Krzysztof Zborowski

Determination of the most stable structures of selected hydroxypyrones and their cations and anions

Equilibrium geometries of selected hydroxypyrones were determined using quantum-mechanical calculations. Computations were performed for all possible structures of pyromeconic acid, maltol, ethylmaltol and kojic acid. Tautomerism of protonated and deprotonated (kojic acid) forms of the studied compounds were also taken into account. For all neutral tautomers of investigated compounds it was shown that the most stable was enolic form. Cationic species were created by protonation of the keto oxygen atom. In order to form stable kojic acid anion deprotonation of hydroxyl group on the pyran ring is preferred. Obtained results confirm that tautomeric equilibria in a gas phase were only slightly influenced by entropy.

Calculation of the HOMA model parameters for the carbon-boron bond

An extension of the harmonic oscillator model of aromaticity (HOMA) model to systems with carbon–boron bonds is presented. Model parameters were estimated using experimental and theoretical bond lengths. It is shown that both approaches produce very similar HOMA models. In the second part of the article, the aromaticity levels of several model compounds containing carbon–boron bonds are calculated using the previously obtained parameters. The results of these calculations are compared with those provided by other aromaticity indices. The aromaticity of boron-containing compounds is also compared with the aromaticity of analogous compounds containing carbon and nitrogen.

Resonance Raman study of Franck-Condon effects in the C10H8+ radical. Ab initio MCSCF calculations for the low-energy 2B2g and 2B3g symmetry states

Abstract Resonance Raman spectra in the region corresponding to the 2 A u ( D o ) → 1 2 B 3 g ( D 2 ) electronic excitation of the naphthalene cation (C 10 H 8 + ) are studied in terms of a model based on Franck-Condon (FC) effects. This simple model correctly accounts for the RRS spectra and yields FC parameters which correspond to those obtained from ab initio MCSCF computations. Computed FC activities of nine totally symmetric modes in the 2 B 3g (D 2 ) and 2 B 2g (D 4 ) states of the C 10 H 8 + radical are discussed and compared to experiment and earlier theoretical based studies on the ROHF/3-21G approach.

HOMA parameters for the boron–boron bond: How the introduction of a BB bond influences the aromaticity of selected hydrocarbons

An extension of the harmonic oscillator model of aromaticity (HOMA) model for systems with boron–boron bonds is presented. For the first time, the parameters of the HOMA model are estimated using only theoretically calculated bond lengths. The HOMA parameters obtained make geometric aromaticity studies possible for a large number of compounds containing the boron–boron bond. The derived HOMA parameters have been used to investigate how the introduction of the boron–boron moiety in the structure of selected hydrocarbons modifies their aromaticity. The conclusion is that the insertion of a boron–boron bond usually strongly decreases the aromaticity of the boron-containing compounds in comparison to their parent hydrocarbons.

Aromatic properties of 8-hydroxyquinoline and its metal complexes

AbstractChelatoaromaticity (aromaticity of chelate complexes) has been recently recognized as an important property influencing the stability of chelate compounds. In this paper, aromaticity of various forms of 8-hydroxyquinoline (anion, neutral molecule, zwitterion and cation) as well as its chelate complexes with magnesium and aluminium ions are investigated. Aromatic properties of these compounds are analyzed using several aromaticity indices based on energetic, geometric, magnetic and electronic physical manifestations of this phenomenon. Results of performed calculations have shown different aromatic properties for the two rings (pyridine and benzene) occurring in the studied ligand. Aromaticity of these rings in metal complexes of 8-hydroxyquinoline is significantly higher than that in corresponding ligand anion. This means that during complexation the aromaticity of the ligand increases and the chelatoaromatic effect stabilizes the studied metal complexes. In contrast, metallocyclic rings of studied metal complexes have non-aromatic properties, and, consequently, the metallocyclic ring is not stabilized by chelatoaromaticity. We conclude that, in the complex, every 8-hydroxyquinoline unit and the metal ion are separated p-electronic systems.

Raman, Surface-Enhanced Raman, and Density Functional Theory Characterization of (Diphenylphosphoryl)(pyridin-2-, -3-, and -4-yl)methanol

This work presents near-infrared Raman spectroscopy (FT-RS) and surface-enhanced Raman scattering (SERS) studies of three pyridine-α-hydroxymethyl biphenyl phosphine oxide isomers: (diphenylphosphoryl)(pyridin-2-yl)methanol (α-Py), (diphenylphosphoryl)(pyridin-3-yl)methanol (β-Py), and (diphenylphosphoryl)(pyridin-4-yl)methanol (γ-Py) adsorbed onto colloidal and roughened in oxidation-reduction cycles silver surfaces. The molecular geometries in the equilibrium state and vibrational frequencies were calculated by density functional theory (DFT) at the B3LYP 6-311G(df,p) level of theory. The results imply that the most stable structure of the investigated molecules is a dimer created by two intermolecular hydrogen bonds between the H atom of the α-hydroxyl group (in up (HOU) or down (HOD) stereo bonds position) and the O atom of tertiary phosphine oxide (═O) of the two monomers. Comparison the FT-RS spectra with the respective SERS spectra allowed us to predict the orientation of the hydroxyphosphonate derivatives of pyridine that depends upon both the position of the substituent relative to the ring N atom (in α-, β-, and γ-position, respectively) and the type of silver substrate.

SELENOMALTOL -SYNTHESIS, SPECTROSCOPY AND THEORETICAL CALCULATIONS

- Synthesis and structure of the seleno derivative of maltol (selenomaltol) is described. Structural and energetical properties of possible selenomaltol structures have been calculated at the B1LYP/6-311++G(d,p) level. The lowest energies are always predicted for the keto-enol tautomer. To verify obtained results several standard experimental methods, namely: elemental analysis, mass spectrometry, infrared and NMR spectroscopies and X-ray crystallography have been used. Investigation of IR and NMR spectra clearly indicate that the oxygen atom of exocyclic keto group on maltol was replaced by selenium. Experimental crystallographic results support this conclusion.

Theoretical study on the molecular tautomerism of the 3-hydroxy-pyridin-4-one system

3-hydroxy-pyridin-4-one is a parent molecule for the family of hydroxypyridinones that are known in coordination chemistry as efficient metal ions chelators. In this work, relative stabilities of some possible tautomers were investigated using several quantum chemical methods: CBS (complete basis set methods), Gn, DFT (density functional theory), Hartree–Fock and MP2. Performed calculations show that the system under consideration exists as a mixture of two tautomers with comparable energies. Among them, the hydroxypyridinone structure of the studied molecular system seems to be a bit more stable than the o-dihydroxypyridine one, by a few kJ/mol only. Aromaticity and intra-molecular hydrogen bonding are the main effects influencing the stability of the studied tautomeric structures. Consequently, aromatic effects were calculated using several indices of aromaticity: HOMA (harmonic oscillator model of aromaticity), NICS (nucleus independent chemical shift), H, PDI (para delocalisation index), MCI (multi-centre index) and ASE (aromatic stabilisation energy). The strength of possible intra-molecular hydrogen bonds (H-bonds) was determined by means of the AIM (atoms-in-molecules) method and by calculating enthalpies for theoretical reactions that do or do not involve H-bonds. The AIM method was employed to understand how variations in atomic energies influence the stability of different tautomeric structures.

The influence of solvent molecules on NMR spectrum of barbituric acid in the DMSO solution

This work shows the modification of barbituric acid (BA) chemical shifts by dimethylsulphoxide (DMSO) molecules. The discussed changes are caused by creation of the H-bonded associates formed by barbituric acid with DMSO in solution. Free molecule of barbituric acid, the cluster of BA with two DMSO molecules and two different clusters of BA with four DMSO units are taken into consideration. The chemical shifts of these systems have been calculated and the obtained results have been compared with experimental data. Theoretical calculations predict a significant downfield shift for imino protons of barbituric acid involved in intermolecular-N-H...DMSO hydrogen bonds. The influence of the solvent molecules on other nuclei chemical shifts, especially protons of barbituric acid methylene group, is also reported.The calculations have involved Hartree-Fock and several Density Functional Theory methods. All methods correctly describe experimental 1H and 13C NMR spectra of barbituric acid. The best consistence between experiment and theory is observed for the BLYP functional. Four approximations of magnetic properties calculations embedded in the Gaussian’98 package have been tested. The results of the performed calculations indicate that from a practical point of view the GIAO method should be preferred.

Quantum mechanical study of the tautomerism and molecular spectra of 2-hydroxy-3-methyl-2-cyclopenten-1-one

2-Hydroxy-3-methyl-2-cyclopenten-1-one (Hmcp) is a natural compound showing many chemical similarities to hydroxypyrones, which have been tested to act as ligands during the formation of novel biologically active metal complexes. In this work, we present an experimental and theoretical study of the molecular structure of such a ligand. It has been documented, based on the total energies and the total energies partitioned into atomic contributions, that the keto–enol tautomer denoted as T1 is the most stable structure of Hmcp. This result is supported by the available structural and spectroscopic data of the studied compound. Crystallographic, vibrational (infrared and Raman), and NMR (1H and 13C chemical shifts) data are compared with the results of quantum-chemical calculations. For this purpose, we calculated the energies, geometries, frequencies and intensities of the vibrational bands as well as the chemical shifts of the studied ligand at the B3LYP/6-311++G(d,p) level of theory. The calculated and experimental data are in good agreement. The results of Potential Energy Distribution analysis serve as a foundation for a thorough interpretation of the vibrational spectra.