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High resolution proton magnetic resonance study in hydrogen bonded systems Namba, Natsuko

Abstract

The keto-enol tautomeric systems of cyclic 1,3 diones have been studied by the proton magnetic resonance method. The presence of both keto and enol forms were confirmed in chloroform and acetonitrile solutions . From the concentration dependence of OH proton chemical shift, the equilibrium among the three forms has been suggested: (formula omitted). A linear relationship was found between the OH proton chemical shift and l/√c. The chemical shift of the hydrogen bonded OH proton was found to be -740 cps from T.M.S. at 60 M.c. by extrapolation to infinite concentration. That of the non-hydrogen bonded OH proton was found to be -440 cps, which gave the most reasonable equilibrium constant, K₁ , for all concentrations. The equilibrium constants were obtained from the observed OH chemical shift and from the ratio of the areas under the specific peaks for each species. Measurements were also made in the temperature range 302°K to 357°K for cyclohexane 1,3 dione in chloroform. The overall heat of conversion from the dimer enol form to the monomer keto form was found to be 2.05 Kcal/mole. The proton magnetic resonance spectra of methyl pyridines have been investigated both in carbontetrachloride and trifluoroacetic acid. The broad triplets which were observed at the pyridine concentration of 6~8 mole %, in trifluorocacetic acid confirmed the presence of the pyridine cations . Almost, all the signals, shifted to low field on protonation. The factors which affect the chemical shift were split into several terms on the basis of Pople's theory.¹²⁵ The chemical shifts of the pyridine and pyridinium systems were compared with the benzene system. Dominating factors were found to be; the inductive and anisotropic effects of the nitrogen atom or the protonated nitrogen atom, the mesomerlc effect of the above, and the inductive and anisotropic effects of the methyl groups. Using these values, which were obtained from the simple compounds, the chemical shifts were calculated and compared with the observed values. They agreed in most of the cases within ± 5 cps at 60 M.c. The ortho coupling constants between the 2 and 3 ring protons in the puridine system are much smaller than those in the benzene system. The slight increases in coupling constants on protonation were only understandable considering the pyridinium cations as odd sequences from the benzene and pyridine systems. It was observed that the C¹³-H¹ coupling constants increased on protonation as a result of the increase in the electronegativity of the nitrogen atom. That is the s character in the carbon atomic orbital increased on protonation.

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