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Keto_Enol Tautomerism in Pyruvic Acid-Theoretical (HF, MP2 and DTF in the Vacuo) Studies

Raczyńska E., Duczmal K., Darowska M.

Polish Journal of Chemistry

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2005

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Vol. 79 / nr 4

689-697

EN

Extended quantum-chemical calculations {HF, MP2, DFT(B3LYP) in vacuo} were performed for neutral pyruvic acid and its enol forms. Among various tautomers-rotamers considered, three keto (Tce, Tte and Cte) and six enol structures (E1-E6) are found to be thermodynamically stable. The stability order for the keto and enol isomers: Tce > Tte > Cte > E1 > E2 > E3, E4, E5 > E6 is the same at each level of computations. The keto Tce structure has the lowest Gibbs free energy (G). The G value of the most stable enol E1 structure is larger than those of the three keto structures by a few kcal mol-1.

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Theoretical (DFT in Vacuo) Studies on the Structure of 2-(_-Aminoethyl)-pyridine - an Agonist of the Histamine H1Receptor. Comparison with Experiment (FT-IR in Apolar Solvent)

Raczyńska E., Darowska M., Rudka T.

Polish Journal of Chemistry

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2003

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Vol. 77 / nr 11

1529-1546

EN

Preliminary quantum-chemical calculations (DFT) were performed for 2-(_-aminoethyl)- pyridine (AEP). In calculations, rotational isomerism around the single C-C and C-N bonds of the chain aminoethyl group in AEPwas considered. Nine stable conformations (four trans and five gauche) with similar Gibbs energies were found. For all of them, vibrational frequencies were calculated at the DFT(B3LYP)/6-31G* level. Infrared (FT-IR) spectra were recorded in apolar solvent (CCl4) for various concentrations of AEP, and compared with those found by computations. The comparison shows that in apolar CCl4 solution AEP exists as a mixture of different conformers. Analysis of the experimental stretching NH and CH vibrations, and the bending in-plane NH vibrations suggests that all nine conformers of AEP can be present in CCl4 solution.

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Variations of Tautomeric Preferences in Histamine Monocation-Ab initio Studies for "Essential" and "Scorpio" Conformations from the Gas Phase to Aqueous Solution

Darowska M., Raczyńska E. D.

Polish Journal of Chemistry

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2002

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Vol. 76 / nr 7

1027-1035

EN

The polarizable continuum model (PCM) for geometries optimized at the RHF/6-31G* was applied to study the variations of the tautomeric preferences in the histamine monocation from the gas phase to aqueous solution. Seven solvents of different polarities (from cyclohexane to water) were chosen and calculations performed. A change of the tautomeric preference takes place already in apolar solvents containing heteroatoms. The ring N-aza protonated form (ImH+) is only favoured in the gas phase and cyclohexane,oe benzene, CCl4. The chain N-amino protonated form (AmH+-T1) predominates in other solvents: CHCl3, THF, acetone, water.