RAMAN AND INFRARED-SPECTRA, CONFORMATIONAL STABILITY, AND AB-INITIO CALCULATIONS OF TRANS-1-FLUORO-2-BUTENE

The Raman (3200 to 10 cm(-1)) and infrared (3200 to 20 cm(-1)) spectra of gaseous and solid trans-1-fluoro-2-butene, trans-CH3HC=CHCH2F, hav e been recorded. Additionally, the Raman spectrum of the liquid with q ualitative depolarization data has also been obtained. Variable temper ature studies have been carried out with the sample dissolved in xenon . These spectral data have been interpreted on the basis that the mole cule exists in the fluid phases as a mixture of the synclinal (fluorin e atom oriented cis to the double bond) and anticlinal (gauche) confor mations and the synclinal conformer is more stable by 135 +/- 30 cm(-1 ) (377 +/- 106 cal/mol). There was a considerable scatter of the data but in the liquid state the anticlinal conformer is more stable and th e only comformer present in the solid state. Ab initio gradient calcul ations employing the RHF/3-21G, RHF/6-31G, and MP2/6-31G* basis sets predict the synclinal conformer to be more stable by 800, 86, and 150 cm(-1), respectively. The fundamental asymmetric torsional mode for th e synclinal comformer is observed at 123.9 cm(-1) with three excited s tates falling to lower frequency. The corresponding mode for the antic linal conformer is observed as a broad nondescript band centered at ab out 93 cm(-1). Utilizing these data along with the Delta H and dihedra l angle for the anticlinal conformer the potential function for the co nformational interchange has been calculated. The determined coefficie nts are V-1, = -246 +/- 5, V-2 = 371 +/- 3, V-3 = 836 +/- 22, V-4 = 86 +/- 3, and V-6 = 85 +/- 8 cm(-1). Complete equilibrium geometries for both conformers have been determined with the RHF/3-21G, RHF/6-31G, and MP2/6-31G basis sets. A normal coordinate analysis utilizing the harmonic force constants obtained with the MP2/6-31G basis set has be en carried out for both conformers. The results are discussed and comp ared with the corresponding quantities obtained for some similar molec ules.