Microwave spectrum, structure and dipole moment of phenylacetylene

Abstract

The microwave spectra of phenylacetylene, C₆H₅CCH, the deuterated species, C₆H₅CCD, and carbon–13 species at positions C(2), C(3), C(4), C(7) and C(8) have been measured and analysed. Inertial defects confirm the expected planarity of the molecule, and pronounced nuclear spin effects in the ground-state and vibrationally excited spectra confirm the C_2v symmetry. The detailed structure is determined to be: C(1)C(2)= 1.388, C(2)C(3)= 1.396, C(3)C(4)= 1.398, C(1)C(7)= 1.448, C(7)C(8)= 1.208, and C(8)H = 1.055 Å; C(6)C(1)c(2)= 120.8°, C(1)C(2)C(3)= 119.8°, C(2)C(3)C(4)= 119.9° and C(3)C(4)C(5)= 119.9°. The dipole moment has been accurately determined using new Stark-effect technique to be µ=µ_a= 0.656 ± 0.005 D and is found to be smaller for the deuterated species by 0.010 ± 0.006 D.

The effect of the acetylenic group on the structure of the benzene ring is quite small, as expected from results of microwave studies for other substituted benzenes, while the acetylenic parameters are close to those of methylacetylene. The dipole moment is also near to that of methylacetylene (and other alkyl acetylenes) and is taken to indicate that it arises essentially from a π-system polarisation of the acetylenic group as suggested by recent molecular orbital calculations.