Solvent effects tautomer equilibrium

NO2, 3-NO2, 4-Br, 4-Ph, 4-Me, 4-MeCONH, and 4-MeO, log = 4.56, 5.06, 4.46, 4.49, 2.08, 4.00, 0.67 and 2.47, respectively) show that in solution the open-chain tautomer 42A is strongly stabilized by solvation effects. An increase in the polarity and polarizability of the solvent shifts the equilibrium toward the open-chain tautomer (y < 0, p < 0). The higher polarity of the open-chain tautomer may be caused by intramolecular hydrogen bonds, as shown in 43 or 44. An increase in the general basicity of the... 

Gas-phase NMR spectroscopy has been used to obtain equilibrium constants and rate constants for many low-energy molecular processes. These data have been used to address questions regarding the relative stability of conformers and tautomers in the gas phase, the kinetics of exchange processes in the gas phase, and the direction and magnitude of solvent effects on these equilibria and processes. Most of the studies have appeared in the last 10 years. Continued progress in NMR instrumentation and techniques as well as considerable recent developments in kinetic theory ensure that the next 10 years will see many novel applications of gas-phase NMR spectroscopy. 

Preferred geometry of the benzene oxide-oxepin system can be predicted by molecular orbital methods. Thus benzene oxide la is predicted to be markedly non-planar (with the epoxide ring at an angle of 73° to the benzene ring), while the oxepin lb has been predicted to prefer a shallow boat structure (MINDO/3) or a planar structure ab initio) As previously mentioned, the proportion of each tautomer present at equilibrium is both temperature and solvent-dependent. Molecular orbital calculations have been used to rationalize the solvent effects, both in terms of the more polar character of the arene oxide that is favored in polar solvents and the strengthening of the oxirane C-C bond upon coordination of the oxygen atom lone pair in polar solvents. Thus values in the range 1.5-2.0 D and 0.76-1.36 D for the dipole moments of arene oxide la and oxepin lb have been calculated. 

The solvent effects of 25 different solvents, including polar, dipolar, aprotic, and amphiprotic examples, on the tautomeric equilibrium of /-butyl quinaldyl ketone 121 (R1 = /-Bu R2 = H) in semi-dilute solutions have been studied by H NMR and UV spectroscopy. An increase in temperature or in solvent polarity shifts the 121a 121c equilibrium toward the ketimine tautomer 121a (82JHC785). 

X-ray analysis showed that isoquinolines 125 (R1 = H, R2 = H, Me R1 = OMe, R2 = Me) exist in the solid state as NH-tautomers 125a. The tautomeric equilibrium does not shift noticeably in a solution, and no solvent effects were observed. The MNDO calculations, however, predict the OH-tautomer 125b to be the most stable, the experimentally observed predominance of 125a being explained by nonspecific solvation effect (93IZV288, 95KG922). 

The stability of the pyrazolinone tautomers was related to their scale of aromaticity.Experimental values for the tautomerism of pyrazolinone derivatives were obtained by flash vacuum pyrolysis (FVP), which allowed the calculation of the effect of the temperature on the equilibrium between tautomers. The variation of Kj with the temperature was determined by H NMR in solution for the model compound l-phenyl-3-methyl-2-pyrazolin-5-one (Scheme 25), showing that at 0 K the only tautomers present are either the NH or the OH ones. Temperature and solvent effects on the prototropic equilibrium of the model compound in solution showed that... 

An NMR study has shown that the amino form (419) is the predominant tautomer of 2-[arylamino(imino)]-4a,5,6,7,8,8a-hexahydro-(47f)-l,3,4-benzoxadiazines. The proton-transfer tautomerism of 4-azabenzimidazole, mediated by hydrogen-bonded complexes, has been studied in the ground and excited states by means of absorption and emission spectroscopy, and it has been shown that 6-nitro and 8-nitro groups in the tetrazolo[l,5-a]pyridine molecule exhibit completely different influences on the tetrazole-azide equilibrium. The solvent effect on the azo-hydrazone tautomeric equilibrium of l-phenylazo-4-naphthol has been modelled by using ab initio quantum-chemical calculations. The results have shown that the hydrazone form is more stable in water, while methanol stabilizes the azo form. ... 

VO(L ° )(OMe)(MeOH)] (2-MeOH) was studied by and NMR spectroscopies and DFT calculations, and the difference between the NMR chemical shifts of the two diastereomers for 1 or 2 is 30 or 28 ppm, respectively. The chemical speciation of 2-amino-N-hydro-(p-alaninohydroxamic acid, HL) and V(v) in aqueous solution was studied using the results obtained by the calculations of the thermodynamic properties and the NMR chemical shifts of the species formed at equilibrium and the experimental NMR measure-ments. The NMR chemical shifts were calculated by DFT approach accounting for relativistic corrections and solvent effects and all the tautomers of the 1 1 and 1 2 V02 /p-ala complexes with different... 

Enol imine-enaminone and phenol—quinone tautomerism in (arylazo) naphthols and in analogous Schiff bases were studied by Fabian et al. [92, 93]. In all these molecules there is a favorable N- -H- -O intramolecular hydrogen bond. Depending on the X-H sigma bond (X = N, O), there are two possible tautomers in solution. The solvent effect was calculated on the equilibrium [92], and a combined effect of the solvent and the benzene substituent was studied in [93]. While the FEP/MC simulations provided consistent organic solvent effects in accord with the experimental results [92], the wide spectrum of the solvent-effect calculation methods could predict rather diverse results for several groups of systems in [93]. 

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