The microwave spectrum, structure and dipole moment of 1,4-pentadiyne

The microwave spectra of 1,4-pentadiyne and 1,4-pentadiyne-1,5- d₂ are assigned and rotational and centrifugal distortion constants obtained. A unique structure could not be determined. However, analysis of the moments of inertia indicates that the bond distances in C ₅H ₄ are close to typical values found in other related compounds. An interaction involving the acetylene moieties is evidenced by the derived bond angles. The data are consistent with either the central CCC angle being close to the tetrahedral value with the acetylene groups pushed away from linearity by approximately 3-4° or opening of the central CCC angle to about 113° along with linear acetylene groups. A range of structures between these two is also possible. The dipole moment is determined to be 0.516(5) D.