Svetlana Marković

Triplet fluoranthenes: Aromaticity versus unpaired electrons

AbstractThree fluoranthenes and one substituted fluoranthene, 2,2-dimethyl-2H-dibenzo[cd,k]fluoranthene, were investigated using the unrestricted symmetry-broken and complete active space methods. It was shown that four Kekuléan hydrocarbons are diradicals, implying that their ground state is a triplet. In the energetically less favorable singlet state these hydrocarbons exhibit pronounced diradical character. This occurance is explained with the tendency of the investigated molecules to delocalize their π-electrons. This leads to aromatic stabilization which is stronger than destabilization due to unpaired electrons. Our results for 2,2-dimethyl-2H-dibenzo[cd,k]fluoranthene are in excellent accord with experimental findings of McMaster et al. concerning this compound. FigureSpin density map of a triplet fluoranthene

Revisiting a simple regularity for benzenoid hydrocarbons. Total π-electron energy versus the number of Kekulé structures

Abstract The linear dependence, previously claimed, of the total π-electron energy (Eπ) of isomeric benzenoid hydrocarbons on the number of Kekule structures (K) is found to be a satisfactory approximation only for benzenoids having eight or fewer six-membered rings (h ⩽ 8), and is violated if h ⩾ 9. For sets of isomeric ubranched catocondensed benzenoids (having fixed values of h, h ⩾ 9), Eπ is well reproduced by the expression a + bK − cK − 1 2 , where a, b and c are constant.

Effect of Bay Regions on the Total π-Electron Energy of Benzenoid Hydrocarbons

Abstract The three main structural factors determining the total π-electron energy (E) of a benzenoid hydrocarbon are the number of C atoms (n), number of CC bonds (m) and the Kekule structure count (K). Nevertheless, the E-values of benzenoid hydrocarbons having the same n, m and K may considerably vary. These variations are studied and found to depend on certain structural features of the perimeter, in particular on the number of bay regions (b). A linear relation between E and b is established, such that E increases with b. Furthermore, the effect of bay regions on E increases with the increasing size of the molecule.

Comparative study of the antioxidative activities of caffeoylquinic and caffeic acids

A detailed conformational analysis was performed to determine the most stable conformers of chlorogenic, cryptochlorogenic, and neochlorogenic acids. The simulated and experimental NMR spectra of caffeoylquinic acids are in excellent agreement. The bond dissociation enthalpies, proton affinities, electron transfer enthalpies, ionisation potentials, and proton dissociation enthalpies for these compounds and caffeic acid in benzene, methanol, and water were used for thermodynamic consideration of the major antioxidative mechanisms: HAT (Hydrogen Atom Transfer), SPLET (Sequential Proton-Loss Electron-Transfer), and SET-PT (Single Electron Transfer - Proton Transfer). All compounds are characterised with very similar values of each enthalpy, suggesting that they will exhibit comparable antioxidative activities. This assumption is in perfect accord with the experimental findings. It was suggested that HAT may be the predominant mechanism in nonpolar solvents, while HAT and SPLET are competitive pathways in polar media. All calculations were performed using the B3LYP-D2/6-311++G(d,p) and M06-2X/6-311++G(d,p) levels of theory and CPCM solvation model.

Vanillic Mannich bases: synthesis and screening of biological activity. Mechanistic insight into the reaction with 4-chloroaniline

One-step multi-component Mannich reaction of vanillin, aromatic amines (aniline and 4-chloroaniline), and cyclohexanone was successfully catalyzed by three chloroacetate ethanolamine based ionic liquids: diethanolammonium chloroacetate, and newly synthesized ethanolammoniumchloroacetate and N,N-diethylethanolammoniumchloroacetate. These reactions were performed in ethanol at room temperature. Mechanistic aspects of the reaction with 4-chloroaniline were considered by using density functional theory. The yield of obtained Mannich bases (MB-Cl and newly synthesized MB-H) was very good, while diastereoselectivity was excellent. These compounds were evaluated for their in vitro antioxidative activity by DPPH free radical scavenging assay. It was shown that both bases exhibit high activity against DPPH. In vitro cytotoxic and antioxidative effects of MB-Cl and MB-H against human breast carcinoma MDA-MB-231 and human colon carcinoma HCT-116 cell lines were also determined. The investigated Mannich bases show moderate or very weak cytotoxic effect on HCT-116 cells, while no cytotoxic effect was observed in the case of MDA-MB-231 cells. On the other hand, the tested substances induced oxidative stress in the treated cancer cell lines.

A Case of Breakdown of the Kekulé–Structure Model

For a long time, Kekulé structures have been used to predict and rationalize the stability, geometry, and π-electron properties of polycyclic conjugated molecules, especially hydrocarbons. We now point out an example, demonstrating that the Kekulé–structure model is not generally applicable. Namely, the molecule of dibenzo[cd,mn]indeno[123, gf]pyrene is perfectly planar and strain-free. Yet, its geometry (determined by means of an unrestricted symmetry–broken UB3LYP/6-311G(d,p) DFT method) is in complete disagreement with what one would expect on the basis of its Kekulé structures.

Triethanolammonium acetate as a multifunctional ionic liquid in the palladium-catalyzed green Heck reaction.

AbstractAn efficient green Heck reaction protocol was performed using a triethanolammonium acetate ionic liquid–palladium(II) catalytic system. The ionic liquid used acts as a reaction medium, base, precatalyst-precursor, and mobile support for the active Pd species. Our experimental investigation indicates that performing the Heck reaction in ionic liquid is superior to the same procedure carried out in triethanolamine. The mechanism of the reaction of triethanolammonium acetate with PdCl2 was examined using density functional theory (M06 method). It was found that two Pd(II) complexes are formed, one of which acts further as a precatalyst yielding catalytically active Pd(0) complex. The calculated activation energies are in agreement with our experimental findings. FigureOptimized geometries of triethanolammonium acetate ([TEA][HOAc]), andtrans-[PdCl2(TEA)2] (Pd-N,N and Pd-N,O) complexes)

The evaluation of the eighth moment for benzenoid graphs

SummaryThe evaluation of the eighth moment of the adjacency matrix of benzenoid graphs is considered. It is found that the eighth moment can be expressed in terms of 7 graphical invariants. By this we extend the recently obtained results of Hall [1] and Dias [2].

DFT study on singlet diradical character of zethrenes

The diradical character of zethrenes was investigated using a symmetry-broken UB3LYP/6-311G(d,p) method. The number of hexagons in the investigated molecules ranges from 6 to 12. It was found that all zethrenes are singlet diradicals, whose diradical character increases with the increasing size of the molecules. A singlet diradical structure provides a possibility for an electron pair to occupy different parts of space, and allows for achieving aromatic stabilization. It can be predicted, on the basis of the singlet-triplet values, that even higher zethrenes will be singlet, but not triplet molecules.

Mechanism of the Kolbe-Schmitt reaction with lithium and sodium phenoxides

The mechanisms of the carboxylation reactions of lithium and sodium phenoxides are investigated by means of the DFT method with a CEP-31 + G(d) basis set. The introduction of diffusion functions does not affect the outcome of the calculations. As a consequence, the results of this investigation are in good agreement with the findings obtained by means of the LANL2DZ basis set. Lithium phenoxide yields only salicylic acid in the Kolbe-Schmitt reaction. The reaction of sodium phenoxide can proceed in the ortho and para positions, but the para-substituted product can be expected at a very low concentration in the reaction mixture. The deviation of lithium and sodium phenoxides from the mechanisms of carboxylations of other alkali metals is a consequence of the small ionic radii of lithium and sodium.