Microwave Spectrum, Barrier to Internal Rotation, and Torsion–Vibration Interaction in ortho-Fluoro Toluene

Abstract

The microwave spectrum of ortho-fluoro toluene, C6H4CH3F, was studied in the frequency range 12–38 GHz. Rotational spectra in four torsional states were assigned. Each state could be fit by an effective rotational Hamiltonian quite well. The change in rotational constants from one state to another could not be explained using the standard torsion–rotation coupled Hamiltonian. A simple model is given which helps explain the deviations in the rotational constants in the excited A levels from their expected positions. The true rotational constants for the ground state were calculated to be: A = 3243.08 GHz, B = 2180.44 GHz, C = 1314.36 GHz, κ = − 0.10234. By standard methods the angle between the top axis and the A axis was found to be 32°. The barrier to internal rotation is 649 cal/mol assuming V6 = 0 and Iα = 3.237 amu·Å2. Including the effects of changing average structure, there is evidence of a negative V6 and also that the angle mentioned above is somewhat smaller in the ground state.