Vibrational Stabilization in Cyclacene Carbon Nanobelts

Cyclacene carbon nanobelts are predicted to be more stable in certain vibrational states. Vibrational simulations using hybrid thermally assisted-occupation density functional theory (TAO–DFT) predict small but consistent singlet–triplet electronic excitation energy changes at the classical harmonic vibrational turning points of the smaller belts. Geometric and vibrational properties are also compared between hybrid Kohn–Sham DFT and TAO–DFT for [n]cyclacene (n = 6–14), where TAO–DFT is found to shorten the carbon–carbon bonds bridging between the two annulene ribbons and causes qualitative changes in the calculated infrared spectra. These geometric changes lower the singlet–triplet transition energies and introduce greater ring strain, while individual vibrational modes are observed to shift by over 200 cm^–1. These findings indicate that including static correlation is important for describing both the geometric and vibrational properties of cyclacenes accurately.