Bistable carbon nanobracelets

Existence of bistability for carbon nanobracelets (cyclic molecules with alternating polycyclic regions and double carbon chains) is predicted using calculations based on density functional theory (DFT). It was found that two stable states have the same topological structure of covalent bonds, but different symmetries, with the total energy of the low-symmetry state lower by 1.0 and 0.8 eV than the energy of the high-symmetry state for the nanobracelets consisting of 4 and 5 monomers, respectively. On the basis of the calculated structural characteristics and electronic properties, we propose that the bistability of the carbon nanobracelets is related to the competition between the electron structure energy and the energy of interaction between the adjacent chains.

Structure optimization of carbon nanobracelets was performed using spin-polarized all-electron DFT calculations with the PBE functional implemented in the Priroda code. Due to chain flexibility, a multi-step procedure was employed. The initial coordinates, derived from molecular mechanics and hand-made designs, were refined via PM3 (MOPAC2016) to generate starting points for high- and low-symmetry states. The final optimization was performed in the Priroda code without symmetry restrictions. Energies included zero-point corrections. Extended triple-n Gaussian basis sets, with kinetically balanced small components, were employed. Molecular symmetry was determined using WebMO algorithms. Positive vibrational frequencies confirmed true energy minima. The GFN-xTB method with van der Waals correction D4 was used to verify DFT-PBE calculations. To investigate the stability of the local minima, we performed ab initio molecular dynamics simulations. The molecular electrostatic potential was visualized using Jmol.