Direct Elucidation of the Vibrationally Averaged Structure of Benzene: A Path Integral Molecular Dynamics Study.

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 , 130537]. 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.