Raman and far-infrared spectra, conformational stability, barriers to internal rotation, vibrational assignment and ab initio calculations of 2-bromo-3-fluoropropene

The far‐infrared spectrum (375–30 cm−1) of gaseous 2‐bromo‐3‐fluoropropene, CH2C(CH2F)Br, was recorded at a resolution of 0.10 cm−1. The fundamental asymmetric torsional mode is observed at 112.6 cm−1 with four excited states falling to lower frequency for the s‐cis (fluorine atom eclipsing the doubel bond) conformer. For the higher energy gauche conformer, the asymmetric torsion is tentatively assigned to a very weak band at 93.0 cm−1. From these data the asymmetric torsional potential was calculated. The potential function coefficients are calculated to be V1 = 306 ± 5, V2 = −120 ± 4, V3 = 928 ± 6, V4 = 118 ± 2 and V6 = 83 ± 2 cm−1, with an enthaply difference between the more stable s‐cis and higher energy gauche conformers of 200 ± 30 cm−1 [572 ± 86 kcal mol−1 (1 kcal = 4.184 kJ)]. This function gives values of 1009 cm−1 (2.88 kcal mole−1), 1024 cm−1 (2.93 kcal mol−1) and 799 cm−1 (2.88 kcal mol−1), for the s‐cis to gauche, gauche to gauche, and gauche to s‐cis barriers, respectively. From the relative intensities of the Raman lines of the gas at 548 cm−1 (gauche) and 678 cm−1 (s‐cis) as a function of temperature, the enthalpy difference is found to be 196 ± 23 cm−1 (560 ± 66 cal mol−1), and a similar study of the liquid gives a value of 66 ± 4 cm−1 (189 ± 11 cal mol−1) with the s‐cis conformer the more stable rotamer in both of these phases. The s‐cis conformer is the rotamer present in the crystalline solid. The Raman spectrum of the gas was recorded from 3500 to 70 cm−1 and, utilizing these data and previously reported infrared data, a complete vibrational analysis is proposed for both conformers. The conformational stability, barriers to internal rotation and fundamental vibrational frequencies, which were determined experimentally, are compared with those obtained from ab initio gradient calculations employing the RHF/LANL1DZ basis set and/or this basis set with electron correlation at the MP2 level, and with the corresponding quantities for some similar molecules.