Ab initio study of 2,4-substituted azolidines. I. Tautomerism

Proton transfer in the benzimidazolone and benzimidazolthione tautomerism process catalyzed by polar protic solvents

The tautomeric equilibrium of benzimidazolone and benzimidazolthione have been studied by the density functional theory method using the CAM-B3LYP functional together with the 6-311G(d,p) basis set. Two separate mechanisms have been investigated: a direct intramolecular transfer using the polarizable continuum model and an indirect proton transfer assisted by a molecule of solvent (C6H12, H2O, CH3OH, and H2O2). In both cases, the results obtained indicate that ketone and thione are the most stable forms. However, the enhanced height of the activation barrier for the four-center mechanisms describing the tautomerism reaction as a direct intramolecular transfer implicates a relatively disadvantaged process. The participation of a polar protic solvent molecule allows the lowering of the activation energy barrier. Potential energy profiles of keto-enol and thio-enol tautomerism assisted by methanol and water are very different. The former one describes a concerted mechanism but the latter does not because it is associated with asynchronous processes that take place during the thio-enol tautomerism.

2-Cyanobenzoic Acids: Tautomeric Equilibria Study

Evidence of ring–chain tautomerism in the gas phase from mass spectrometry and in solution determined by means of 1H NMR and 13C NMR is reported for 2-cyanobenzoic acids. The analysis of the corresponding spectra has allowed specific assignment of fragment ions to tautomers. The predictive value of this methodology is supported by the influence of the substitution pattern of these compounds on this equilibrium.

TAUTOMERISM IN NEUTRAL AND DEPROTONATED 6-AMINO-5-FORMYLURACILS: A PM3-COSMO APPROACH

In order to identify the structure of the most stable tautomers of 6-amino-5-formyluracil and its N-methylated derivatives in different pH conditions, relative stabilities of potential tautomers in aqueous phase have been calculated taking into account the entropy effects over the tautomeric equilibria. In each medium, the tautomer with lower energy must be the most representative form at the corresponding pH and the knowledge of the effect of the medium in the tautomerization energies allows evaluating the possible effect of the medium over the molecular stability. The results show that, in aqueous phase, the order of basicity of protonation sites in the 6-amino-5-formyluracil derivatives is N 9 > ( N 3 > N 1) > ( O 8 > O 4 > O 2). The 6-amino-5-formyluracil derivatives act as an amino-formyl-keto tautomer with a strong imino character, although, upon successive deprotonations, a growing contribution of enolyzed forms which explain the increasing labilization of carbonyl bonds must be pointed out.

Theoretical investigation on tautomerism and NBO analysis of 3-hydroxy-1,2,5-thiadiazole derivatives: solvent and substituent effects

Thiadiazole compounds including 3-hydroxy-4-substituted-1,2,5-thiadiazole are known as non-nucleoside inhibitors of HIV-1. Density functional calculations at the B3LYP/6-311++G(d,p) level were employed to determine the stability, tautomerism, and substituent effects of 1,2,5-thiadiazole, a widely used skin-whitening, antibrowning, and antibacterial agent, and its derivatives (4-NO

Ab initio study of 2,4-substituted azolidines. II. Amino-imino tautomerism of 2-aminothiazolidine-4-one and 4-aminothiazolidine-2-one in water solution

The relative stabilities of the tautomers of 2-aminothiazolidine-4-one and 4-aminothiazolidine-2-one were calculated at the MP2/6-31+G(d,p) level by considering their mono- and trihydrated complexes. Single-point calculations at the MP4/6-31+G(d,p)//MP2/6-31+G(d,p) level of theory were performed to obtain more accurate energies. The values of proton transfer barriers in the isolated, mono- and trihydrated tautomers of 2-aminothiazolidine-4-one (2AT) and 4-aminothiazolidine-2-one (4AT) were calculated for two different mechanisms of tautomerisation. In the absence of water, the process of proton transfer should not occur. Addition of water molecules decreases the barrier making the process faster, as the participation of two water molecules in a proton transfer reaction is more favorable than the participation of only one water molecule. To estimate the effect of the medium (water) on the relative stabilities of the tautomers of the studied compounds we applied the polarizable continuum model (PCM). (13)C NMR chemical shieldings were calculated using the GIAO approach at MP2/6-31+G(d,p) optimized geometries. HF and the DFT B3LYP functional with 6-31+G(d,p) basis set were employed. The quantum chemical results for the chemical shifts in gas phase and in polar solvents (water and DMSO) were compared with experimental data. TD DFT B3LYP/aug-cc-pVTZ calculations were performed to predict the absorption maxima of tautomers A and B of 2AT and 4AT.