The nature of the intramolecular hydrogen bond in the enol tautomer of 2,4-pentanedione

Citation

Dea, Phoebe Kin-Kin (1972) The nature of the intramolecular hydrogen bond in the enol tautomer of 2,4-pentanedione. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/WST8-CK73. https://resolver.caltech.edu/CaltechETD:etd-02032005-155729

Abstract

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The nature of the intramolecular hydrogen bond in the enol tautomer of 2,4-pentanedione has been studied in detail using high resolution proton, deuteron and [...] magnetic resonance spectroscopy. An unusually large deuterium isotope effect on the chemical shift of the bridge hydrogen has been observed. This unexpected result, together with the observation of a pronounced temperature dependence for both the bridge proton and deuteron resonances, as well as a temperature dependence of the carbonyl resonances, suggest that two states with different chemical shifts for the bridge hydrogen are involved in rapid equilibrium and that the anomalous deuterium isotope effect has its origin in the effect of deuterium substitution on the energy separation between these states. The results of a least-squares analysis of the OH and the OD temperature data in terms of a two-state model indicate that the energy separation for the proton and deuterated systems in the two states are [...] and [...]; but essentially identical chemical shifts were obtained for the OH proton as for the OD deuteron in each state (-14.15 and -10.10 ppm from the enol methyl group respectively).

It is proposed that these states correspond to the symmetric and asymmetric structures of the intramoleculax hydrogen bond. Results of our CNDO/2 calculations on the enol tautomer have indicated that these two structures have quite similar energies. In particular, it is suggested that the potential function associated with the vibrational motion of the bridge hydrogen is symmetric double minimum in nature when the donor-acceptor vibration (symmetric stretch) is in its zeroth vibrational state and the potential function becomes asymmetric when the symmetric stretch is excited by one or more vibrational quanta. By exciting the symmetric [...] stretch, we expect a charge shift in the [...] system which can alter the effective potential function for the bridge hydrogen. The [...] transition of the symmetric stretch is observed at [...] in the Raman spectrum of the normal compound and [...] in the deuterated molecule. This difference of [...] in the energy separation of the ground and first excited vibrational states of the symmetric stretch would be expected to give rise to the large deuterium isotope effect observed in our magnetic resonance experiments.

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