Vladimír Lukeš

The thermodynamics of protonation equilibria and equilibria between enol and keto tautomers occurring in naturally found hydroxy derivatives of trans-resveratrol analogues were investigated. Reaction Gibbs free energies at room temperature were obtained using the B3LYP functional together with a continuum solvation model for water. From these data, acidity constants (pKa) and tautomerization constants (pKT) were calculated for the electronic ground state (S0) and the lowest excited singlet state (S1). The results reveal a pronounced dependence of both equilibria on the position of the hydroxy substituents. Upon photoexcitation, the compounds become considerably stronger acids and indicate, in many cases, a shift toward the keto tautomers rather than the parent enol forms. Moreover, our results support the feasibility of proton transfer from a hydroxy group on one aromatic ring to the adjacent ring via the central −CH═CH– bridge. Such processes may be relevant for nonradiative de-excitation pathways and for the protective biochemical functions of these compounds in plants.

1. Copper ions play a vital role in numerous biological processes. The interaction between copper ions and drugs used for therapy is a topic of significant interest [1]. Copper(II) ions are vital f...

The present study investigates the catalytic upgrading of bioethanol to higher-value chemicals through heterogeneous catalysis, with a focus on the production of 1-butanol as a versatile compound widely used in many applications. Copper-impregnated Li-Al mixed metal oxides were synthesized and characterized as catalysts for ethanol transformation. Comprehensive characterization techniques were involved to correlate catalyst properties with their performance and describe catalyst stability. The catalysts showed promising activity in the formation of higher alcohols under a high weight hourly space velocity (WHSV) and pressure. Theoretical calculations were incorporated into the study to describe the feasibility of individual steps in the Guerbet reaction with main side reactions involved and to supplement information regarding the influence of the catalyst itself. The temperature effects on the reactions were illustrated. In this way, we attempted to explain the role of the catalyst in the reaction pathways, contributing to a more precise tuning of the catalytic system.

The systematic ab initio G4 study of para- and di-meta-substituted phenol derivatives is presented for the gas phase and the model water environment. The thermodynamics of proton addition was considered based on calculated proton affinities for unprotonated and monoprotonated compounds. The effect of substitution on the enthalpy balance shows a linear dependence with Hammett constants for O-protonation of the neutral compounds where the aromatic character of the molecules is preserved. Depending on the substitution and the extent of protonation, O-protonation proves to be thermodynamically less favorable than C-protonation in the monoprotonated species. It was demonstrated that the highest exothermic character of protonation was found for the gas-phase and neutral keto tautomers forming under flash photolysis that have the best ability to accept a proton. The theoretical reaction Gibbs free energies were used for the quantification of prototropic tautomerism between C-protonated species and enol-keto equilibria occurring in the suggested reaction scheme. The obtained results indicate a significant change in the tautomeric equilibria between the neutral and protonated compounds. Presented theoretical calculations could be helpful in the identification of intermediates occurring in various chemical reactions of phenols in strong acidic environments.

A theoretical study of a set of unsubstituted and fluorinated small aromatic monocyclic and arene-type condensed molecules is presented. The quantum chemical calculations were performed at the density functional theory level. The fluorination effect on the Wibberg bond order and structural HOMA indices is discussed for monocyclic and bicyclic molecules with arene units. Changes in the electronic structure in the vicinity of the atoms forming the aromatic ring were analyzed using the sum of negative and positive partial atomic charges. The global electron-rich or electron-deficient character of investigated small molecules was also quantified using vertical ionization potentials and vertical electron affinities. For selected tricyclic and pentacyclic condensed molecules, the synergy of central ring modification and fluorination was investigated for the electrochemical and lowest energy optical band gaps. The geometric pattern of these compounds is either linear or angular, and it is based on possible combinations of benzene moieties with a six- or five-membered central aromatic ring. Theoretical results were compared with experimental data. The obtained data indicate that the fluorinated angular pentacyclic molecules with a central thiophene and pyridazine moiety are expected to be promising candidates for the construction of organic n-type semiconductors with respect to the setting of electronic structure as well as internal reorganization energies.

An extensive thermochemical study of γ-undecanolactone and δ-undecanolactone has been developed using two complementary calorimetric techniques. The combustion energy of each compound was determined by static-bomb combustion calorimetry, and the corresponding enthalpy of vaporization was determined by high-temperature Calvet microcalorimetry, in which both properties of each compound are reported at T = 298.15 K. The standard molar enthalpy of formation in the gas phase of each lactone was derived by the combination of the experimental results. Additionally, high-level computational calculations were carried out, using composite ab initio G4 and G4(MP2) methods, as well as DFT M06-2X/6-311++G(d,p) approach, to estimate the corresponding enthalpy of formation in the gas phase. The experimental and computational results are in good agreement. The G4 and G4(MP2) methods show the best accordance with experimentally determined gas phase enthalpies of formation. The experimental results are discussed in terms of structural contributions to the energetic properties of the lactones studied, as well as to other alkylated γ- and δ-lactones, and empirical correlations are suggested for the estimation of the standard molar enthalpies of formation, at T = 298.15 K, for other alkylated γ- and δ-lactones, both in the liquid and gaseous phases, as well as for the respective enthalpies of vaporization. Finally, the thermochemistry of individual steps of lactone ring opening and successive decarboxylation mechanism, including the identification of transition states, was studied using the M06-2X/6-311++G(d,p) approach.

The distortions and instability of high-symmetry configurations of polyatomic systems in nondegenerate states are usually ascribed to the pseudo-Jahn–Teller effect (PJTE). The geometries of hypericin, isohypericin, and fringelite D were optimized within various symmetry groups. Group-theoretical treatment and (TD-)DFT calculations were used to identify the corresponding electronic states during the symmetry descent. The symmetry descent paths (up to the stable structures without imaginary vibrations) were determined using the corresponding imaginary vibrations as their kernel subgroups starting from the highest possible symmetry group. The vibronic interaction between the ground and excited electronic states relates to an increasing energy difference of both states during the symmetry decrease. This criterion was used to identify possible PJTE. We have shown that the PJTE in these naturally occurring compounds could explain only the symmetry descent paths C_2v → C₂ and C_2v → C_s in hypericin, and the D_2h → C_2v, D_2h → C_2v → C₂, and D_2h → C_2h ones in fringelite D. The electric dipole moments of hypericin and its analogs were determined prevailingly by the mutual orientations of the hydroxyl groups. The same held for the energies of frontier orbitals in these systems, but their changes during the symmetry descent were less significant.