Relative Energies Of Tautomers Research Articles

Effect of methylation on relative energies of tautomers and on the intramolecular proton transfer barriers of protonated nitrosamine: A MR-CISD study.

MR-CISD, MR-CISD+Q, and MR-AQCC calculations have been performed on the minima and transition states (corresponding to intramolecular proton transfer between the protonation sites) of the ground state of protonated nitrosamine and N,N-dimethylnitrosamine. Our highest level results (MR-AQCC/cc-pVTZ) for the smaller system indicate that protonation on the N amino (2a) is practically as favorable as the most favorable protonation on the O atom (1a). They also suggest that protonation on the nitroso N atom (2c) is ∼14.5 kcal/mol less favorable than 1a. Results obtained at the MR-CISD+Q/cc-pVTZ level indicate that the effect of methylation on the relative energies of the tautomers is, in order of importance, 2a > 2c and increases their energies by ∼17.5 and 4.8 kcal/mol, respectively. They also indicate that methylation alters significantly the intramolecular proton transfer barriers. The largest differences between the common geometric parameters of both systems have been found for 2a.

Density functional study of isoalloxazine and C4a-hydroperoxidihydroisoalloxazine

The structures of isoalloxazine and C4a-hydroperoxidihydroisoalloxazine have been computed at B3LYP/6-31G ∗ level. Solvent effects on relative energies of tautomers and protonation equilibria have been investigated using the SCI-PCM model. The results have been discussed in relation to biochemical properties of flavin coenzymes.

Electron correlation effects in the ab initio study on tautomerism of guanine

The molecular structures of three quanine tautomers are examined using ab initio method at the MP2/6–31G**//HF/6–31G level. The study predicts amino—oxo Gua N(7)H tautomer (1) to be the global minimum. The second amino—oxo Gua N(9)H (2) and amino—hydroxy Gua* N(9)H (3) tautomers have relative energies of 0.5 and 4.6 kJ mol−1 (MP2/6–31G**+ZPE), respectively. These results explain well recent experimental IR matrix-isolation and UV photoelectron studies on guanine. The shift of tautomeric equilibria towards 2 on going from the isolated molecules towards polar solvents is predicted. The influence of the basis set and electron correlation contributions on the relative energies of tautomers is discussed.