Abstract

We report a combined solid-state (¹H, ²H, ¹³C, ¹⁷O) NMR and plane-​wave DFT computational study of the O···H···O low-​barrier hydrogen bonds (LBHBs) in two 1,3-diketone compounds: dibenzoylmethane (1) and curcumin (2). In the solid state, both 1 and 2 exist in the cis-keto-enol tautomeric form, each exhibiting an intramolecular LBHB with a short O···O distance (2.435 Å in 1 and 2.455 Å in 2). While numerous experimental (structural and spectroscopic) and computational studies have been reported for the enol isomers of 1,3-diketones, a unified picture about the proton location within a LBHB is still lacking. This work represents the first time that solid-​state 17O NMR data are reported for the O···H···O LBHBs in 1,3-diketones. The central conclusion of this work is that detailed information about the probability density distribution of the proton (nuclear zero-​point motion) across a LBHB can be obtained from a combination of solid-​state NMR and plane-​wave DFT computations (both NMR parameter calculations and ab initio MD simulations). We propose that the precise proton probability distribution across a LBHB should provide a common basis on which different and sometimes seemingly contradicting experimental results obtained from complementary techniques such as X-ray diffraction, neutron diffraction, and solid-​state NMR can be reconciled.