Polarizable Continuum Approach Research Articles

Charge transfer optical absorption and fluorescence emission of 4-(9-acridyl)julolidine from long-range-corrected time dependent density functional theory in polarizable continuum approach

A long-range-corrected time-dependent density functional theory (LC-TDDFT) in combination with polarizable continuum model (PCM) have been applied to study charge transfer (CT) optical absorption and fluorescence emission energies basing on parameterized LC-BLYP xc-potential. The molecule of 4-(9-acridyl)julolidine selected for this study represents typical CT donor–acceptor dye with strongly solvent dependent optical absorption and fluorescence emission spectra. The result of calculations are compared with experimental spectra reported in the literature to derive an optimal value of the model screening parameter ω. The first absorption band appears to be quite well predictable within DFT/TDDFT/PCM with the screening parameter ω to be solvent independent (ω≈0.245Bohr−1) whereas the fluorescence emission exhibits a strong dependence on the range separation with ω-value varying on a rising solvent polarity from about 0.225 to 0.151Bohr−1. Dipolar properties of the initial state participating in the electronic transition have crucial impact on the effective screening.

Polarizable continuum approach to solvated transient anions

Solvent effects on transient negative ions are surveyed with a model that combines the polarizable continuum approach with the Schwinger multichannel method for electron scattering. The solvent reaction field is allowed to act on the many-body wave function of the anion and stabilizes the transient states. The model is applied to π* temporary ions of several molecules.

Absorption and Fluorescence Spectra of Heterocyclic Isomers from Long-Range-Corrected Density Functional Theory in Polarizable Continuum Approach

Long-range-corrected (LC) DFT/TDDFT methods may provide adequate description of ground and excited state properties; however, accuracy of such an approach depends much on a range separation (exchange screening) representing adjustable model parameter. Its relation to a size or specific of molecular systems has been explored in numerous studies, whereas the effect of solvent environment is usually ignored during the evaluation of state properties. To benchmark and assess the quality of the LC-DFT/TDDFT formalism, we report the optical absorption and fluorescence emission energies of organic heterocyclic isomers, DPIPQ and PTNA, calculated by LC-BLYP DFT/TDDFT method in the polarizable continuum (PCM) approach. The calculations are compared with the optical absorption and fluorescence spectra measured in organic solvents of different polarity. Despite a considerable structural difference, both dyes exhibit quite similar range separations being somewhat different for the optical absorption and fluorescence emission processes. Properly parametrized LC-BLYP xc-potential well reproduces basic features of the optical absorption spectra including the electronic transitions to higher excited states. The DFT/TDDFT/PCM analysis correctly predicts the solvation trends although solvatochromic shifts of the electronic transition energies appear to be evidently underestimated in most cases, especially for the fluorescence emission. Considering the discrepancy between the experiment and theory, evaluated state dipole moments and solvation corrections to the exchange screening are analyzed. The results of the present study emphasize the importance of a solvent-dependent range separation in DFT/TDDFT/PCM calculations for investigating excited state properties.

Alkaline hydrolysis of ethylene phosphate: An ab initio study by supermolecule model and polarizable continuum approach

The alkaline hydrolysis reaction of ethylene phosphate (EP) has been investigated using a supermolecule model, in which several explicit water molecules are included. The structures and single-point energies for all of the stationary points are calculated in the gas phase and in solution at the B3LYP/6-31++G(df,p) and MP2/6-311++G(df,2p) levels. The effect of water bulk solvent is introduced by the polarizable continuum model (PCM). Water attack and hydroxide attack pathways are taken into account for the alkaline hydrolysis of EP. An associative mechanism is observed for both of the two pathways with a kinetically insignificant intermediate. The water attack pathway involves a water molecule attacking and a proton transfer from the attacking water to the hydroxide in the first step, followed by an endocyclic bond cleavage to the leaving group. While in the first step of the hydroxide attack pathway the nucleophile is the hydroxide anion. The calculated barriers in aqueous solution for the water attack and hydroxide attack pathways are all about 22 kcal/mol. The excellent agreement between the calculated and observed values demonstrates that both of the two pathways are possible for the alkaline hydrolysis of EP.

Predicting the UV–vis spectra of oxazine dyes

In the current work we have investigated the ability of time-dependent density functional theory (TD-DFT) to predict the absorption spectra of a series of oxazine dyes and the effect of solvent on the accuracy of these predictions. Based on the results of this study, it is clear that for the series of oxazine dyes an accurate prediction of the excitation energy requires the inclusion of solvent. Implicit solvent included via a polarizable continuum approach was found to be sufficient in reproducing the excitation energies accurately in the majority of cases. Moreover, we found that the SMD solvent model, which is dependent on the full electron density of the solute without partitioning into partial charges, gave more reliable results for our systems relative to the conductor-like polarizable continuum model (CPCM), as implemented in Gaussian 09. In all cases the inclusion of solvent reduces the error in the predicted excitation energy to <0.3 eV and in the majority of cases to <0.1 eV.

Theoretical Study of the Effects of Solvation, Substitution, and Structure on the Properties of Imidazolines, Oxazolines, and Thiazolines

Different isomers and tautomers of methylphosphino- and phenylphosphino-substituted cyclic imidazoline, oxazoline, and thiazoline have been investigated theoretically in gas and aqueous phases. Special emphasis is put on the relative total energies and on the changes in the structure due to substitution or solvation. The calculations were carried through using the B3LYP/6-31+G(d,p) method. To include the effects of the solvent, we used the polarizable-continuum approach both without and with the inclusion of explicit water molecules. Lacking experimental information on the systems, the results were compared with those for nitrogen-containing compounds. In gas phase the cyclic moiety of these molecules show clear deviations from planarity. Only moderate changes in the structure due to solvation were found. On the other hand, the solvent affects strongly the relative stability of different tautomers and isomers. The inclusion of explicit water molecules changes the order of stability due to the presence of intermolecular hydrogen bonds. On the basis of the Bader theory of atoms in molecules, we identify the critical points of the hydrogen bonds as well as properties of the electron density at those points. Thereby, we can quantify the strength of the hydrogen bonds. Finally, we report energies of solvation for the systems of our study.

The thermodynamic stability of hydrogen bonded and cation bridged complexes of humic acid models—A theoretical study

Hydrogen bonded and cation bridged complexation of poly(acrylic acid) oligomers, representing a model compound for humic acids, with acetic acid and the herbicide (4-chloro-2-methylphenoxy) acetic acid (MCPA) have been studied by means of density functional theory. Solvation effects were computed by means of a combination of microsolvation (explicit insertion of water molecules) and global solvation (polarizable continuum approach). The stability of hydrogen bonded complexes in solution is characterized by a strong competition between solute and solvent molecules. The cation bridged complexes of the negatively charged (deprotonated) ligands were found to be strongly favored explaining the capability of humic acids to fixate anionic species from soil solutions and the ability to form cross-linking structures within the humic acid macromolecules.

A polarizable continuum approach for the study of heterogeneous dielectric environments

We present a computational method, exploiting some features of the polarizable continuum model (PCM) to describe heterogeneous media; it belongs to the family of electrostatic embedding mixed methods, such as the more common quantum-mechanical (QM)/molecular mechanics approaches, with the electrostatic long range effects accounted for by a polarized continuum instead of atomic point charges. Provided effective dielectric constants are determined for the various parts of the system, the method is much faster than its atomistic counterpart, and allows for high-level QM calculations on the fragment of interest, using all the highly efficient computational tools developed for homogeneous PCM. Two case studies (the calculation of the pKa of solvent exposed acidic residues in a model protein, and the calculation of the electron spin resonance spectrum of a typical spin probe partially embedded in a membrane) are analyzed in some detail, to illustrate the application of the method to complex systems.

The acidity of tert-butyl alcohol in near- and supercritical water: a polarizable continuum approach.

We use a polarizable continuum approach to study the acidity of tert-butyl alcohol in water at ambient, near-critical, and supercritical conditions. In the most straightforward calculation, the bare ionic species (the tert-butoxy anion and the hydronium cation) are placed in cavities surrounded by a dielectric continuum, using a dielectric constant corresponding to the state point. A second method is first to solvate these ions with a small number of explicit water molecules and then surround this cluster with the dielectric. This is the cluster-continuum approach. We discuss the advantages and disadvantages of these two schemes and we also discuss various ways in which the second method can be implemented. No method showed quantitative agreement with all available experimental results but the first, straightforward method was the most successful in predicting the correct trends. From both a numerical and a theoretical point of view, we believe this is the method of choice.

Synthesis of anticonvulsant sulfamides. Theoretical study of the related mechanism

A theoretical study of the mechanisms associated with the synthesis of anticonvulsant symmetric N,N’-substituted sulfamides is presented. Two possible synthetic routes are compared, which mainly differ in the use of pyridine as a nucleophilic agent in the reaction mechanism. Geometry optimization techniques and transition-state detection at the B3LYP/6-31G** level, modeling the solvent by means of an isodensity polarizable continuum approach, allow the most suitable method for the experimental process to be discerned.

An integrated effective fragment—polarizable continuum approach to solvation: Theory and application to glycine

A new discrete/continuum solvation model has been developed by combining the effective fragment potential (EFP) for the discrete part and the polarizable continuum model (PCM) for the continuum part. The usefulness of this model is demonstrated by applying it to the calculation of the relative energies of the neutral and zwitterionic forms of glycine. These calculations were performed by treating glycine with ab initio wave functions. Water clusters were treated with both ab initio and EFP methods for comparison purposes, and the effect of the continuum was accounted for by the PCM model. The energy barrier connecting the zwitterionic and neutral three-water clusters was also examined. The computationally efficient EFP/PCM model gives results that are in close agreement with the much more expensive full ab initio/PCM calculation. The use of methods that account for electron correlation is necessary to obtain accurate relative energies for the isomers of glycine.

Theoretical study of the thio-Claisen rearrangement. Can vinylthioethanimine undergo a [3,3]-sigmatropic shift?

An ab initio study of the [3,3]-sigmatropic rearrangements of allyl vinyl sulfide 1 and vinylthioethanimine 2 has been carried out. To this end, geometries of stationary points were optimized at the HF, DFT (B3LYP) and MP2 (fc) levels using the 6-31G* basis set, while relative energies were estimated using post-HF calculations up to ∼ QCISD(T)/6-311 + G(2df,2p)//MP2/6-31G* level and corrected for zero-point energies. Progress of the charge distributions and delocalization interactions (NBO analysis) along the reaction have been investigated by employing the intrinsic coordinate method. Solvent effects on these processes have been simulated by the mean of SCRF computations associated to a continuum model. The results show that 2 is predicted to be less reactive than 1, the calculated enthalpy of activation for cis-2 and tras-2 rearrangement being respectively 4.5 and 10.0 kcal mol–1 higher than for the rearrangement of compound 1. The thio-Claisen rearrangement is calculated to be slightly exothermic (ΔrH=–1.5 kcal mol–1 at our best level of calculation) while the rearrangement from vinylthioethanimine is calculated to be endothermic [ΔrH(cis-2)= 7.0 kcal mol–1; ΔrH(trans-2)= 9.1 kcal mol–1]. Unlike the thio-Claisen rearrangement, the results obtained considering the polarizable continuum approach indicate that no significant rate enhancement of rearrangement from compound 2 can be expected on going from the gas phase into solution. NBO analysis is a tool for the mechanistic study of these rearrangements and is a guideline in the research of electronic effects in order to improve their feasibility.

Interpretation of partition between phases and chromatographic retention for a series of oxygen compounds

The Gibbs free energies of solvation in water and in 1-octanol were calculated for a series of 13 oxygen compounds using the polarizable continuum approach. The values obtained were correlated with the experimental distribution constants in the water-1-octanol system and with the capacity factors related to HPLC retention. Statistically significant correlations were found between the calculated and experimental data, indicating that the model used and the methods applied describe the physical nature of the studied properties of substances in a qualitatively correct manner.