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The influence of pure solvent and binary solvent mixtures on the optical properties of non-steroidal anti-inflammatory drug (NSAID) molecule, specifically flufenamic acid (FLA), has been studied using absorption and fluorescence spectroscopic techniques. A bathochromic shift is observed in the emission wavelength of FLA with an increase in the solvent polarity of pure solvents, attributed to the highest stability of the excited state. The spectral properties are correlated with Lippert–Mataga, solvent polarity parameter, Kamlet, and Catalan solvent polarity parameters, suggesting that both general and specific solvent effects influence the spectral properties, with general solvent effects dominating over specific solvent effects. Preferential solvation studies have been carried out in tetrahydrofuran (THF) and water solvent mixture, revealing variations in the preference for solvation of FLA as a function of the mole fraction of water. Solvatochromic data are utilized to estimate the excited state dipole moment in pure solvents using different solvatochromic methods, revealing that the excited state dipole moment of FLA is higher than its ground state counterpart. The increase in dipole moment in the excited state compared to the ground state is explained based on intramolecular charge transfer (ICT), with elaboration and discussion on the possible resonance structure corresponding to ICT, inferring the more polar nature of the excited state compared to the ground state. The effect of solute polarizability on the excited state dipole moment and change in dipole moment is also discussed.

Understanding the acid-base interactions is important in chemistry, biology and material science as it helps to rationalize materials properties such as interfacial properties, wetting, adhesion and adsorption. Quantitative relation between changes in enthalpy (ΔH) and frequency shift (Δν) during the acid-base complexation is particularly important. We investigate ΔH and Δν of twenty-five complexes of acids (methanol, ethanol, propanol, butanol and phenol) with bases (benzene, pyridine, DMSO, Et₂O and THF) in CCl₄ using intermolecular perturbation theory calculations. ΔH and Δν of complexes of all alcohols with bases except benzene fall in the range from -14 kJ mol^-1 to -30 kJ mol^-1 and 215 cm^-1 to 523 cm^-1, respectively. Smaller values of ΔH (-2 kJ mol^-1 to -6 kJ mol^-1) and Δν (23 cm^-1 to 70 cm^-1) are estimated for benzene. Linear correlations are found between theoretical and experimental values of ΔH as well as Δν. For all the studied complexes, ΔH varies linearly (R² ≥ 0.97) with Δν concurrent with the Badger–Bauer rule yielding the average slope and intercept of 0.053(± 0.002) kJ mol^-1 cm and 2.15(± 0.56) kJ mol^-1, respectively.

An approach for computing accurate redox potentials in enzymes is developed based on the free energy perturbation technique in a QM/MM framework. With an appropriate choice of the QM level and QM/MM coupling scheme, the intermolecular interaction between the redox center and the protein environment can be adequately described; the speed of QM/MM methods also allows a sufficient configurational sampling for the convergence of free energy derivatives. Following the implementation into the simulation package CHARMM, the method was tested with an application to the first reduction potential of FAD in cholesterol oxidase (Chox). In addition to an accurate QM level and adequate conformational samplings, the effect of long-range electrostatic interactions due to the bulk solvent was also found to be essential. Using a semi-empirical density functional theory (SCC-DFTB) as the QM level, and a multi-stage charge-scaling scheme based on Poisson–Boltzmann calculations for the solvation effect, satisfactory agreements with experimental measurements were obtained. The study of Chox also indicates that large errors in the calculated redox potential might arise if changes in the conformational properties of the protein during the redox process are not taken into account, such as in energy minimization type of studies based on only the X-ray structure of the enzyme in one redox state.

The optimized geometries of the methoxy, ethoxy, iso-propoxy and tert-butoxy anions were calculated using the B3LYP/6-311++G(d, p) theoretical level. Thereby, these structures were used to explore their hydration clusters by means of the AGOA methodology. Analyzing the molecular electrostatic potential of the solute and using the TIP4P model to locate the water molecules over the alkoxide anions, it was observed that the energies of the hydration clusters decrease in order from methoxy to the tert-butoxy. These results corroborate the experimental acidity of the correspondent conjugate alcohols, indicating that the AGOA methodology can be used as an efficient computational methodology to appraise the structures of molecules in aqueous media.

Studying the solvation of an extra proton is important for understanding the proton transfer mechanism in polymer electrolyte membrane fuel cell. We study the interaction of hydronium ion and Zündel ion with water molecules in their first solvation shell using density functional method. A many-body analysis approach has been used to know the contribution of many-body energies to the binding energy of the hydronium ion–(water)₃ and Zündel ion–(water)₄ hydrogen bonded complex. It was observed that not only two-body energies but three-body and four-body energies also contribute significantly to the binding energy of the hydronium ion–(water)₃ and Zündel ion–(water)₄ complexes. The binding energy for the former is -32.14 kcal/mol whereas that for the latter is -48.48 kcal/mol. The percentage contributions of the many-body energies to the binding energies for these complexes are reported. The contribution from the relaxation energy to the binding energy of hydronium ion–(water)₃ and Zündel ion–(water)₄ complexes is 6% and 4.58%, respectively.

Formate ion and formic acid are linked in water by the equilibrium for the acidic dissociation of formic acid, which as the simplest carboxylic acid is an important model system. In this study, the microscopic details of the solvation around a formate ion and around a formic acid molecule in aqueous solution are explored by ab initio molecular dynamics simulations, at 300, 500, 700, and 900 K. The formate ion exerts a strong influence on the surrounding solvent molecules by hydrogen bonding, which restricts the access of other water molecules. With rising temperature, the hydrogen bonds are disrupted, and the space around formic acid becomes more accessible. Solvation of the formic acid is marked by its partial dissociation to produce a proton, and the hydrogen bond interaction around a formic acid is not as strong as that around a formate ion. The acidic dissociation becomes less favorable as temperature rises, which indicates a lesser catalytic role for the water molecules in the thermal dissociation of formic acid.

Methods for computing electrostatic interactions often account implicitly for the solvent, due to the much smaller number of degrees of freedom involved. In the Poisson–Boltzmann (PB) approach the electrostatic potential is obtained by solving the Poisson–Boltzmann equation (PBE), where the solvent region is modeled as a homogeneous medium with a high dielectric constant. PB however is not exempt of problems. It does not take into account for example the sizes of the ions in the atmosphere surrounding the solute, nor does it take into account the inhomogeneous dielectric response of water due to the presence of a highly charged surface. In this paper we review two major modifications of PB that circumvent these problems, namely the size-modified PB (SMPB) equation and the Dipolar Poisson–Boltzmann Langevin (DPBL) model. In SMPB, steric effects between ions are accounted for with a lattice gas model. In DPBL, the solvent region is no longer modeled as a homogeneous dielectric media but rather as an assembly of self-orienting interacting dipoles of variable density. This model results in a dielectric profile that transits smoothly from the solute to the solvent region as well as in a variable solvent density that depends on the charges of the solute. We show successful applications of the DPBL formalism to computing the solvation free energies of isolated ions in water. Further developments of more accurately modified PB models are discussed.

Understanding the thermodynamic driving forces underlying any chemical process requires a description of the underlying free energy surface. However, computation of free energies is difficult, often requiring advanced sampling techniques. Moreover, these computations can be further complicated by the evaluation of any long-ranged interactions in the system of interest, such as Coulomb interactions in charged and polar media. Local molecular field theory is a promising approach to avoid many of the conceptual and computational difficulties associated with long-ranged interactions. We present frameworks for performing alchemical free energy calculations and non-Boltzmann sampling with local molecular field theory. We demonstrate that local molecular field theory can be used to perform these free energy calculations with accuracy comparable to traditional methodologies while eliminating the need for explicit treatment of long-ranged interactions in simulations.

This paper summarizes what was discussed in a lecture at the IXth International Conference on Porphyrin Chemistry held in Suzdal (Russia) in September 2003. The development of porphyrin chemistry in former Soviet Union countries and over the world are discussed. Some ideas which were cultivated during three conditional porphyrin periods (1930-1960, 1960-1980 and 1980-2000) are also discussed.

Theoretical studies on porphyrin-ethanol solvates containing from 1 to 8 ethanol molecules have been carried out by density functional theory (DFT) calculations using hybrid (B3LYP) and meta-GGA (M06-L) functionals. It was established that porphirin molecule can be considered as two centered protones acceptor. Specific solvation is at the base of porphyrin-(EtOH)n interactions and results in a distortion of macrocycle planar structure. The nucleophilic solvation energy of the porphirin pyrrole demonstrates dependence on the number of molecules in the ethanol solvate, being the highest for the first two ethanol molecules and decreasing with ethanol association in the solvation shell. Results obtained by this way are in good agreement with data derived from ¹H NMR.

We studied the interaction of water-soluble cationic metalloporphyrins, $p$Py and TMPyP, with the DNA double helix in ionic liquid solutions using CD and absorption spectra. The dispersibility of their porphyrin complexes was improved by electrostatic interactions of the anions of the ionic liquid. In the case of Cu(II) and Zn(II) complexes, the $p$Py complexes with pyridinium cations attached to the meso-phenyl groups via methylene groups intercalated into the DNA double helix, while the compact TMPyP complexes with pyridinium cations at the meso-position could not interact with DNA. Thus, it was suggested that the solvation behavior by ionic liquids depends on the structure of the porphyrin and also affects its interaction with DNA. Neither Mn(III) complexes with axial ligands could interact with DNA, regardless of the presence of the ionic liquids, due to their steric hindrance.

The stability of an organic compound depends on its nature and the environment in which it is placed. It depends on the presence or absence of reagents (acid, base, oxidizing agent, reducing agent, light, etc.) and catalysts. The stability may not be the same in the solid, liquid or gaseous state. In a homogenous solution, the stability might be affected by the polarity of the solvent and its concentration. For instance a polar solvent favors ionization. In a non-polar solvent ionization is difficult. In the gas phase ionization never occurs. In the gas phase and in solution stability depends on pressure and the presence of impurities. We are interested here in the thermal stability of pure compounds in the gas phase or in non-polar solvents under one atmosphere…

The wide scattering of experimental data shows unambiguously that Nafion^®117 conductivity is very sensitive to climatic conditions, temperature and relative humidity. Conductivity measurements have been carried out by impedance spectroscopy between 10°C to 95°C in abroad domain of relative humidity i.e. 10 to 98%RH. These accurate data enable a power relationship to be proposed at constant temperature between conductivity and relative humidity. This suggests that the solvation process of a sulfonic group involves four water molecules. Assuming a protonic mobility weakly dependent on temperature, a solvation enthalpy of a perfluorosulfonic acid group by water of −135 kJ·mol⁻¹ is deduced from conductivity variations with the temperature.