Polarizable Continuum Model Method Research Articles

Tuning the Reactivity and Bonding Properties of Metal Square-Planar Complexes by the Substitution(s) on the Trans-Coordinated Pyridine Ring.

The kinetics of the hydration reaction on trans-[Pt(NH3)2(pyrX)Cl]+ (pyr = pyridine) complexes (X = OH–, Cl–, F–, Br–, NO2–, NH2, SH–, CH3, C≡CH, and DMA) was studied by density functional theory calculations in the gas phase and in water solution described by the implicit polarizable continuum model method. All possible positions ortho, meta, and para of the substituent X in the pyridine ring were considered. The substitution of the pyr ligand by electron-donating X’s led to the strengthening of the Pt–N1(pyrX) (Pt–NpyrX) bond and the weakening of the trans Pt–Cl or Pt–Ow bonds. The electron-withdrawing X’s have exactly the opposite effect. The strengths of these bonds can be predicted from the basicity of sigma electrons on the NpyrX atom determined on the isolated pyrX ligand. As the pyrX ring was oriented perpendicularly with respect to the plane of the complex, the nature of the X···Cl electrostatic interaction was the decisive factor for the transition-state (TS) stabilization which resulted in the highest selectivity of ortho-substituted systems with respect to the reaction rate. Because of a smaller size of X’s, the steric effects influenced less importantly the values of activation Gibbs energies ΔG⧧ but caused geometry changes such as the elongation of the Pt–NpyrX bonds. Substitution in the meta position led to the highest ΔG⧧ values for most of the X’s. The changes of ΔG⧧ because of electronic effects were the same in the gas phase and the water solvent. However, as the water solvent dampened electrostatic interactions, 2200 and 150 times differences in the reaction rate were observed between the most and the least reactive mono-substituted complexes in the gas phase and the water solvent, respectively. An additional NO2 substitution of the pyrNO2 ligand further decelerated the rate of the hydration reaction, but on the other hand, the poly-NH2 complexes were no more reactive than the fastest o-NH2 system. In the gas phase, the poly-X complexes showed the additivity of the substituent effects with respect to the Pt–ligand bond strengths and the ligand charges.

Natural pigments in dye-sensitized solar cell (DSSC): a DFT-TDDFT study

The molecular structures and photophysical properties of twelve types of the dyes from natural pigments of chlorophyll, flavonoid, carotenoid and anthocyanin were calculated with density functional theory (DFT) and time-dependent DFT (TD-DFT). Based on the comparison and better agreement between experimental wavelengths of maximum absorption and their calculated values with functionals of B3LYP, CAM-B3LYP and BHand-H, the B3LYP/6-31G** level was chosen for the process of our studies. The dyes’ properties in solvent environment were carried out using conductor-like polarizable continuum model methods (CPCM-DFT and TD-CPCM-DFT). The results show that pigment type affects the photophysical properties, and the dyes of a specific pigment type have nearly the same properties. Anthocyanins have the highest values of ionization potential (IP), electron affinity (EA), electronic chemical potential (µ) (their absolute values), chemical hardness (η), electrophilicity index (ω), electroaccepting power (ω+) and electrodonating power (ω−), which can lead to high efficiency of these dyes type for dye-sensitized solar cells (DSSCs). Energy gap of ELUMO and EHOMO of the dyes respectively with the conduction band edge of TiO2 (ECB) and Eredox of $$\left(\tt {I^ - }/I_{3}^{ - }\right)$$ couple (ΔEL and ΔEH), the free energy difference for electron injection (ΔGinject) and the driving force for dye regeneration (ΔGreg), the maximum absorption wavelengths of peaks (λmax) and oscillator strength values (f) for all the dyes in gas, acetonitrile and water phases were calculated and explained. These parameters show that carotenoids, chlorophylls and anthocyanins are more appropriate pigments than flavonoid pigment and the estimated open-circuit photo-voltage values (eVoc) showed that flavonoids are the desirable pigment. We expect these results could be helpful for DSSC producers to choose highly efficient natural dyes according to their optical and electronic properties.

Dependence of the Substituent Effect on Solvent Properties

The influence of a solvent on the substituent effect (SE) in 1,4-disubstituted derivatives of benzene (BEN), cyclohexa-1,3-diene (CHD), and bicyclo[2.2.2]octane (BCO) is studied by the use of polarizable continuum model method. In all X-R-Y systems for the functional group Y (NO2, COOH, OH, and NH2), the following substituents X have been chosen: NO2, CHO, H, OH, and NH2. The substituent effect is characterized by the charge of the substituent active region (cSAR(X)), substituent effect stabilization energy (SESE), and substituent constants σ or F descriptors, the functional groups by cSAR(Y), whereas π-electron delocalization of transmitting moieties (BEN and CHD) is characterized by a geometry-based index, harmonic oscillator model of aromaticity. All computations were carried out by means of B3LYP/6-311++G(d,p) method. An application of quantum chemistry SE models (cSAR and SESE) allows to compare the SE in water solutions and in the gas phase. Results of performed analyses indicate an enhancement of the SE by water. The obtained Hammett-type relationships document different nature of interactions between Y and X in aromatic and olefinic systems (a coexistence of resonance and inductive effects) than in saturated ones (only the inductive effect). An increase of electric permittivity clearly enhances communications between X and Y for BEN and CHD systems.

Evaluation of Acid Dissociation Constants in DMSO and DMF by Quantum-Chemical Methods

We have calculated total electronic energies (E) and Gibbs energies (G) of a large number of acids and their anions in water, dimethylsulfoxide, and dimethylformamide using the hybrid B3LYP functional DFT method in the 6-31++G(d,p) basis set, taking into account the solvent effect by the conductor-like polarizable continuum model method. A linear correlation has been found between the experimental values of acid dissociation constants (pKa) of different nature and the difference between anion and acid E values, and between pKa and the difference between anion and acid G values. The obtained correlations allowed us to evaluate the pKa values of both inorganic and organic acids. Such an evaluation is of special importance for nonaqueous solvents as it is quite problematic to determine these dissociation constants.

Thermodynamics-antioxidant activity relationships of some 4-benzylidenamino-4, 5-dihydro-1h-1,2,4-triazol-5-one derivatives: Theoretical evaluation

ABSTRACTDensity functional methods were used to predict the antioxidative efficiency of thirteen 4-benzylidenamino-4, 5-dihydro-1H-1,2,4-triazol-5-one derivatives in the gas phase and in the solution phase (water and benzene). Optimized geometries of molecules and reaction thermodynamic energies (enthalpies and reaction-free energies) of three main antioxidant mechanisms (hydrogen atom transfer, single electron transfer-proton transfer, and sequential proton loss electron transfer) were studied at B3LYP/6-31G (d,p) level. Solvent contributions to thermodynamic energies were computed employing integral equation formalism integral equation formalism polarized continuum model method. Obtained results revealed that the three main working mechanisms were endothermic, but not spontaneous especially in the gas phase. We found that the single electron transfer process from the anionic form was more preferable than that from the neutral form in the gas phase. The comparison of the ionization potentials of 4-benzylidenamino-4, 5-dihydro-1H-1,2,4-triazol-5-one derivatives to those of classical antioxidants (gallic acid, caffeic acid, ferulic acid, and ascorbic acid) indicated that the electron transfer mechanism was more predominant in the thirteen 4-benzylidenamino-4, 5-dihydro-1H-1,2,4-triazol-5-one derivative compounds. Thermodynamically, single electron transfer process from the anionic form was the most preferable mechanism in the gas phase. Solvent effect drastically modified thermodynamic energies of mechanisms. The proton transfer process was the thermodynamically favored mechanism as compared to other mechanisms in both solvents. It is worth mentioning that all the mechanisms were found not to be spontaneous in the solution phase except the proton transfer process.

The benchmark of 31P NMR parameters in phosphate: a case study on structurally constrained and flexible phosphate.

A benchmark for structural interpretation of the 31P NMR shift and the 2JP,C NMR spin-spin coupling in the phosphate group was obtained by means of theoretical calculations and NMR measurements in diethylphosphate (DEP) and 5,5-dimethyl-2-hydroxy-1,3,2-dioxaphosphinane 2-oxide (cDEP). The NMR parameters were calculated employing the B3LYP, BP86, BPW91, M06-2X, PBE0, KT2, KT3, MP2, and HF methods, and the 6-31+G(d), Iglo-n (n = II, III), cc-pVnZ (n = D, T, Q, 5), aug-cc-pVnZ (n = D, T and Q), and pcS-n and pcJ-n (n = 1, 2, 3, 4) bases, including the solvent effects described with explicit water molecules and/or the implicit Polarizable Continuum Model (PCM). The effect of molecular dynamics (MD) on NMR parameters was MD-calculated using the GAFF force field inclusive of explicit hydration with TIP3P water molecules. Both the optimal geometries and the dynamic behaviors of the DEP and cDEP phosphates differed notably, which allowed a reliable theoretical benchmark of the 31P NMR parameters for highly flexible and structurally constrained phosphate in a one-to-one relationship with the corresponding experiment. The calculated 31P NMR shifts were referenced employing three different NMR reference schemes to highlight the effect of the 31P NMR reference on the accuracy of the calculated 31P NMR shift. The relative Δδ(31P) NMR shift calculated employing the MD/B3LYP/Iglo-III/PCM method differed from the experiment by 0.16 ppm while the NMR shifts referenced to H3PO4 and/or PH3 deviated from the experiment notably more, which illustrated the superior applicability of the relative NMR reference scheme. The 2JP,C coupling in DEP and cDEP calculated employing the MD/B3LYP/Iglo-III(DSO,PSO,SD)/cc-PV5Z(FC)/PCM method inclusive of correction due to explicit hydration differed from the experiment by 0.32 Hz and 0.15 Hz, respectively. The NMR calculations demonstrated that reliable structural interpretation of the 31P NMR parameters in phosphate must involve both the structural and the dynamical components.

Photochemistry of Dithiocarbamate Cu(S2CNEt2)2 Complex in CHCl3. Transient Species and TD-DFT Calculations.

Nanosecond laser flash photolysis was used to study the mechanism of photochemical transformations of the diethyldithiocarbamate Cu(II) complex (Cu(dtc)2, where dtc- ≡ -S2CNEt2 anion) in chloroform solutions. The electron transfer from the excited Cu(dtc)2 complex to a solvent molecule leads to the appearance of the primary intermediate, the [ClCu(dtc)(dtcCHCl2)] complex, where a dtcCHCl2 molecule is coordinated with a copper ion via one sulfur atom. In the fast reaction (k = 2.1 × 109 M-1 s-1) with Cu(dtc)2, this complex forms a long-lived dimer [ClCu(dtc)(dtcCHCl2)Cu(dtc)2]. This intermediate decays during several seconds (k = 5.6 × 10-2 s-1) into the final product, i.e., a diamagnetic dimer [ClCu(dtc)Cu(dtc)2]. To determine the structure of intermediate complexes the quantum chemical calculations were carried out using DFT, TD-DFT, and PCM (Polarizable Continuum Model) methods.

Density functional theory computational study on solvent effect, molecular conformations, energies and intramolecular hydrogen bond strength in different possible nano-conformers of acetaminophen

The molecular modeling analysis via density functional theory has been performed on all possible conformers of acetaminophen. Computations performed at B3LYP/6-311++G(d,p) level using the Gaussian 09 program package. The polarizable continuum model method was used to investigate solvent effects on the electronic and structure properties. The optimized structures, total energies, energy gap, chemical potential, global hardness, softness, global electrophilicity and dipole moments were calculated. From 4 different obtained conformers, A1 is the most stable one with intramolecular C–H⋯O hydrogen bonding. The investigated structures contain intramolecular hydrogen bonded (HB) ring which was analyzed in detail by topological parameters in terms of electron density (ρ) and its Laplacian (∇2ρBCP) at the bond critical points and ring critical points from Bather's atom in molecular quantum theory. These parameters could be treated as measure of HB strength. Results show that the C–H⋯O and C–H⋯C bonds possess low ρ and non-covalent character (∇2ρBCP>0) whereas C–H and C–O have covalent character (∇2ρBCP<0). The energy of HB for the most stable conformer was −16.11kJ/mol. Charge density distributions and chemical reactive sites of studied conformers have been obtained by molecular electrostatic potential surface.

Microhydration effects on geometric properties and electronic absorption spectra of ortho-aminobenzoic acid

TD-DFT and a combination of polarized continuum model (PCM) and microhydration methods helped to simulate the optical electronic absorption spectrum of ortho-aminobenzoic acid (o-Abz). The microhydration method involved the use of different numbers, from 1 to 5, of first solvation layer water molecules. We examined how implicit and explicit water affected the energies of the HOMO–LUMO transition in the o-Abz/water systems. Adding until five water molecules, the theoretical spectrum becomes closer to the experimental data. Microhydration combined with the PCM method leads to agreement between the theoretical result for five water molecules and the experimentally measured absorption bands.

A theoretical study of ternary indole-cation-anion complexes.

The simultaneous interactions of an anion and a cation with a π system were investigated by MP2 and M06-2X theoretical calculations. Indole was chosen as a model π system for its relevance in biological environments. Two different orientations of the anion, interacting with the N-H and with the C-H groups of indole, were considered. The four cations (Na(+), NH4(+), C(NH2)3(+) and N(CH3)4(+)) and the four anions (Cl(-), NO3(-), HCOO(-) and BF4(-)) included in the study are of biological interest. The total interaction energy of the ternary complexes was calculated and separated into its two- and three-body components and all of them are further divided into their electrostatic, exchange, repulsion, polarization and dispersion contributions using the local molecular orbital-energy decomposition analysis (LMO-EDA) methodology. The binding energy of the indole-cation-anion complexes depends on both ions, with the cation having the strongest effect. The intense cation-anion attraction determines the geometric and energetic features in all ternary complexes. These structures, with both ions on the same side of the π system, show an anti-cooperative interaction. However, the interaction is not only determined by electrostatics, but also the polarization contribution is important. Specific interactions like the one established between the anion and the N-H group of indole or the proton transfer between an acidic cation and a basic anion play a significant role in the energetics and the structure of particular complexes. The presence of the polar solvent as modelled with the polarizable continuum model (PCM) does not seem to have a significant effect on the geometry of the ternary complexes, but drastically weakens the interaction energy. Also, the strength of the interaction is reduced at a faster rate when the anion is pushed away, compared to the results obtained in the gas phase. The combination of PCM with the addition of one water molecule indicates that the PCM method properly reproduces the main energetic and geometrical changes, even at the quantitative level, but the explicit hydration allows refining the solvent effect and detecting cases that do not follow the general trend.

Quantum chemical studies on molecular conformations, energetic and intramolecular hydrogen bonding in ground and electronic excited state of (thioxosilyl) ethyleneselenol

In this work, a conformational analysis of (thioxosilyl) ethyleneselenol was performed using several computational methods, including density-functional theory (DFT) (B3LYP), MP2 and G2MP2. Harmonic vibrational frequencies were estimated at the same levels to confirm the nature of the stationary points found and also to account for the zero point vibrational energy correction. MES-1 and TES-1 conformers exhibit hydrogen bonding. This feature, although is not the dominant factor in the stability of conformers, appears to be of foremost importance to define the geometry of the molecule. Two intramolecular hydrogen bonds established between the polar groups were identified by the structural geometric parameters. These involved the thiol and selenol functional groups and were identified and characterized by the frequency shift in their stretching vibration modes. Furthermore, the excited-state properties of intramolecular hydrogen bonding have been investigated theoretically using the time-dependent DFT method. The influence of the solvent on the stability order of conformers and the strength of intramolecular hydrogen bonding was considered using the PCM (polarizable continuum model), SCI-PCM (self consistent isodensity-polarizable continuum model) and IEF-PCM (integral equation formalism-polarizable continuum model) methods. The “atoms in molecules” theory of Bader was used to analyze critical points and to study the nature of hydrogen bond in these systems. Natural bond orbital (NBO) analysis was also performed for better understanding the nature of intramolecular interactions. The calculated highest occupiedmolecular orbital and lowest unoccupied molecular orbital energies show that charge transfer occur within the molecule. Further verification of the obtained transition state structures was implemented via intrinsic reaction coordinate analysis. Calculations of the 1H NMR chemical shift at the GIAO/B3LYP/6–311++G** level of theory are also presented.

UV-induced formation of the thymine-thymine pyrimidine (6-4) pyrimidone photoproduct — a DFT study of the oxetane intermediate ring opening

The mechanism by which the hypothetical oxetane/azetidine intermediate formed during the photochemical process leading to pyrimidine (6-4) pyrimidone photoproducts when DNA is submitted to UV radiation opens is investigated computationally by DFT using a 5'-TT-3' dinucleoside monophosphate as a structural model. First, the feasibility of an intramolecular mechanism involving one proton transfer inducing opening of the oxetane ring is examined. It results in a very high Gibbs energy of activation (+166 kJ mol(-1)) and quite a low Gibbs energy of reaction (-35 kJ mol(-1)). The protonation state of the phosphate group is shown to have little effect while the bulk effect of an aqueous environment modeled by the Polarizable Continuum Model method lowers slightly the activation barrier (by about 10-20 kJ mol(-1)), not enough to explain the fact that the oxetane intermediate is not observed experimentally. Then the catalytic effect of water molecules on the reaction pathway is studied by including either 1 or 2 assisting water molecules in the chemical system. The resulting activation barrier is considerably lowered and in the most favorable situation - a phosphate group deprotonated and 2 assisting water molecules - the Gibbs energy activation is as low as +44 kJ mol(-1) and the Gibbs energy of reaction is quite favorable: -79 kJ mol(-1), suggesting that in biological systems the oxetane ring opening process proceeds with explicit intervention of water molecules from the environment.

6-Phenyl-3-(4-pyridyl)-1,2,4-triazolo-[3,4-b][1,3,4]thiadiazole: Synthesis, experimental, theoretical characterization and biological activities

The molecular geometry, vibrational frequencies, and gauge including atomic orbital (GIAO) 1H and 13C NMR chemical shift values of the title compound in the ground state have been calculated using the Hartree–Fock (HF) and density functional theory (DFT) methods with 6-31G(d) basis sets, and compared with the experimental data. The calculated results show that the optimized geometries can well reproduce the crystal structural parameters and the theoretical vibrational frequencies, and 1H and 13C NMR chemical shift values show good agreement with experimental data. To determine conformational flexibility, molecular energy profile of the title compound was obtained by HF/6-31G(d) and (DFT/B3LYP) calculations with respect to selected degree of torsional freedom, which was varied from −180° to +180° in steps of 10°. The energetic behavior of the title compound in solvent media was examined using the B3LYP method with the 6-31G(d) basis set by applying the Onsager and the polarizable continuum model (PCM). The results obtained with these methods reveal that the PCM method provided more stable structure than Onsager’s method. The title compound has been tested in vitro for biological effects.

Pharmacological classification and activity evaluation of furan and thiophene amide derivatives applying semi-empirical ab initio molecular modeling methods.

Pharmacological and physicochemical classification of the furan and thiophene amide derivatives by multiple regression analysis and partial least square (PLS) based on semi-empirical ab initio molecular modeling studies and high-performance liquid chromatography (HPLC) retention data is proposed. Structural parameters obtained from the PCM (Polarizable Continuum Model) method and the literature values of biological activity (antiproliferative for the A431 cells) expressed as LD50 of the examined furan and thiophene derivatives was used to search for relationships. It was tested how variable molecular modeling conditions considered together, with or without HPLC retention data, allow evaluation of the structural recognition of furan and thiophene derivatives with respect to their pharmacological properties.

QSAR Analysis of Compounds Exhibiting General Anesthetics’ Properties

A set of 28 compounds with inhaled general anesthetics properties and with the literature values of their biological activity was used to search for relationships between structural parameters obtained from PCM (Polarizable Continuum Model) method. Ab initio calculations have been done for molecules in vacuo and also placed in the aquatic environment and the environment of an organic solvent (ethanol). Finally the data examined the biological activity of compounds was related to their structural multiparametric indicators using stepwise, progressive regression analysis and principal component analysis (PCA) of structural descriptors for the selected environment. Obtained results showed that values of total three descriptors: dipole moment (TDM), energy gap (EG) between energy of HOMO and LUMO and isotropic polarizability (IPOL) depends the most on the biological activity data for the checked general anesthetics in all examined environments. Keywords: General anesthetics, Molecular modeling, Structural analysis, Vacuo, PCM method, Total dipole method

A theoretical study on photomagnetic fluorescent protein chromophore coupled diradicals and their possible applications

We have designed and theoretically studied three different pairs of green fluorescent protein chromophores and their different homologue-based diradicals coupled with imino nitroxides. To begin with, the geometries of all these diradicals have been optimized at high spin (HS) state in the gas phase, in a water medium and in a blood plasma medium. The process of calculations is straightforward and well-established in the case of the gas phase. However, for calculations in water, we have adopted our own N-layer integrated molecular orbital and molecular mechanics (ONIOM) method. Similarly for the blood phase calculations, the polarized continuum model (PCM) method has been adopted. With these optimized geometries the magnetic exchange coupling constant (J) values are estimated for these diradicals in different media using the broken symmetry (BS) approach in an unrestricted DFT framework. In order to obtain the BS solutions in the ONIOM method, we have carried out ONIOM-BS, where the BS calculations are done for the inner high-level layer (diradical system) keeping the outer water layer at low level. In a similar fashion, a PCM-BS technique has also been adopted for the BS calculations in the PCM method. We have found that these diradicals have an ability to change their magnetic nature from antiferromagnetic in the trans form to ferromagnetic in the cis form upon irradiation of light with the appropriate wavelength. Using a time-dependent DFT (TDDFT) technique, the required wavelengths of light by which non-fluorescent dark trans diradicals turn into their corresponding bright fluorescent cis isomers are determined for each pair of diradicals for the gas and water media. This color change is indeed a signature of the change in magnetic state of the diradicals concerned. Here, we have also calculated the zero field splitting (ZFS) parameter (D), rhombic ZFS parameter (E) and ZFS magnitude (a2). From our calculations we ambitiously expect that if these diradicals are synthesized then they might be used as a successful, non-hazardous magnetic resonance imaging contrast agent (MRICA) in place of other metal-based contrast agents.

Combined experimental and computational modeling studies on 4‐[(2‐hydroxy‐3‐methylbenzylidene) amino]‐1,5‐dimethyl‐2‐phenyl‐1,2‐dihydro‐3H‐pyrazol‐3‐one

AbstractThe Schiff base compound, 4‐[(2‐hydroxy‐3‐methylbenzylidene)amino]‐1,5‐dimethyl‐2‐phenyl‐1,2‐dihydro‐3H‐pyrazol‐3‐one, has been synthesized and characterized by IR, UV–vis, and X‐ray single‐crystal determination. Molecular geometry from X‐ray experiment of the title compound in the ground state have been compared using the density functional method (B3LYP) with 6‐31G(d,p) basis set. Calculated results show that density functional theory (DFT) can well reproduce the structure of the title compound. The energetic behavior of the title compound in solvent media has been examined using B3LYP method with the 6‐31G(d,p) basis set by applying the Onsager and the polarizable continuum model (PCM). The results obtained with these methods reveal that the PCM method provided more stable structure than Onsager's method. By using TD‐DFT method, electronic absorption spectra of the title compound have been predicted and a good agreement with the TD‐DFT method and the experimental one is determined. The predicted nonlinear optical properties of the title compound are much greater than ones of urea. In addition, DFT calculations of the title compound, molecular electrostatic potential and NBO analysis were performed at B3LYP/6‐31G(d,p) level of theory. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

An ab initio quantum chemical characterization of structure and vibrational spectra of anthranilic acid

AbstractHigh‐level ab initio methods are used to analyze the electronic structure and vibrational spectra of the anthranilic acid or ortho‐aminobenzoic acid (o‐Abz), both in gas phase and aqueous solution. The geometries of the stationary points for the two lowest energy conformations (I and II) of o‐Abz are optimized at the MP2/6‐31++G** and MP2/aug‐cc‐pVDZ levels of theory. Conformation I is found to be more stable, less polar but more polarizable than conformation II. The same levels of calculation are used to obtain the harmonic vibrational frequencies. Polarizable continuum model method is used later to study the solvent effect on the structure and vibrational spectra of o‐Abz. Several electronic, electric, and thermodynamical properties are calculated and compared with the available experimental results and other theoretical calculations. The aqueous solution produces red‐shift for most of the vibrational frequencies but increases the intensity of vibrations for all modes of vibration of o‐Abz. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Ab initio and semiempirical computational studies on 1‐{(2E)‐2‐[(aminocarbonothioyl)hydrazono]‐2‐(3‐mesityl‐3‐methylcyclobutyl)ethyl}‐pyrrolidine‐2,5‐dione

AbstractThe molecular geometry, vibrational frequencies, and gauge including atomic orbital (GIAO) 1H‐ and 13C NMR chemical shift values of the title compound in the ground state have been calculated using the Hartree‐Fock (HF) and density functional theory (DFT) methods with 6‐31G(d) basis sets, and compared with the experimental data. The calculated results show that the optimized geometries can well reproduce the crystal structural parameters and the theoretical vibrational frequencies, and 1H‐ and 13C NMR chemical shift values show good agreement with experimental data. To determine conformational flexibility, the molecular energy profile of the title compound was obtained by semiempirical (AM1) calculations with respect to the selected torsion angle, which was varied from −180° to +180° in steps of 10°. The energetic behavior of the title compound in solvent media was examined using the B3LYP method with the 6‐31G(d) basis set by applying the Onsager and the polarizable continuum model (PCM). The results obtained with these methods reveal that the PCM method provided more stable structure than Qnsager's method. By using TD‐DFT method, electronic absorption spectra of the title compound have been predicted and a good agreement with the TD‐DFT method and the experimental one is determined. The predicted nonlinear optical properties of the title compound are much greater than ones of urea. In addition, the molecular electrostatic potential (MEP), frontier molecular orbitals (FMO) analysis, NBO analysis and thermodynamic properties of the title compound were investigated using theoretical calculations. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012

Solvent effects on the antioxidant activity of 3,4-dihydroxyphenylpyruvic acid : DFT and TD-DFT studies

In the human body, free radicals or reactive oxygen species (ROS) can be generated excessively and lead thereafter to many diseases. However, it was proved that 3,4-dihydroxyphenylpyruvic acid (3,4-DHPPA) presents an antioxidant activity sufficient to fight against these harmful molecules through different mechanisms. The antioxidant activity of this phenolic acid (PhA) was previously evaluated in the vacuum. Since most biological processes occur in solvent, in this work, solvent effects on the antioxidant activity of this PhA and its UV–Vis absorption spectrum were investigated using B3LYP/6-31+G *, 6-311++G ** and Integral Equation Formalism Polarizable Continuum Model (PCM) methods. The solvation is described in a hybrid manner, since PCM methods fail in describing solute–solvent short range interactions. The antioxidant activity of four of the twelve conformers of this PhA is evaluated thermodynamically in different media and it is pointed out that, the less stable representative conformer of 3,4-DHPPA is the most responsible of the ROS scavenging. Solvent effects on bond dissociation enthalpy is very weak (0–25 kJ mol −1), but sufficient to influence hydrogen bonds and to invert the order of the preferential sites of hydrogen atom cleavage. In addition, solvent notably influences and changes the nature of the scavenging mechanism of ROS, favouring by this way the sequential proton loss electron transfer in polar solvents and the homolytic hydrogen atom transfer in non-polar solvents. Finally, it is worth mentioning that solvent induces a considerable red shift of the absorption maximum of 3,4-DHPPA going from the vacuum, and a slight blue shift of the transition S 0 → S 1 going from less polar to more polar solvents.