Tautomerism stabilities

Table VI collects the theoretical quantities appropriate to the description of the tautomeric stability of cytosine, computed by methods including both -n- and cr-electrons. We have added to it the results of a 77-SCF MO approach (including cr-polarization effects) on heats of atomization of different tautomeric forms of cytosine. Bodor et al.liB have related the heat of atomization to the stability of the tautomers, and found that the most stable structure of the molecule is the amine-lactam 2 having the greatest heat of atomization (59.707 eV). They predict on this basis the following order of stability 2 > 3 > 6 > 1.

For other CNDO/2 calculations on tautomeric stability of cytosine, see refs. 14, 152, 153. In all these calculations the same geometry of the ring has been used for different tautomers. 

Theoretical Quantities0 Characterizing the Tautomeric Stability of Uracils and Thymines Method References Compound Quantity6 (kcal/mole)... 

Quantum-mechanical studies on the tautomeric stabilities of the rare pyrimidine bases are very scarce. Tentative CNDO/2 calculations have been carried out156,440 on the energies of some of them. They showed that in t.hiouracils the most stable form in each case is form 51. In 2-thiocytosine, two forms, 52 and 55, are nearly equivalent (form 52 is more stable than 55 by 4 kcal/mole), and isothiocytosine form 56... 

TheokbticaIj Quautities Chahaotbbizestg the Tautomeric Stability op Ueacils ahd Thymines... 

Using the same geometrical positions for heavy atoms of the two tautomers. For other CNDO/2 study of tautomeric stability of uracil, see refs. 153, 156. 

Phosphorus nuclear magnetic shielding anisotropy in (1-hydroxyalkyl)dimethyl-phosphine sulfides has been studied using the IGLO method, and the tautomeric stability, molecular structure, and internal rotation of methylphosphonic dicyanide MeP(0)(CN)2, dicyanomethoxyphosphine MeOP(CN)2, and their isocyano analogues have been extensively followed using ab initio calculations. ... 

Heterocyclic /J-enamino esters differ in many respects from enamines,2 open-chain /S-enamino esters,2 and heterocyclic enamines,3 all of which point to a specific electron distribution in their enaminocarbonyl chromophore. In the following, particular attention is paid to spectroscopic studies of these title compounds and to the consequences for their structures, protonation behavior, tautomerism, stability, and mesomeric forms. 

A recent study on relative thermodynamic contributions to the tautomeric stabilities of NABs concluded that the temperature-dependent enthalpy and entropy contributions are relatively small, and inclusion of these contributions is necessary only when experimental data indicate that the... 

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