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Tautomeric Equilibria in Solutions of 1-Phenacylpyridazin-1-ium and 1-(o-Fluorophenacyl)pyridin-1-ium Cations. DFT and NMR Studies

Gawinecki R., Dobosz R., Ośmiałowski B., Zakrzewska A.

Polish Journal of Chemistry

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2009

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Vol. 83, nr 9

1623-1628

EN

Although 1-(2-oxo-2-phenylethyl)pyridazin-1-ium and 1-[2-oxo-2-(o- fluoro phenyl - ethyl)]pyridin-1-ium cations may equilibriate with the respective enol and enaminone tautomeric forms, both experimental NMR data and results of ab in itio calculations show the keto forms to be the only species present in DMSO solution. Intramolecular hydrogen bonds and pi electron delocalization seem to have negligible effect on stabilization of the tautomers. On the other hand, electrostatic repulsions between the onium nitrogen and hydroxy oxygen atoms in the compounds studied are considered to be responsible for the in stability of the enol and enaminone tautomeric forms.

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Formyl Group as a Source of AGIBA Effect in Mono-, Di- and Tri- Formylo Derivatives of Benzene

Frączak B. T., Krygowski T. M., Cyrański M. K., Korybut-Daszkiewicz B

Polish Journal of Chemistry

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2008

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Vol. 82, nr 10

1999-2007

EN

Experimental geometry of 1,3,5-triformylobenzene and the optimized geometry of mono, meta- and para- diformylo - and 1,3,5-triformylo- benzene deriva tives were used for analysis of structural and energetic consequences of angular group induced bond alternation (AGIBA) effect of substituent. The effect is mostly observed by a substantial imbalance of the Kekulé structures of the ring in molecules in questions - the cis-type bonds in the ring in respect to CO bond in the formyl groups are al ways longer than the trans ones. Energetic differences between molecules with and with out AGIBA ef fects are rather small. Key words: substituent effect, pi-electron delocalization, canonical structures, AGIBA

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Consequences of Covalent and Non-Covalent Interactions on Conformational Preferences and pi-Electron Delocalization for the Substrate (Pyruvate) and Inhibitor (Oxamate) of Lactate Dehydrogenase - DFT Studies

Raczyńska E. D., Hallmann M., Duczmal K.

Polish Journal of Chemistry

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2008

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Vol. 82, nr 5

1077-1090

EN

Conformational preferences and pi-electron delocalization for the isolated substrate (pyruvate) and inhibitor (oxamate) of lactate dehydrogenase (LDH), their protonated forms, adducts with alkali metal cations, and complexes with imidazole and/or the guanidinium cation (models for His195 and Arg171 of the LDH pocket, respectively) have been investigated at the DFT(B3LYP)/6-31++G(d,p) level. Covalent and non covalent bonds in acids, adducts and complexes cause greater conformational changes for pyruvate than for oxamate. Delocalization of pi electrons for the OCO frag ments is similar for similar structures. For isolated anions, adducts and complexes, the HOMED(OCO) values are between 0.9 and 1.0. For protonated forms, the HOMED(OCO) val ues are re duced to 0.55-0.75. Delocalization of pi electrons for the CCO fragment in pyruvate and for the NCO fragment in oxamate in creases when the anions form covalent and non-covalent bonds with other ions and molecules. The HOMED(CCO) values vary from 0.1 to 0.5 forsigma-pi conjugated pyruvate, whereas the HOMED(NCO) values vary from 0.9 to 1.0 for n-pi conjugated oxamate.

4

Tautomeric Equilibria and pi-Electron Delocalization for the Substrate (Pyruvate) and Inhibitor (Oxamate) of Lactate Dehydrogenase - DFT Studies

Raczyńska E. D., Duczmal K., Hallmann M.

Polish Journal of Chemistry

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2007

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Vol. 81, nr 9

1655-1666

EN

Keto-enol tautomerism for the substrate (pyruvate) of lactate dehydrogenase (LDH) and amideiminol tautomerism for its inhibitor (oxamate) were studied at the DFT(B3LYP)/6-31++G(d,p) level. Both anions (and also both radicals) prefer the C=O forms, i.e. the keto and amide form, respectively. The OH forms (enolpyruvate and iminoloxamate) have higher Gibbs free energies. Their amounts in the tautomeric mixtures are larger than 0.01% for anions and lower than 0.001% for radicals. pi-Electron delocalization for OCO fragments is greater than that for XCO fragments for both anions and radicals.

5

Intramolecular O–H…O Hydrogen Bonds – the Influence of External Agents on Their Properties

Grabowski S.

Polish Journal of Chemistry

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2007

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Vol. 81, nr 5-6

799-811

EN

Intramolecular O–H…O hydrogen bond in malonaldehyde is analyzed. Ab inito MP2/6- 311++G(d,p) calculations were carried out for this isolated moiety as well as for malonaldehyde assisted by external systems: Li+, Na+ ions and HF molecule. The latter moieties act as Lewis acids interacting with the oxygen carbonyl atom or with the oxygen hydroxyl atom. The influence of these external agents on the characteristics of H-bond, the -electron delocalization within six-member pseudo-ring and on the proton transfer process is also analyzed. The Bader theory is also applied to characterize intramolecular O–H…O hydrogen bond.