Determination of the molecular structure of diacetylene from high-resolution FTIR spectroscopy

The high-resolution infrared absorption spectra of eight²H or¹³C substituted isotopomers of diacetylene have been recorded, and the bands corresponding to the v₄ fundamental and v₆ combination of the major isotopomer have been analyzed using a Loomis-Wood-type program. Effective ground-state rotational constants have been obtained from ground-state combination differences. A number of r₀, r_s, r_m^ς, and (r_m^ς)_corr structures have been calculated from the available data and are compared to those obtained by ab initio methods. The (r_m^ς)_corr structure, which is a reliable near-equilibrium structure of diacetylene, is r_C-H=106.131(13) pm;r_C-C=137.081(16) pm;r_C-C=120.964(14). (r_m^ς)_corr structures of the related molecules cyanogen, cyanoacetylene, and cyanodiacetylene have been calculated, and near-equilibrium structures of triacetylene and dicyanoacetylene have been predicted.