AM1 studies on the prototropic tautomerism of 6-thioguanine

Contreras, J. G.; Alderete, Joel B.

doi:10.1016/0166-1280(93)87139-5

Cited by 16 publications

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“…In this sense, azaadenine behaves as azapurine. 8 Since it has been found that the AM1 method does not provide a good description of atomic charge distributions and energies in this kind of molecule, [9][10][11] we applied another elimination criterion, i.e. all species possessing relative energies less than 7 kcal mol À1 at the HF/6-31G* //HF/6-31G* level will survive.…”

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Theoretical study of the tautomerism of 8-azaadenine

Contreras

Madariaga

Alderete

1998

J. Phys. Org. Chem.

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The prototropic tautomerism of 8-azaadenine (azaade) was studied theoretically by means of ab initio methods, in both the gas phase and aqueous solution. A number of tautomeric forms were not included in the calculations after applying a stepwise elimination procedure based on both AM1 and HF/6-31G* energy values. The tautomers 9H-azaade, 8H-azaade and 7H-azaade survived to this elimination and their optimized geometries and energies were calculated at the MP2/6-31*//HF/6-31G* level. To include the solvent effects, two self-consistent reaction field method were used: (1) Onsager's SCRF with multipolar expansion up to the hexadecapolar term and (2) the isodensity polarizable continuum method (IPCM). Both methods produce similar results, although the latter represents better the situation in aqueous solution. The stability order in solution, 8H-b 9H-b 7H-azaade, differs slightly from that found in the gas phase, implying that in general the electrostatic effects in solution are important, but the intrinsic stability of these species in the gas phase overcomes the solvent effect.

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confidence: 99%

Theoretical study of the tautomerism of 8-azaadenine

Contreras

Madariaga

Alderete

1998

J. Phys. Org. Chem.

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“…Because of the limitation in IPCM calculations, only single‐point calculations are possible with this method. Therefore, the gas phase molecular geometries optimized at the B3LYP/6‐311++G** level were used for the IPCM calculations and we assumed that the structures are not changed from gas phase to solutions 25, 26. The solvation free energies were taken as the difference between the energies in solution and gas phase.…”

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Theoretical investigation of the tautomerism of isoorotic acid in gaseous and aqueous phases

Cho

Mohamed

Elroby

2006

Int J of Quantum Chemistry

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In the present investigation, the tautomeric and conformational equilibrium of isoorotic acid have been studied using Møller-Plesset second-order (MP2) and density functional theory (DFT) methods in the gas phase and aqueous solution ( ϭ 78.5) using the IPCM model. The relative energies of these tautomers have been calculated at the two levels of theory using 6-311ϩϩG** basis set. Energetics and relative stabilities of the tautomers were compared and analyzed in both the gaseous and aqueous phases. The results indicate that the diketo tautomer (iso) is the most stable form in the gas phase and water. The carboxylic substitution in the uracil ring does not alter its relative stability order of the tautomers. The proton affinity of the oxygen atoms and the deprotonation enthalpy of the NH bonds of isoorotic acid have been compared with recent data of uracil. The relative stability of both syn-and anticonformations was investigated and the syn form was found to be more stable by 17.65 kcal/mol. It was determined in ab initio calculations that an electron can attach to isoorotic acid, forming a stable anion better than uracil. INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY 69 12. Karelson, M. M.; Katritzky, A. R.; Szafran, M.; Zener, M. C.

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