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.

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.

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.