Refinement of labile hydrogen positions based on DFT calculations of 1H NMR chemical shifts: comparison with X-ray and neutron diffraction methods.

摘要: Numerous gas phase electron diffraction, ultra-fast X-ray and neutron diffraction experiments on β-dicarbonyl compounds exhibiting enol-enol tautomeric equilibrium, with emphasis acetylacetone dibenzoylmethane, have so far been reported conflicting results the structural details of O-HO intramolecular hydrogen bond resulted in alternative hypotheses potential function either a double minimum corresponding to two forms equilibrium or single symmetrical one. We demonstrate herein, firstly, that DFT calculated OH 1H NMR chemical shifts dibenzoylmethane exhibit strong linear dependence computed OO length ∼-50 ppm A-1 as angle ∼1 per degree, upon transfer atom from ground state toward transition state. Secondly, refinement labile atomic positions bonds based root-mean-square deviation between experimentally determined solution can provide high resolution structures O-H O(H)O lengths angles an accuracy ∼10-2 A ∼0.5°, respectively. Thirdly, tautomers are excellent agreement experimental value, even for moderate basis sets energy minimization. In contrast, structure strongly delocalized system is which deviate value. Fourth, quantitative literature data take into account effect quantum nuclear motion. The critically discussed respect state-of-the-art variable temperature methods.