THE CONFORMATIONAL COMPOSITION OF 4-FLUORO-1-BUTENE AS STUDIED BY MICROWAVE SPECTROSCOPY AND ABINITIO COMPUTATIONS

The microwave spectrum of 4-fluoro-1-butene has been reinvestigated in the 10-26.5 GHz spectral range. The ground and several vibrationally excited states have been assigned for two 'new' rotamers denoted Skew-Gauche I and Skew-Gauche II. The dipole moments of the 'new' conformers have been determined. Additional data have been obtained for the already assigned conformer denoted Skew-Anti. The energy differences between the three conformers have been established. Skew-Gauche II is the more stable rotamer. It is 1.9(2) kJ mol-1 more stable than Skew-Anti, and Skew-Gauche II is 2.1(2) kJ mol-1 more stable than Skew-Gauche I. Further conformers are at least 4 kJ mol-1 less stable than Skew-Gauche II. Ab initio calculations with the 3-21G* basis were made for five selected conformations in order to assist the microwave work.