Semi-empirical calculation of electronic absorption wavelengths of polyynes, monocyano- and dicyanopolyynes. Predictions for long chain compounds and carbon allotrope carbyne

Absorption wavelengths and oscillator strengths have been calculated using the ZINDO method for the allowed low-energy electronic transition 1Σ+←1Σ+ of monocyanopolyynes (HCyN, y=1–13), 1Σu+←1Σg+ of polyynes and dicyanopolyynes (HCyH and NCyN, y=1–40). For y>18, the geometries were extrapolated from DFT optimization of the shortest members. Extrapolation formulas have been drawn up for longer chains, the asymptotic values of those yield an estimation of the absorption wavelength (ca. 400 nm) of the hypothetical carbon allotrope carbyne.