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1 Polish Journal of Chemistry

1 1998

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Isotope effects on chemical shift in tautomeric systems with double proton transfer. Citrinin

Hansen P.E., Langgard M., Bolvig S.

Polish Journal of Chemistry

1998

Vol. 72, nr 2

269-276

Primary and secondary deuterium isotope effects on 1H and 13C chemical shifts are measured in citrinin, a tautomeric compound with an unusual doubly intramolecularly hydrogen bonded structure. The isotope effects are to a large extent dominated by equilibrium contributions and deuteration leads to more of the deuterated enol forms rather than the deuterated acid form. 1H, 13C and 17O nuclear shieldings are calculated using density functional ab initio methods. A very good correlation between calculated nuclear shieldings and experimental 1H and 13C chemical shifts is obtained. The tautomeric equilibrium can be analyzed based on the isotope effects on C-6 and C-8 carbons and shows an increase in the o-quinone form on lowering the temperature. Furthermore, upon deuteration the largest equilibrium shift is found for deuteration at OH-8 and the shift in the tautomeric equilibrium upon deuteration is increasing at lower temperature.