THE MOLECULAR-STRUCTURE, CONFORMATION, POTENTIAL TO INTERNAL-ROTATIONAND FORCE-FIELD OF 2,2,2-TRICHLOROACETAMIDE AS STUDIED BY GAS ELECTRON-DIFFRACTION AND QUANTUM-CHEMICAL CALCULATIONS

2,2,2-Trichloroacetamide (TCA) has been studied by electron diffractio n (ED) and ab initio Hartree-Fock (HF) calculations using several basi s sets and density functional theory (DFT). The HF calculations give e ssentially a syn conformation while the DFT calculations predict an un symmetrical conformation with one of the Cl atoms perpendicular to the amide skeleton. ED support the DFT predictions. A molecular force fie ld has been determined, and the assignments of the fundamental frequen cies have been discussed. Refined structural parameters were determine d using constrained ED, i.e. ab initio results are included in the ana lysis, are: r(g)(N-H-4) = 1.033(8), r(g)(C=O) = 1.213(3), r(g)(C-N) = 1.347(4), r(g)(C-Cl-7) = 1.761(2), angle(alpha)OCN = 127.7(6), angle(a lpha)CCN = 114.1(9), angle(alpha)CCCl(7) = 109.5(1). Bond distances ar e in Angstrom and bond angles in degrees. Uncertainties are one standa rd deviation from least-squares refinement using a diagonal weight mat rix, but corrected for uncertainties in the electron wave length, The structural parameters have been compared with related amides, and the most unusual feature is the central sp(2)-sp(3) C-C bond length which is determined to be as long as r(g) = 1.591(5) Angstrom. The Fourier c oefficients V-3 and V-6 in the potential to internal rotation of the C Cl3 group, V(phi) = 1/2 x V-3(1 - cos(3 phi)) + 1/2 x V-6(1 - cos(6 ph i)), are determined to be -0.8(14) and -1.1(4) kJ mol(-1), respectivel y. (C) 1997 Elsevier Science B.V.