Interaction Site Model Self-consistent Field Research Articles

Coupled Cluster Theory Combined with Reference Interaction Site Model Self-Consistent Field Explicitly Including Spatial Electron Density Distribution.

Calculating the geometry and energy of a molecule in a solution is one of the most important tasks in chemistry. However, performing an accurate calculation in a solution is still a difficult task because the electronic structure and solute-solvent interactions are required to be accurately evaluated with an efficient computational cost. To overcome this difficulty, we proposed the coupled cluster with single and double excitations and perturbative triple excitations combined with the reference interaction site model (RISM) by employing our fitting approach. Our method correctly reproduced the relative stabilities of 1,2,3-triazole, isonicotinic acid, cytosine, and 6-chloro-2-pyridone in the aqueous phase, whereas the dielectric continuum model provided incorrect results for isonicotinic acid and 6-chloro-2-pyridone. Our method provided accurate results because the RISM captured the local solvation structure, such as hydrogen bonds.

A computational scheme of pKa values based on the three-dimensional reference interaction site model self-consistent field theory coupled with the linear fitting correction scheme.

A scheme for quantitatively computing the acid dissociation constant, pKa, of hydrated molecules is proposed. It is based on the three-dimensional reference interaction site model self-consistent field (3D-RISM-SCF) theory coupled with the linear fitting correction (LFC) scheme. In LFC/3D-RISM-SCF, pKa values of target molecules are evaluated using the Gibbs energy difference between the protonated and unprotonated states calculated by 3D-RISM-SCF and the parameters fitted by the LFC scheme to the experimental values of training set systems. The pKa values computed by LFC/3D-RISM-SCF show quantitative agreement with the experimental data.

Statistical Mechanics-Based Theoretical Investigation of Solvation Effects on Glucose Anomer Preferences.

The importance of solvation effects on the stability of glucose anomers has been studied by the combination of quantum mechanics and statistical mechanics, namely, the reference interaction site model self-consistent field spatial electron density distribution. The preferences of α- and β-glucose in H2O are well reproduced with the obtained ratio of 35:65 for α- and β-glucose, respectively. Indirect interactions and bulk effects, described by the Onsager model, are relatively small compared to the direct solute-solvent interactions, especially in [DMIM]Cl and dimethyl sulfoxide. From the decomposition of solvation free energy and solvation structures, it can be seen that the interactions with the solvent molecules greatly contribute to the anomer preferences.

Solvatochromism and preferential solvation of Brooker's merocyanine in water-methanol mixtures.

The excitation energy of Brooker's merocyanine in water-methanol mixtures shows nonlinear behavior with respect to the mole fraction of methanol, and it was suggested that this behavior is related to preferential solvation by methanol. We investigated the origin of this behavior and its relation to preferential solvation using the three-dimensional reference interaction site model self-consistent field method and time-dependent density functional theory. The calculated excitation energies were in good agreement with the experimental behavior. Analysis of the coordination numbers revealed preferential solvation by methanol. The free energy component analysis implied that solvent reorganization and solvation entropy drive the preferential solvation by methanol, while the direct solute-solvent interaction promotes solvation by water. The difference in the preferential solvation effect on the ground and excited states causes the nonlinear excitation energy shift. © 2017 Wiley Periodicals, Inc.

Photo absorption of p-coumaric acid in aqueous solution: RISM-SCF-SEDD theory approach.

Photo absorption properties of p-coumaric acid, the chromophore of photoactive yellow protein, in aqueous solution were investigated by means of reference interaction site model self-consistent field with spatial electron density distribution (RISM-SCF-SEDD) method. RISM-SCF-SEDD is a combination methodology of electronic structure theory and statistical mechanics for molecular liquids. Here, time-dependent density functional theory was coupled with RISM equation to study the electronic structure of p-coumaric acid in aqueous system. Excitation energies of the chromophore in its neutral, two monoanionic and dianionic forms were computed to elucidate the effect of the deprotonation and solvation on the spectroscopic properties. We found that solvation strongly affects the excitation character of the chromophore, especially for phenolate anion and dianion. The free energy difference among the four protonation states is also discussed. © 2017 Wiley Periodicals, Inc.

Time-dependent density functional theory (TD-DFT) coupled with reference interaction site model self-consistent field explicitly including spatial electron density distribution (RISM-SCF-SEDD).

Theoretical approach to design bright bio-imaging molecules is one of the most progressing ones. However, because of the system size and computational accuracy, the number of theoretical studies is limited to our knowledge. To overcome the difficulties, we developed a new method based on reference interaction site model self-consistent field explicitly including spatial electron density distribution and time-dependent density functional theory. We applied it to the calculation of indole and 5-cyanoindole at ground and excited states in gas and solution phases. The changes in the optimized geometries were clearly explained with resonance structures and the Stokes shift was correctly reproduced.

Theoretical analysis of complex formation of p-carboxybenzeneboronic acid with a monosaccharide

Changes in the acid dissociation constant, pKa, of p-carboxybenzeneboronic acid (PCBA) upon complex formation with monosaccharide are considered by using the three-dimensional reference interaction site model self-consistent field theory. The pKa of PCBA is lowered through complex formation, which is consistent with experimental observations. Free energy component analysis of the dissociation reaction was performed to investigate the details of the contribution to the pKa shift. The magnitudes of the changes in both the electronic energy and the solvation free energy were smaller for the PCBA-complex than for PCBA. These smaller changes can be attributed to the delocalization of the excess charge and to a reduction of the solvent-accessible area near the boric acid group because of the steric bulk of the monosaccharide.

Glucose transformation to 5-hydroxymethylfurfural in acidic ionic liquid: A quantum mechanical study.

Isomerization and transformation of glucose and fructose to 5-hydroxymethylfurfural (HMF) in both ionic liquids (ILs) and water has been studied by the reference interaction site model self-consistent field spatial electron density distribution (RISM-SCF-SEDD) method coupled with ab initio electronic structure theory, namely coupled cluster single, double, and perturbative triple excitation (CCSD(T)). Glucose isomerization to fructose has been investigated via cyclic and open chain mechanisms. In water, the calculations support the cyclic mechanism of glucose isomerization; with the predicted activation free energy is 23.8 kcal mol(-1) at experimental condition. Conversely, open ring mechanism is more favorable in ILs with the energy barrier is 32.4 kcal mol(-1) . Moreover, the transformation of fructose into HMF via cyclic mechanism is reasonable; the calculated activation barriers are 16.0 and 21.5 kcal mol(-1) in aqueous and ILs solutions, respectively. The solvent effects of ILs could be explained by the decomposition of free energies and radial distribution functions of solute-solvent that are produced by RISM-SCF-SEDD.

Three-dimensional reference interaction site model self-consistent field analysis of solvent and substituent effects on the absorption spectra of Brooker's merocyanine.

Solvent and substituent effects on the absorption spectra of Brooker's merocyanine (BM) are investigated using the three-dimensional reference interaction site model self-consistent field method and time-dependent density functional theory. The π-π* excitation energies are computed for BM and its derivative 2,6-di-tert-butyl (di-t-Bu) BM. The behaviors of the computed excitation energies with increasing solvent polarity are in good agreement with those of the corresponding experimental measurements. In addition, analysis of the solute-solvent interaction energies and spatial distribution functions reveals that the effects of the solvent on the absorption spectra are reduced by the steric hindrance of the t-Bu groups. Furthermore, from the difference in the solute-solvent interaction energies of BM and di-t-Bu BM, it is shown that the effect of the t-Bu substituents on the absorption spectrum is greater in high-polarity solvents.

A hybrid framework of first principles molecular orbital calculations and a three-dimensional integral equation theory for molecular liquids: multi-center molecular Ornstein-Zernike self-consistent field approach.

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.

Exchange repulsive potential adaptable for electronic structure changes during chemical reactions.

Hybrid methods combining quantum mechanical (QM) and classical calculations are becoming important tools in chemistry. The popular approach to calculate the interaction between QM and classical calculations employs interatomic potentials. In most cases, the interatomic potential is constructed of an electrostatic (ES) potential and a non-ES potential. Because QM treatment is employed in the calculation of the ES potential, the electronic change can be considered in this ES potential. However, QM treatment of the non-ES potential is difficult because of high computational cost. To overcome this difficulty of evaluating the non-ES potential, we proposed an exchange repulsive potential as the main part of the non-ES potential on the basis of a QM approach. This potential is independent of empirical parameters and adaptable for electronic structure. We combined this potential with the reference interaction site model self-consistent field explicitly including spatial electron density distribution and successfully applied it to the chemical reactions in aqueous phase.

Theoretical study of cellobiose hydrolysis to glucose in ionic liquids

The SN1-type hydrolysis reaction of cellobiose in ionic liquids (ILs) was theoretically investigated. First principles and ab initio quantum chemical methods were used in conjunction with the ‘reference interaction site model self-consistent field with spatial electron density distribution’ (RISM-SCF-SEDD) method. Reaction mechanism pathways are discussed and compared to calculations in gas phase and in aqueous solution. Analysis of solvation effects indicates strong interaction between hydrogen atoms of glucose hydroxyl groups and the anions in ILs, contributing to large stabilization of the reaction product. The calculated activation energy in ILs (24.5kcal/mol) agrees quantitatively with the experimental value (26.5kcal/mol).

Theoretical Study of One-Electron Oxidized Mn(III)– and Ni(II)–Salen Complexes: Localized vs Delocalized Ground and Excited States in Solution

One-electron oxidized Mn(III)- and Ni(II)-salen complexes exhibit unique mixed-valence electronic structures and charge transfer (CT) absorption spectra. We theoretically investigated them to elucidate the reason why the Mn(III)-salen complex takes a localized electronic structure (class II mixed valence compound by Robin-Day classification) and the Ni(II)-analogue has a delocalized one (class III) in solution, where solvation effect was taken into consideration either by the three-dimensional reference interaction site model self-consistent field (3D-RISM-SCF) method or by the mean-field (MF) QM/MM-MD simulation. The geometries of these complexes were optimized by the 3D-RISM-SCF-U-DFT/M06. The vertical excitation energy and the oscillator strength of the first excited state were evaluated by the general multiconfiguration reference quasi-degenerate perturbation theory (GMC-QDPT), including the solvation effect based on either 3D-RISM-SCF- or MF-QM/MM-MD-optimized solvent distribution. The computational results well agree with the experimentally observed absorption spectra and the experimentally proposed electronic structures. The one-electron oxidized Mn(III)-salen complex with a symmetrical salen ligand belongs to the class II, as experimentally reported, in which the excitation from the phenolate anion to the phenoxyl radical moiety occurs. In contrast, the one-electron oxidized Ni(II)-salen complex belongs to the class III, in which the excitation occurs from the doubly occupied delocalized π1 orbital of the salen radical to the singly occupied delocalized π2 orbital; the π1 is a bonding combination of the HOMOs of two phenolate moieties and the π2 is an antibonding combination. Solvation effect is indispensable for correctly describing the mixed-valence character, the geometrical distortion, and the intervalence CT absorption spectra of these complexes. The number of d electrons and the d orbital energy level play crucial roles to provide the localization/delocalization character of these complexes.

Theoretical Study of Salt Effects on the Diels–Alder Reaction of Cyclopentadiene with Methyl Vinyl Ketone Using RISM-SCF Theory

Salt effects on the Diels-Alder reaction of cyclopentadiene with methyl vinyl ketone are investigated using reference interaction site model self-consistent field (RISM-SCF) theory. The rate of the reaction is accelerated by adding LiCl to the water solvent. The structures of four transition states, endo-cis, endo-trans, exo-cis, exo-trans, were found by geometry optimization of the cyclopentadiene and methyl vinyl ketone complexes. The endo-trans structure shows the lowest energy in both water and LiCl solution. The activation barrier of the reaction in LiCl solution is lower than that in water, and the difference is in good agreement with that from experiments. The decrease in the activation barrier arises from destabilization of the reactant species. The salt effect of LiCl makes all species concerning the reaction unstable by the hydrophobic effect; however, the increased hydrophobic effect in the TS complexes is suppressed by making the hydrogen bond, which is stronger compared with the reactant methyl vinyl ketone.

New fitting approach of electrostatic potential for stable quantum mechanical calculations using the reference interaction site model

We proposed a new fitting approach of electrostatic potential (ESP) for stable quantum mechanical (QM) calculations using the reference interaction site model. In this approach, the approximated density matrix is fitted so that the ESP computed from QM calculations can be reproduced. We introduced two conditions: conservation of the number of electrons and positive semi-definite condition of the electron density matrix. This approach was introduced in a linear response approximation of the reference interaction site model self-consistent field explicitly including the spatial electron density distribution. By considering the two conditions, we overcame the instability inherent in a previous approach.

Theoretical study on the ionization of aniline in aqueous solutions

Abstract The ionization and excitation processes of aniline in aqueous solutions are investigated by the method of RISM-SCF-SEDD (reference interaction site model self-consistent field with the spatial electron density distribution). Four different models are employed to characterize the response of the solvation upon the ionization and excitation. A simple treatment for estimation of the spectral width is also proposed.

Three-Dimensional Reference Interaction Site Model Self-Consistent Field Study of the Electronic Structure of [Cr(H2O)6]3+ in Aqueous Solution

The three-dimensional reference interaction-site model self-consistent-field (3D-RISM-SCF) method was applied to the electronic structure of hexahydrated chromium trication [Cr(H2O)6](3+) in aqueous solution. The 3D distribution around [Cr(H2O)6](3+) showed 12 maximum density points arising from hydrogen bonds, as well as eight local maximum points at the hollow sites, in the second hydration shell. The 3D distribution function also indicated 20 local maximum sites in the third hydration shell. The low-lying excited states of vertical transitions were computed using time-dependent density functional theory (TD-DFT), including the electric field from the solvent. The electronic configurations and excitation energies calculated using DFT and 3D-RISM-SCF were very similar, although the orbital energies involved in the transition were rather different. The two lowest excited states (1(4)T and 2(4)T) were roughly assigned to the d-d transitions, and the third and fourth excited states (3(4)T and 4(4)T) were assigned to ligand-to-metal charge-transfer transitions. The computed excited energies were in good agreement with the experimental values.

Excited-state free energy surfaces in solution: Time-dependent density functional theory/reference interaction site model self-consistent field method

Constructing free energy surfaces for electronically excited states is a first step toward the understanding of photochemical processes in solution. For that purpose, the analytic free energy gradient is derived and implemented for the linear-response time-dependent density functional theory combined with the reference interaction site model self-consistent field method. The proposed method is applied to study (1) the fluorescence spectra of aqueous acetone and (2) the excited-state intramolecular proton transfer reaction of ortho-hydroxybenzaldehyde in an acetonitrile solution.

Linear response approximation to reference interaction site model self-consistent field explicitly including spatial electron density distribution. Free energy

We proposed linear response approximation to reference interaction site model self-consistent field explicitly including spatial electron density distribution (RISM-SCF-SEDD). Because the electrostatic potential induced by solvent molecules is approximated by linear response to the density change of solute molecule, new algorithm becomes significantly simple compared to the original RISM-SCF-SEDD. The present method was applied to simple molecules and p-nitroaniline (PNA). The maximum relative error of the linear response approximation is less than 1.0% in the solvation free energies of the simple molecules. The present method also achieved a stable calculation of PNA.

A 3D-RISM-SCF method with dual solvent boxes for a highly polarized system: application to 1,6-anhydrosugar formation reaction of phenyl α- and β-d-glucosides under basic conditions

One of the difficulties in application of the usual reference interaction site model self-consistent field (RISM-SCF) method to a highly polarized and bulky system arises from the approximate evaluation of electrostatic potential (ESP) with pure point charges. To improve this ESP evaluation, the ESP near a solute is directly calculated with a solute electronic wavefunction, that distant from a solute is approximately calculated with solute point charges, and they are connected with a switching function. To evaluate the fine solvation structure near the solute by incorporating the long-range solute-solvent Coulombic interaction with low computational cost, we introduced the dual solvent box protocol; one small box with the fine spacing is employed for the first and the second solvation shells and the other large box with the normal spacing is employed for long-range solute-solvent interaction. The levoglucosan formation from phenyl α- and β-d-glucosides under basic conditions is successfully inspected by this 3D-RISM-SCF method at the MP2 and SCS-MP2 levels, though the 1D-RISM-SCF could not be applied to this reaction due to the presence of highly polarized and bulky species. This 3D-RISM-SCF calculation reproduces the experimentally reported higher reactivity of the β-anomer. The 3D-RISM-SCF-calculated activation free energy for the β-anomer is closer to the experimental value than the PCM-calculated one. Interestingly, the solvation effect increases the difference in reactivity between these two anomers. The reason is successfully elucidated with 3D-RISM-SCF-calculated microscopic solvation structure and decomposition analysis of solute-solvent interaction.