Molecular orbital constrained gas electron diffraction study of N-acetyl N′-methyl alanine amide

Abstract

Gas electron diffraction (GED) data were recorded at about 510 K for , N-acetyl N′-methyl alanine amide (ALA) and analysed using the Molecular Orbital Constrained Electron Diffraction procedure. The HF/6-31G * geometries of seven conformations were optimized (C 7 eq ( 1), C 5 ( 2), β 2 ( 3), C 7 ax ( 4), α R ( 5), α L ( 6), and α′ ( 7) and used as constraints of the GED data analysis. HF/6-31G * force constants were determined for each conformation and used to calculate mean amplitudes of vibration and shrinkage corrections. It is found that, in the case of ALA, the customarily applied linear approximation for calculating shrinkages yields values which are unacceptably large. Therefore data analyses were performed both without shrinkage and with shrinkages calculated at a lower temperature. Subject to the selected constraints our interpretation of the GED data supports the following conclusions for vapors of ALA at about 500 K: ( 1) the compound exists in a conformational equilibrium with several significantly populated states; ( 2) the C 5 form is not less populated than the global energy minimum, C 7 eq: ( 3) the helical form, α R, is not a significantly populated conformer.