Conformational and structural studies of 2,2,2 trifluoroethylamine from temperature dependent Raman spectra of xenon solutions and ab initio calculations

Variable temperature (−60 to −100 °C) studies of the Raman spectra (4000–300 cm−1) of 2,2,2 trifluoroethylamine, F3CCH2NH2 dissolved in liquid xenon have been carried out. From these data both conformers have been identified and their relative stabilities obtained. The enthalpy difference has been determined to be 267 ± 27 cm−1 (3.19 ± 0.32 kJ mol−1) with the trans conformer the more stable form. The percentage of the gauche conformer is estimated to be 35 ± 3% at ambient temperature. The conformational stabilities have been predicted from ab initio calculations utilizing several different basis sets up to aug-cc-pVTZ for both MP2(full) and density functional theory calculations by the B3LYP method. By utilizing previously reported microwave rotational constants along with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r0 parameters have been obtained for the trans conformer. The determined heavy atom structural parameters are for the distances (Å): C1–C2 = 1.513(3), C2–N3 = 1.447(3), C1–F4 = 1.344(3), C1–F5,F6 = 1.347(3) and angles in degrees (°) ∠ N3C2C1 = 115.2(5), ∠ F4C1C2 = 111.4(5), ∠ C2C1F5, F6 = 111.6(5), ∠ F4C1F5 = 107.6(5), and ∠ F51F6 = 106.9(5). Vibrational assignments have been provided for the observed bands which have been supported by MP2(full)/6-31G(d) ab initio calculations to predict harmonic force fields, frequencies, infrared intensities, Raman activities and depolarization ratios for both conformers. The results are discussed and compared to the corresponding properties of some similar molecules.