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The P, aman (3200 to 10 cm-i) and infrared (3200 to 20 cm-i) spectra of gaseous and solid trans-1-fluoro-2-butene, trans-CH3HC=CHCH2 F, have been recorded. Additionally, the Raman spectrum of the liquid with qualitative depolarization data has also been obtained. Variable temperature studies have been carried out with the sample dissolved in xenon. These spectral data have been interpreted on the basis that the molecule exists in the fluid phases as a mixture of the synclinal (fluorine atom oriented cis to the-double bond) and anticlinal (gauche) conformations and the synclinal conformer is more stable by 135 + 30 cm -I (377 + 106 cal/mol). There was a considerable scatter of the data but in the liquid state the anticlinal conformer is more stable and the only comformer present in the solid state. Ab initio gradient calculations employing the RHF/3-21G, PHF/6-31G*, and MP2/6-31G* basis sets predict the synclinal conformer to be more stable by 800, 86, and 150 cm -I, respectively. The fundamental asymmetric torsional mode for the synclinal comformer is observed at 123.9 cm -~ with three excited states falling to lower frequency. The corresponding mode for the anticlinal conformer is observed as a broad nondescript band centered at about 93 cm -I, Utilizing these data along with the AH and dihedral angle for the anticlinal conformer the potential function for the conformational interchange has been calculated. The determined coefficients are VI = -246 + 5, V 2 = 371 + 3, V 3 = 836 + 22, I/4 = 86 + 3, and V 6 = 85 + 8 cm-J. Complete equilibrium geometries for both conformers have been determined with the RHF/3-21 G, RHF/6-31G*, and MP2/6-31 G* basis sets. A normal coordinate analysis utilizing the harmonic force constants obtained with the MP2/6-3 IG* basis set has been carried out for both conformers. The results are discussed and compared with the corresponding quantities obtained for some similar molecules.

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