Ewa D Raczyńska | PubMed Health

PubMed2006

Tautomeric equilibria and pi electron delocalization for some monohydroxyarenes--quantum chemical studies.

Keto-enol tautomeric interconversions and variations of the pi-electron distribution were studied for 11 isolated monohydroxyarenes at the DFT(B3LYP)/6-311++G(2df,2p) level. For two monohydroxyarenes (phenol and 9-anthrol), the PCM model of solvation (water) was also applied to the DFT geometries. The geometry-based HOMA index was applied to estimate pi-electron delocalization in the keto and enol tautomeric forms. Thermodynamic parameters of tautomeric interconversions (DeltaET, DeltaGT, TDeltaST, pKT) were calculated to estimate relative stabilities of individual tautomers and their percentage contents in the tautomeric mixtures. In almost all cases, the aromatic enol forms are strongly favored. An exception is 9-anthrol, which prefers its keto form. The resonance stabilization of this form comes from the central ring. Generally, aromaticity is the main factor that influences tautomeric equilibria in monohydroxyarenes. Hydration effect is considerably smaller and it does not change the tautomeric preference.

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PubMed2004

Experimental and theoretical evidence of basic site preference in polyfunctional superbasic amidinazine: N(1),N(1)-dimethyl-N(2)-beta-(2-pyridylethyl)formamidine.

The gas-phase basicity (GB) of the flexible polyfunctional N(1),N(1)-dimethyl-N(2)-beta-(2-pyridylethyl)formamidine (1) containing two potential basic sites (the ring N-aza and the chain N-imino) is obtained from proton-transfer equilibrium constant measurements, using Fourier-transform ion-cyclotron resonance mass spectrometry. Comparison of the experimental GB obtained for 1 with those reported for model amidines and azines indicates that the chain N-imino in the amidine group is the favored site of protonation. Semiempirical (AM1) and ab initio calculations (HF, MP2, and DFT), performed for 1 and its protonated forms, confirm this interpretation. These results are in contrast to those found previously for N(1),N(1)-dimethyl-N(2)-azinylformamidines (containing the amidine function directly linked to the azinyl ring), in which the ring N-aza is the most basic site in the gas phase. The separation of the two potential basic sites in 1 by the ethylene chain interrupts the resonance conjugation between the two functions and changes their relative basicities and, thus, the preferable site of protonation. It also increases the chelation effect against the proton and the gas-phase basicity of 1 in such a magnitude that consequently 1 may be classified as a superbase (GB = 241.1 kcal mol(-)(1)). A transition state corresponding to the internal transfer of the proton (ITP) between the ring N-aza and the chain N-imino in 1 is investigated at the DFT(B3LYP)/6-31G level. The energy barrier calculated for the ITP between the two basic sites is small and vanishes when zero-point vibrational terms and thermal corrections are applied to obtain the enthalpy or Gibbs energy of activation for the proton transfer. Additional calculations at the DFT(MPW1K)/6-31G level confirm this behavior. This indicates that the quantum-chemical ITP in 1 has a single-well character. The proton is located on the N-imino site, and the H-bond is formed with the N-aza site.

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