Theoretical study of the structure and properties of polyynes and monocyano- and dicyanopolyynes:: Predictions for long chain compounds

Geometrical parameters and harmonic vibration frequencies of polyynes HC2nH (n = 1-8), cyanopolyynes HC2n-1N (n = 1-7), and dicyanopolyynes NC2nN (n = 1-8) have been calculated with various density functionals using the Dunning triple-zeta basis set. For selected data, we propose extrapolation formulas for longer molecules of the series. Bond lengths and electron localization function analysis indicate that a marked bond alternation persists in molecules as long as HC30H. The evolution along the series of some vibration frequencies and their accuracy for the identification of long molecules in extraterrestrial systems are also discussed.