THE MOLECULAR-STRUCTURE, CONFORMATION, POTENTIAL TO INTERNAL-ROTATIONAND 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 diffractio n (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 f or MP2/6-311 + + G*, which predicts a slightly pyramidale NH2 group. A molecular force field has been determined, and the fundamental frequ encies have tentatively been assigned. The refined structural paramete rs were determined using constrained ED, i.e. ab initio results are in cluded 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)=1.5 62(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(1 1). Bond distances are in Angstrom and bond angles in degrees. Uncerta inties are one standard deviation from least squares refinement using a diagonal weight matrix and inclusion of the uncertainty in the elect ron wavelength. The structural parameters have been compared with rela ted amides. The Fourier coefficients V-3 and V-6 in the potential to i nternal rotation of the CF3 group, V(alpha)=1/2V-3*(1 - cos(3*alpha)) + 1/2V-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 calculat ions. (C) 1998 Elsevier Science B.V.