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 (<i>n</i> = 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.