ACETAMIDE, A CHALLENGE TO THEORY AND EXPERIMENT - ON THE MOLECULAR-STRUCTURE, CONFORMATION, POTENTIAL TO INTERNAL-ROTATION OF THE METHYL-GROUP AND FORCE-FIELDS OF FREE ACETAMIDE AS STUDIED BY QUANTUM-CHEMICAL CALCULATIONS

The molecular geometry has been optimized without any constraints usin g different basis sets and levels of theory as: Hartree-Fock with basi s sets 6-31+G*, 6-311++G**, cc-pVTZ and aug-cc-pVTZ, MP2 with basis s ets 6-311++G* and cc-pVTZ, MP3 with basis set 6-311++G**, and density functional theory with basis sets 6-311++G* and cc-pVTZ. Small basis sets up to 6-31G predict the syn conformation of the methyl group to be the most stable conformation. Larger basis sets predict an unsymmet rical conformation with one of the Fl atoms perpendicular to the amide skeleton or an anti-like conformation. Dunnings correlation consisten t polarized valence triple zeta, cc-pVTZ, basis set including MP2 pred ict two conformations, one perpendicular and one anti to be the most s table. The DFT calculations predict anti-like conformations. The most accurate calculations predict anti-like conformations which have not b een predicted previously. The vibrational frequencies have been calcul ated for several basis sets and compared to the observed frequencies. The wagging frequency of the NH2 is very dependent on the basis sets a nd levels of theory. Most calculations predict a planar NH2 group in a greement with experiment. A scaled molecular force field has been dete rmined by fitting the calculated frequencies to the observed ones for the perpendicular conformation using MP2/cc-pVTZ. The barrier heights for the methyl group have been calculated. The rotational constants, I -A + I-B - I-C values and dipole moments are compared with experimenta l values. (C) 1998 Elsevier Science B.V.