RSHE method, we applied the present approach to water, carbonic acid, and their ionic species in aqueous solution. Molecular properties of the solutes were evaluated by the present approach with four different types of initial wave functions and compared with those by the original (MC‐MOZ‐SCF). We found that an initial wave function considering solvation effects is needed to appropriately reproduce the properties by MC‐MOZ‐SCF. Furthermore, a benchmark test for 32 solute molecules was performed to evaluate the accuracy of the present approach for solvation free energy (SFE) and measure the speedup ratio for MC‐MOZ‐SCF. The error of SFE for MC‐MOZ‐SCF does not correlate with the SFE but increases in proportion to the electronic reorganization energy. Similar to water and carbonic acid, an initial wave function with solvation effects is also important to make the error small. From the averaged speed up ratio, the present approach is 13.5 times faster than MC‐MOZ‐SCF. © 2019 Wiley Periodicals, Inc.

Ornstein-Zernike (OZ)-type theory is a powerful tool to obtain 3-dimensional solvent distribution around solute molecule. Recently, we proposed multi-center molecular OZ method, which is suitable for parallel computing of 3D solvation structure. The distribution function in this method consists of two components, namely reference and residue parts. Several types of the function were examined as the reference part to investigate the numerical robustness of the method. As the benchmark, the method is applied to water, benzene in aqueous solution and single-walled carbon nanotube in chloroform solution. The results indicate that fully-parallelization is achieved by utilizing the newly proposed reference functions.

pKa is one of the fundamental properties of molecules and its accurate prediction by theoretical method is indispensable in the current era. At present, the most common approach is based on the free energy difference evaluated in dielectric continuum model for pure water, epsilon=80, which completely ignores ionic influence, i.e., ionic strength. In the present paper, a molecular level theory to predict pKa is proposed based on the reference interaction site model, which is a statistical mechanics for molecular liquids. By regarding an acidic aqueous solution as a three component system including water, proton species (cation), and anion, aqueous solutions with desired pH can be theoretically realized by controlling the number density of the proton species. Using computed free energy changes on the deprotonation of glycine at various pH conditions, a titration curve and pKa can be obtained from the first principle, showing excellent agreement with experimental data. To our best knowledge, this is the first theoretical attempt to directly evaluate pKa under the condition where ionic influence is explicitly taken into account by using statistical molecular theory.

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