Abstract
New experimental measurements [Sun et al., Nature 2023, 623, 972] of the cyclic C(10) reveal a cumulenic pentagon-like D(5h) structure at ∼5 K. However, the long-standing presumption that a large zero-point vibrational energy combined with an extremely flat D(5h) ↔ D(10h) ↔ D(5h) isomerization pathway washes out the pentagonal D(5h) structure and yields a symmetric D(10h) decagon remains at odds with the experiment. We resolve this issue with our fitting approach based on a bond-order charge-density matrix expressed in permutationally invariant polynomials. We train the model on τHCTH/cc-pVQZ data morphed to reproduce a relativistic all-electron CCSDT(Q)/CBS D(5h)-D(10h) potential energy barrier (benchmarked previously by others). Large scale diffusion Monte Carlo simulations in full dimensionality show that the vibrational ground state of C(10) has compositional character of more than 96% D(5h), fully reflecting the experimental imaging data. Quantum mechanical variational calculations in 1-D further suggest persistence of the D(5h) symmetry structure at higher temperatures.