Dagmar Štellerová

This work provides a systematic theoretical study on gas-phase bond dissociation enthalpies for homolytic C—H bond cleavage in linear and branched alkanes. Quantum chemical calculations were performed using the density functional theory (DFT) and ab initio composite (G4(MP2), G4) methods. In case of DFT calculations, the M06-2X and B3LYP functionals combined with 6-311++G**, aug-cc-pVDZ, aug-cc-pVTZ, and aug-cc-pVQZ basis sets were covered. In linear molecules, the best agreement with experiment was observed for B3LYP/aug-cc-pVQZ. In both composite approaches, as the number of C atoms increases, the BDE values are affected by the systematic error of the method. For example, G4 BDE of terminal —CH3 group of hexane is 422.6 kJ·mol ⁻¹ while for undecane it is 415.5 kJ·mol ⁻¹ . On the contrary, the composite methods are in better agreement with experiments compared to the DFT approaches for simple branched hydrocarbons.

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.

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 any 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 explains only the symmetry descent paths C2v → C2 and C2v → Cs in hypericin, and the D2h → C2v, D2h → C2v → Cs, and D2h → C2h ones in fringelite D. The electric dipole moments of hypericin and its analogs are determined prevailingly by the mutual orientations of the hydroxyl groups. The same holds for the energies of frontier orbitals in these systems, but their changes during the symmetry descent are less significant.

Aflatoxins are naturally occurring compounds produced by fungi, mainly of Aspergillus species. All aflatoxins are proved to cause acute toxicity to human health, some even causing chronic diseases such as cancer. These molecules clearly proved that even natural molecules can be directly related to cancer and the formation of tumours. In general, aflatoxins can be characterised as organic compounds; B1, G1, B2, and G2 are produced directly by fungi and these four are subsequently metabolised in biological systems of microbes, animals or humans into other forms, such as M1, EB1, AFL etc. This manuscript provides a brief overview of 14 aflatoxins, their molecular structure and its possible relationship to aflatoxins biological activity. This information, in combination with additional calculations, offers the possibility to investigate the mentioned compounds and their properties using the QSAR approach.

The molecular structure of mycotoxins enniatin B and beauvericin, which are used as ionophores, was studied using density functional theory in various symmetry groups and singly charged states. We have shown that the charge addition or removal causes significant structural changes. Unlike the neutral C₃ molecules, the stability of the charged C₁ structures was explained by the Jahn–Teller or Pseudo-Jahn–Teller effect. This finding agrees with the available experimental X-ray structures of their metal complexes where electron density transfer from the metal can be expected. Hence, the membrane permeability of metal sandwich-structure complexes possessing antimicrobial activities is modulated by the conformational changes.

In this paper, the molecular and electronic structure of curcumin is studied. High-symmetric gas-phase tautomers and their deprotonated forms in various symmetry groups are identified. The stability of lower-symmetry structures was explained by using the Pseudo-Jahn-Teller (PJT) effect. This effect leads to stable structures of different symmetries for the neutral enol and keto forms. The presented analysis demonstrated the potential significance of the PJT effect, which may modulate the setting of electronic and vibrational (vibronic) energy levels upon photodynamic processes. The PJT effect may rationalize the photoprotection action and activity of naturally occurring symmetric dyes.

Theoretical studies on aniline, phenol, benzenethiol, benzeneselenol, and their corresponding adducts with hydroxyl radical in possible positions on a hydrocarbon ring are presented. Bond dissociation enthalpies, related to radical scavenging of primary antioxidants, were calculated using the M06–2X/6–311+G** method. Calculated data were compared with available experimental data. Preferable homolytic bond dissociation of the presented molecules with OH ^• through functional groups X—OH followed by the m –OH ones has been confirmed. The highest antioxidant activity among the investigated positions is predicted for benzeneselenol. Also, the formation of non-covalent van der Waals structures has been shown as important in radical scavenging.