Microwave spectra of 2-and 3-fluorobenzaldehydes have been investigated in the frequency region 26.5-40.0GHz. Only the 0-trans form of 2fluorobenzaldehyde was detected in the gas phase. Analysis of 3-fluorobenzaldehyde confirmed the existence of two rotational isomers: 0-cis and 0-trans. From relative intensity measurements the 0-cis form was shown to be more stable than the 0-trans form by 300 * 200 cal mol-'. The inertia defect values and the variation of the rotational constants with the torsional quantum number demonstrate that the equilibrium geometry is planar for all rotamers. Molecular structures which are in very good agreement with experimental rotational constants have also been derived.

No abstract