Low-barrier hydrogen bond in curcumin and hemicurcuminoids are studied. Path integral molecular dynamics simulations reveal that the correlation between the proton migration and the reorganization of the π-delocalized structure is not so strong.

The compounds Fujikurin A, B, and D, recently isolated from Fusarium fujikuroi , possess intramolecular low-barrier hydrogen bonds (LBHBs), which are hydrogen bonds with a very low-energy barrier for proton transfer. The isolated compounds have a hydrogen-bonded proton that appears to rapidly switch between two equilibrium states via a transition state (TS). To understand the characteristics of these intramolecular LBHBs in detail, we performed path integral molecular dynamics (PIMD) simulations, which can consider nuclear quantum effects (NQEs) under a finite temperature. The PIMD simulations predicted that the NQE completely washed out the energy barrier for the proton transfer reaction. Consequently, a single-well shape emerged in the results, along with the effective free-energy potential surface for the hydrogen-bonded proton distribution. Thus, we conclude that the hydrogen-bonded proton in Fujikurin does not in fact transfer between two equilibrium structures but widely delocalizes around the global minimum structure involving the TS region.

Path integral molecular dynamics (PIMD) simulations for C 6 H 6 , C 6 D 6 , and C 6 T 6 have been carried out to directly estimate the distribution of projected C−H(D,T) bond lengths onto the principal axis plane. The average values of raw C−H(D,T) bond lengths obtained from PIMD simulations are in the order of ⟨R C−H ⟩ > ⟨R C−D ⟩ > ⟨R C−T ⟩ due to the anharmonicity of the potential energy curve. However, the projected C−H(D,T) bond lengths are almost the same as those reported by Hirano et al. [J . Mol. Struct. 2021, 1243. Our PIMD simulations directly and strongly support the explanation by Hirano et al. for the experimental observations that almost the same projected C−H(D) bond lengths are found for C 6 H 6 and C 6 D 6 . The PIMD simulations also predicted the same projected bond lengths for C 6 T 6 as those of C 6 H(D) 6 . In addition to the previous local mode analysis, the present PIMD simulations predicted, for benzene isotopologues, that the vibrationally averaged structure is planar but non-flat.