The Structure of Trifluoromethyl Acetylene from the Microwave Spectrum and Electron Diffraction Pattern

Abstract

Measurements, in the microwave region, of the J = 3→4 rotational transitions of trifluoromethyl acetylene and several of its isotopic modifications have been made, and the moments of inertia, IB (in a.m.u.-Å2), have been found for the ground vibrational state: for CF3CCH, 175.613, CF3C13CH, 177.024, CF3CC13H, 181.302, and for CF3CCD, 187.462. Three lines corresponding to J = 3→4 transitions of CF3CCH in the excited vibrational state v10 = 1, and two lines corresponding to v10 = 2 and v10 = 3, have also been measured and interpreted according to Nielson's theory of l-type doubling in symmetric tops. From Stark effect measurements at different electric field strengths the dipole moment of CF3CCH in the ground vibrational state has been found to be 2.36±0.04 Debye units. For the ground vibrational state the microwave data lead to the following bond distances: C–H, 1.056±0.005Å; C≡C, 1.201±0.002A. The C–C and C–F distances were calculated from the measured moments of inertia for several assumed values of the FCF angle. Electron diffraction experiments were also made, and the intensity curves calculated for the assumed microwave models were compared with the observed visual curve. From the combination of microwave and electron diffraction results the best agreement was obtained with the following set of parameters: ∠FCF, 107.5°±1°; C–C, 1.464±0.02A; C–F, 1.335±0.01A.