Molecular Structure of Diacetylene as Studied by Gas Electron Diffraction

The molecular structure of diacetylene has been studied by gas electron diffraction. Thermal-average bond distances are determined to be: rg(C–C)=1.3837±0.0019 Å, rg(C≡C)=1.2176±0.0014 Å, and rg(C–H)=1.0935±0.010 Å. The C–C single bond is about 0.007 Å shorter than that in cyanogen, while the C≡C triple bond is about 0.005 Å longer than that in acetylene. The observed mean amplitudes are slightly larger than the parallel amplitudes based on harmonic calculations. A suggestion has been made to account for the discrepancies by the use of empirical constants representing the interaction between bending and bond-stretching vibrations, which makes the bond distances slightly longer, on the average, in a momentary bent structure than in a linear structure. This interaction also seems to account for a slight difference between the rotational constant B0 estimated from the above rg distances and those derived from spectroscopic experiments.