The Raman (3500-10 cm - 1 ) and infrared (3200-50 cm - 1 ) spectra have been recorded of the fluid and solid phases of but-2-enoyl fluoride (crotonyl fluoride) trans-CH 3 CH CHCFO, where the methyl group is trans to the CFO group. From the variable temperature studies of the infrared spectrum of the sample dissolved in liquified Xe, the conformer pair at 836 cm - 1 has been used to determine a ΔH value of 135 +/- 11 cm - 1 (387 +/- 30 cal mol - 1 ), with the s-cis (syn) form (two double bonds oriented cis to one another) the more stable form. In the Raman and infrared spectra of the solid, the s-cis conformer seems predominant, but even with repeated annealing a spectrum free of signals from the s-trans (anti) conformer could not be obtained. In fact, in some of the infrared spectra, there appeared to be a slight preference for the trans conformer. The asymmetric torsional fundamental of the s-trans conformer was observed at 104.3 cm - 1 with two hot bands and that for the s-cis rotamer at 97.0 cm - 1 with one hot band. From these data the potential function governing the conformational interchange was determined, and the potential coefficients are:V 1 = - 122 +/- 1, V 2 = 1993 +/- 27, V 3 = 21 +/- 1 and V 4 = - 88 +/- 8 cm - 1 . The s-trans to s-cis and s-cis to s-trans barriers were determined to be 2044 and 1942 cm - 1 , respectively, with an enthalpy difference between the conformers of 102 +/- 29 cm - 1 (292 +/- 83 cal mol - 1 ). The barriers governing the internal rotation of the CH 3 group for thes-trans and s-cis conformers are calculated to be 1060 +/- 17 cm - 1 (3.03 +/- 0.05 kcal mol - 1 ) and 1042 +/- 23 cm - 1 (2.98 +/- 0.07 kcal mol - 1 ), respectively. A complete vibrational assignment of the normal modes is provided. The structural parameters, force constants, and vibrational frequencies have been determined from ab initio RHF/3-21G, RHF/6-31G * and MP2/6-31G * calculations, and the theoretical results are compared with the experimental values when appropriate. These results are compared with the corresponding quantities obtained for some similar molecules.