The molecular structure, conformation, potential to internal rotation and force field of 2,2,2-trifluoroacetamide as studied by gas electron diffraction and quantum chemical calculations

2,2,2-Trifluoroacetamide (TFA) has been studied by electron diffraction (ED), ab initio Hartree-Fock (HF), density functional theory (DFT), and MP2 calculations. The calculations give one conformation with one of the C-F bonds anti to the C=O bond and a planar NH2 group, except for MP2/6-311 + + G**, which predicts a slightly pyramidale NH2 group. A molecular force field has been determined, and the fundamental frequencies have tentatively been assigned. The refined structural parameters were determined using constrained ED, i.e. ab initio results are included as constraints in the analysis. The structural parameters are: r(g)(N-H-4)=1.040(4), r(g)(C=O)=1.211(2), r(g)(C-N)=1.362(4), r(g)=1.562(1), r(g)(C-F-7)=1.347(1), angle(alpha)OCN=126.5(2), angle(alpha)CCN=116.3(4), angle(alpha)CCF(7)=111.9(1), and angle(alpha)CNH(4)=118.5(11). Bond distances are in Angstrom and bond angles in degrees. Uncertainties are one standard deviation from least squares refinement using a diagonal weight matrix and inclusion of the uncertainty in the electron wavelength. The structural parameters have been compared with related amides. The Fourier coefficients V-3 and V-6 in the potential to internal rotation of the CF3 group, V(alpha)=1/2*V-3*(1 - cos(3*alpha)) + 1/2*V-6*(1 - cos(6*alpha)), are determined to be 2.7(4) and -0.7(3) kJ/mol, respectively. The syn barrier is experimentally determined to be 2.6(4) kJ/mol, which is in good agreeent with theoretical calculations. (C) 1998 Elsevier Science B.V.