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The Raman spectra (3300-10 cm -1 ) of gaseous, liquid, and solid 3,3-difluorobutene, CH 2 CHCF 2 CH 3 , and the infrared spectra (3400-60 cm -1 ) of the gas, xenon solution, and solid have been recorded. The spectra of the fluid phases are consistent with two stable conformers in equilibrium at ambient temperature. Utilizing two conformer pairs from the mid-infrared spectra of the xenon solution as a function of temperature (-100 to -55 °C), the enthalpy difference has been determined to be 68 ( 4 cm -1 (0.81 ( 0.05 kJ/mol) where the cis conformer, where the CH 3 group is cis to the double bond, is the more stable form. These spectral data indicate that the cis conformer is more stable than the gauche form in both the gaseous and liquid phases, but it is possible to obtain either conformer in the solid. The fundamental asymmetric torsion for the gauche conformer was observed at 84.27 cm -1 , and the overtone of the corresponding mode for the cis conformer was observed at 150.37 cm -1 . The potential function governing the conformer interconversion has been obtained and the constants are V 1 ) 284 ( 15, V 2 ) -164 ( 13, V 3 ) 632 ( 7, V 4 ) -30 ( 5, V 6 ) -39 ( 3. The gauche to cis barrier is 489 cm -1 (5.85 kJ/mol) and the gauche to gauche barrier is 848 cm -1 (10.14 kJ/mol). Vibrational assignments for the 30 normal modes for both the cis and gauche conformers are proposed. The structural parameters, dipole moments, conformational stability, vibrational frequencies, infrared intensities, and Raman activities have been determined from ab initio calculations. The predicted conformational stability at all levels of calculation up to MP2/6-311+G(2d,2p) is not consistent with the experimental results. These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.

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