Reference Interaction Site Model Self-consistent Field Research Articles

Assessment of the applicability of the LFC/3D-RISM-SCF scheme for pKa prediction in methanol solutions

Abstract The applicability of the linear fitting correction with the three-dimensional reference interaction-site model self-consistent field (LFC/3D-RISM-SCF) scheme, a pKa prediction scheme, for methanol solutions was investigated. The correlation between experimental and predicted pKa values of dissociative molecules with phenol, amine, and carboxyl derivatives was examined. The pKa values of the LFC/3D-RISM-SCF results showed a good linear correlation with the experimental pKa. This result demonstrates that the LFC/3D-RISM-SCF method can be applied to a variety of solvents other than water.

Application of the reference interaction site model self-consistent field method based on the Dirac-Hartree–Fock wave function to a chemical reaction

The reference interaction site model self-consistent field (RISM-SCF) method is a combined method of the electronic structure theory of molecules and the integral equation theory of molecular liquids. The RISM-SCF method based on the Dirac-Hartree-Fock wave function, recently proposed, is applied to a chemical reaction, specifically, a Menshutkin reaction in aqueous solution. The Helmholtz energy profile along the reaction coordinate is calculated and the characteristics of the reaction are discussed based on energy component analysis.

Implementation of solvent polarization in three-dimensional reference interaction-site model self-consistent field theory

The three-dimensional reference interaction-site model self-consistent field (3D-RISM-SCF) theory is an electronic structure theory of solvated molecules, which can handle the electronic polarization of the solute molecule induced by the interaction with the solvent, whereas the electronic polarization of solvent molecules is ignored. Here, the solvent-polarizable model is implemented to take into account the electronic polarization of solvent molecules. It is applied to the water molecule in an aqueous solution and the p-nitroaniline molecule in an aqueous solution, and the effects of the solvent polarization on the properties of these solutes are demonstrated.

Nuclear magnetic shielding of molecule in solution based on reference interaction site model self-consistent field with spatial electron density distribution.

A new method for calculating nuclear magnetic shielding in solutions is developed based on the reference interaction site model self-consistent field (RISM-SCF) with spatial electron density distribution (SEDD). In RISM-SCF-SEDD, the electrostatic interaction between the solute and the solvent is described by considering the spread of electron to obtain more realistic electronic structure in solutions. It is thus expected to allow us to predict more quantitative chemical shifts of a wide variety of chemical species in solutions. In this study, the method is applied to a water molecule in water and is validated by examining the dependence of the solvent temperature and density on chemical shifts. The dependence of solvent species is also investigated, and more accurate results are obtained for polar solvents compared to the previous RISM-SCF study. Another application example of this method is the 15N chemical shifts of two azines in water, which is difficult to predict with the polarizable continuum model (PCM). Our results are in good agreement with the previous quantum mechanical/molecular mechanics study and experimental results. It is also shown that our method gives more realistic results for methanol and acetone than the PCM.

Toward Accurate Solvation Free Energy Calculation with the Reference Interaction Site Model Self-Consistent Field: Introduction of a New Bridge Function.

The reference interaction site model self-consistent field (RISM-SCF) has the potential to become a powerful tool for studying solution chemistry because of its low computational cost and the capability of calculating local solvation structures accurately, such as hydrogen bonding. However, RISM-SCF has a critical weakness: it is not possible to accurately reproduce the experimental data for solvation free energy. In this study, we proposed a new bridge function and combined it with the repulsive bridge correction (RBC) proposed by Kovalenko and Hirata. To verify the effectiveness of our approach, we computed the hydration free energies of 70 organic molecules. Our method successfully improved the overestimation of the cavitation free energy inherent in the original RISM-SCF.

Theoretical analysis of co-solvent effect on the proton transfer reaction of glycine in a water-acetonitrile mixture.

The co-solvent effect on the proton transfer reaction of glycine in a water-acetonitrile mixture was examined using the reference interaction-site model self-consistent field theory. The free energy profiles of the proton transfer reaction of glycine between the carboxyl oxygen and amino nitrogen were computed in a water-acetonitrile mixture solvent at various molar fractions. Two types of reactions, the intramolecular proton transfer and water-mediated proton transfer, were considered. In both types of the reactions, a similar tendency was observed. In the pure water solvent, the zwitterionic form, where the carboxyl oxygen is deprotonated while the amino nitrogen is protonated, is more stable than the neutral form. The reaction free energy is -10.6 kcal mol(-1). On the other hand, in the pure acetonitrile solvent, glycine takes only the neutral form. The reaction free energy from the neutral to zwitterionic form gradually increases with increasing acetonitrile concentration, and in an equally mixed solvent, the zwitterionic and neutral forms are almost isoenergetic, with a difference of only 0.3 kcal mol(-1). The free energy component analysis based on the thermodynamic cycle of the reaction also revealed that the free energy change of the neutral form is insensitive to the change of solvent environment but the zwitterionic form shows drastic changes. In particular, the excess chemical potential, one of the components of the solvation free energy, is dominant and contributes to the stabilization of the zwitterionic form.

Theoretical analysis of salt effect on intramolecular proton transfer reaction of glycine in aqueous NaCl solution

The salt effect on the intramolecular proton transfer reaction of glycine in aqueous NaCl solution was considered using the reference interaction-site model self-consistent field theory. The free energy profiles were computed for various salt concentrations. The set of profiles clearly showed that the free energy gap between the zwitterionic form (ZW) and the neutral form (NF) became larger as the NaCl concentration increased. The transition-state structure of the solute glycine approaches the NF structure, and the reaction barrier height is reduced. From the energy decomposition analysis of the free energy of solvation, we found that the changes in the solvation free energy are determined by the balance between the electronic distortion energy, the electrostatic interaction, the solvent reorganization, and the entropic effects caused by the NaCl addition. The salt effect of NaCl makes all the species associated with the reaction unstable; however, the destabilization of NFs caused by NaCl addition is stronger than that of ZWs. In the ZW, the penalty coming from the solvent reorganization and the entropic effects are compensated by the strong solute–solvent electrostatic interaction.

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.

Solvent dependence of Stokes shift for organic solute–solvent systems: A comparative study by spectroscopy and reference interaction-site model–self-consistent-field theory

The Stokes shift magnitudes for coumarin 153 (C153) in 13 organic solvents with various polarities have been determined by means of steady-state spectroscopy and reference interaction-site model-self-consistent-field (RISM-SCF) theory. RISM-SCF calculations have reproduced experimental results fairly well, including individual solvent characteristics. It is empirically known that in some solvents, larger Stokes shift magnitudes are detected than anticipated on the basis of the solvent relative permittivity, ɛr. In practice, 1,4-dioxane (ɛr = 2.21) provides almost identical Stokes shift magnitudes to that of tetrahydrofuran (THF, ɛr = 7.58), for C153 and other typical organic solutes. In this work, RISM-SCF theory has been used to estimate the energetics of C153-solvent systems involved in the absorption and fluorescence processes. The Stokes shift magnitudes estimated by RISM-SCF theory are ∼5 kJ mol(-1) (400 cm(-1)) less than those determined by spectroscopy; however, the results obtained are still adequate for dipole moment comparisons, in a qualitative sense. We have also calculated the solute-solvent site-site radial distributions by this theory. It is shown that solvation structures with respect to the C-O-C framework, which is common to dioxane and THF, in the near vicinity (∼0.4 nm) of specific solute sites can largely account for their similar Stokes shift magnitudes. In previous works, such solute-solvent short-range interactions have been explained in terms of the higher-order multipole moments of the solvents. Our present study shows that along with the short-range interactions that contribute most significantly to the energetics, long-range electrostatic interactions are also important. Such long-range interactions are effective up to 2 nm from the solute site, as in the case of a typical polar solvent, acetonitrile.

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.

Proton‐coupled electron transfer of the phenoxyl/phenol couple: Effect of Hartree‐Fock exchange on transition structures

Proton-coupled electron transfer (PCET) and hydrogen atom transfer (HAT) reactions of the phenoxyl/phenol couple are studied theoretically by using wave function theory (WFT) as well as DFT methods. At the complete active space self-consistent field (CASSCF) level, geometry optimization is found to give two transition states (TSs); one is the PCET type with two benzene rings being nearly coplanar, and the other is the HAT type with two benzene rings taking a stacking structure. Geometry optimization at the (semilocal) DFT level, on the other hand, is found to give only one transition state (i.e., the PCET-type one) and fail to obtain the stacking TS structure. By comparing various levels of theories (including long-range corrected DFT functionals), we demonstrate that the Hartree-Fock exchange at long range plays a critical role in obtaining the sufficient stacking stabilization of the present open-shell system, and that the sole addition of empirical dispersion correction to semilocal DFT functionals may not be adequate for describing such a stacking interaction. Next, we investigate the solvent effect on the PCET and HAT TS thus obtained using the reference interaction site model self-consistent field (RISM-SCF) method. The results suggest that the free energy barrier increases with increasing polarity of the solvent, and that the solvent effects are stronger for the PCET TS than the stacking HAT TS pathway. The reason for this is discussed based on the dipole moment of different TS structures in solution.

Solvent effect on (2,2,6,6-Tetramethylpiperidine-1-yl)oxyl (TEMPO): a RISM-SCF-SEDD study

The solvent dependence of several properties of (2,2,6,6-Tetramethylpiperidine-1-yl)oxyl (TEMPO) is investigated by the reference interaction site model self-consistent field (RISM-SCF) theory. Time-dependent density functional theory (TDDFT) coupled with RISM-SCF-SEDD (spatial electron density distribution) is used to evaluate the n → π* transition energies and the results are compared to the reported experimental values.

Theoretical study on aquation reaction of cis-platin complex: RISM–SCF–SEDD, a hybrid approach of accurate quantum chemical method and statistical mechanics

The ligand exchange process of cis-platin in aqueous solution was studied using RISM-SCF-SEDD (reference interaction site model-self-consistent field with spatial electron density distribution) method, a hybrid approach of quantum chemistry and statistical mechanics. The analytical nature of RISM theory enables us to compute accurate reaction free energy in aqueous solution based on CCSD(T), together with the microscopic solvation structure around the complex. We found that the solvation effect is indispensable to promote the dissociation of the chloride anion from the complex.

Consequences of Strong Coupling between Solvation and Electronic Structure in the Excited State of a Betaine Dye

The electronic ground and excited-state structures of the betaine dye molecule pyridinium- N-phenoxide [4-(1-pyridinio)phenolate] are investigated both in the gas phase and in aqueous solution, using the reference interaction site model self-consistent-field (RISM-SCF) procedure within a CASSCF framework. We obtain the total free energy profiles in both the ground and excited states with respect to variation in the torsion angle between the phenoxide and pyridinium rings. We analyze the effect of solvent on the variation of the solute dipole moment and on the charge transfer character in the excited state. In the gas phase, it is shown that the potential energy profile in the excited-state decreases monotonically toward a perpendicular ring orientation and the dipole moment decreases along with decreasing charge localization. In water, the free energy surface for twisting is better characterized as nearly flat along the same coordinate for sterically accessible angles. These results are analyzed in terms of contributions of the solvation free energy, the solute electronic energy, and their coupling. Correspondingly, the dependence of the charge transfer character on solute geometry and solvation are analyzed, and the important roles in the excitation and subsequent relaxation processes for the betaine dye are discussed. It is found that there is considerable solute electronic reorganization associated with the evolution of solvation in the excited state, and it is suggested that this reorganization may contribute significantly to the early time evolution of transient spectra following photoexcitation.

Polyatomic molecules in condensed phase: bond order index and solvation energy studied by RISM-SCF theory

The reference interaction site model - self-consistent field (RISM-SCF) theory was applied to a series of polyatomic molecules in aqueous solution, including CH4, NH3, H2O and HF. The bond order index, which characterizes the chemical bond, and its change by the solvation were studied by comparison with a dielectric continuum model. The change in the electronic structure was also associated with the solvation free energy. We find that the bond order indexes are not sensitive to the solvation effect even if the charge assigned to the atom is considerably changed. The distortion in the electronic energy is in proportion to the change in the solvation energy, showing that the linear response regime is a good expression of the solvation process examined here, in spite of solving the non-linear RISM-SCF equation. It is well-known that the electronic structure of a molecule is significantly changed when the molecule is dissolved into solvent. In the last few decades, theory for the electronic structure of solvated molecules has attracted a lot of researchers' attention, and various types of combination methods have been developed. These can be roughly classified into three categories, namely, the dielectric continuum model such as polarizable continuum model (PCM), quantum mechanical- molecular mechanics (QM-MM), and the method based on the integral equation theory for liquids. RISM-SCF (1,2), which is a representative of the third category, compiles two fundamental methods in theoretical chemistry: one is the reference interaction site model (RISM) (3,4), and the other is ab initio molecular orbital theory. The method determines the electronic structure and the statisti- cal solvent distribution around the solute in a self-consistent manner. It is a remarkable advantage that the RISM-SCF method can provide information on the microscopic solvation structure based on the statistical mechanics. We would like to emphasize that the RISM-SCF theory can be re- garded as an extension of RISM theory as well as that of ab initio molecular orbital theory. The RISM-SCF has been successfully applied to numerous molecular phenomena including chemical reactions, chemical equilibria, charge transfer processes and so on (5). The aim of this review is to characterize the chemical bond in condensed phase, which is definitely one of the central subjects in physical chemistry. The solvation effect by all means modifies the character of a chemical bond in the solute molecule, and, at the same time, this modification in the electronic structure affects the structure of the solvent molecules surrounding the solute. For the system we choose a series of hydrogenated compounds of the second period elements in aqueous solution, which are the most fundamental and ubiquitous molecules in the universe. Since all of them have only 'single' bond and their electronic structures in the gas phase are well known, they are thought to be the most suitable for a systematic comparison for the preset purpose.

Electronic structure and solvation structure of [Ru(CN) 6] 4−/3− in aqueous solution: A RISM-SCF study

The electronic structure and solvation structure of metal complexes, [Ru(CN) 6] 4− and [Ru(CN) 6] 3−, in aqueous solution are studied by using the reference interaction site model self-consistent field (RISM-SCF). Although the electronic structure of these complexes in gas phase is similar, the solvation considerably affects the electronic structure of [Ru(CN) 6] 4− compared to [Ru(CN) 6] 3−. Microscopic solvation structure is also presented.

New generation of the reference interaction site model self-consistent field method: Introduction of spatial electron density distribution to the solvation theory

The authors propose the new generation of the reference interaction site model self-consistent field (RISM-SCF) method for the solvation effect on the electronic structure of a solute molecule, in which the procedure proposed by Gill et al. [J. Chem. Phys. 96, 7178 (1992)] is adopted. Main improvements are the introduction of spatial electron density distribution and the removal of the grid dependency that is inherent in the original RISM-SCF. The procedure also provides very stable determination of the effective charges even if a buried atom exists in the target molecule and eventually extends the applicability of the RISM-SCF. To demonstrate the superiority of our method, sample calculations for H2O, C2H5OH, and HLi in aqueous solution are presented.

Locating the lowest free-energy point on conical intersection in polar solvent: Reference interaction site model self-consistent field study of ethylene and CH2NH2+

We present a theoretical method for locating the lowest free-energy points on conical intersections (CIs) in solution using the reference interaction site model self-consistent field (RISM-SCF) theory. Based on the linear-response theory, the nonequilibrium free energy is defined as a quadratic function of solvation coordinates, the parameters in which are directly obtained by ab initio RISM-SCF calculations. This free energy is easily incorporated into an efficient CI optimization procedure in gas phase. The present method is applied to the cis-trans photoisomerizations of ethylene and methaniminium cation (CH2NH2(+)) in polar solvents. We show that the geometries and energies of CIs are largely affected by the solute-solvent electrostatic interaction. In particular, the hydrogen migration of ethylene observed at CIs in the gas phase disappears in protic solvents due to the large stabilization of the zwitterionic state.

Intramolecular Charge-Transfer State Formation of 4-(N,N-Dimethylamino)benzonitrile in Acetonitrile Solution: RISM-SCF Study

Intramolecular charge-transfer (ICT) state formation of 4-(N,N-dimethylamino)benzonitrile in acetonitrile solution is studied by the reference interaction site model self-consistent field (RISM-SCF) method. Geometry optimizations are performed for each electronic state in solution with the complete-active-space SCF wave functions. Dynamic electron correlation effects are taken into account by using the multiconfigurational quasidegenerate perturbation theory. Two-dimensional free energy surfaces are constructed as the function of the twisting and wagging angles of the dimethylamino group for the ground and locally excited (LE) states. The calculated absorption and fluorescence energies are in good agreement with experiments. The validity of the twisted ICT (TICT) model is confirmed in explaining the dual fluorescence, and the possibility of the planar ICT model is ruled out. To examine the mechanism of the TICT state formation, a "crossing" seam between the LE and charge-transfer (CT) state surfaces is determined. The inversion of two electronic states occurs at a relatively small twisting angle. The effect of solvent reorganization is also examined. It is concluded that the intramolecular twisting coordinate is more important than the solvent fluctuation for the TICT state formation, because the energy difference between the two states is minimally dependent on the solvent configuration.

A Theoretical Investigation of the Shape and Hydration Properties of Aqueous Urea: Evidence for Nonplanar Urea Geometry

To model the structure of a urea molecule in aqueous solution including adaption of the solute electronic structure, we have used the reference interaction site model−self-consistent-field (RISM-SCF) method, describing the solute electronic structure at the ab initio level and hydration via an integral equation for the solvent distribution. The RISM-SCF model for aqueous urea gives a clearly nonplanar urea structure, with more than seven waters located within a contact distance defined for hydrogen bonding. The pyramidalization at the urea nitrogen sites is reduced in water relative to the gas phase, but the structure is closer to the gas phase structure than to the planar crystal structure. It is further shown that the solvent produces substantial polarization of the urea solute, with the dipole moment increasing from 4 to 7 D as the geometry and electronic structure are optimized to the aqueous environment. The overall result is to favor a high density of hydrogen bonds between urea and the surrounding ...