Study of internal rotation of the vinyl group in 1-, vinylpyridones by the AM-1 method

Abstract

By the AM-1 method in the semirigid model approximation, the potential energy of internal rotation of the vinyl group has been calculated for 1-vinyl-2-pyridone, 1-vinyl-6-methyl-2-pyridone, and 1-vinyl-4-pyridone. The potential energy of internal rotation in the first and third of these molecules has two minima corresponding to torsion angles φ around the N-Cα bond approximately 0 and 180°, separated by barriers to internal rotation 3.99 and 2.94 kcal/mole, respectively. For the 1-vinyl-2-pyridone, the energy minimum at φ=180°, the s-cis(O) conformation, is 1.83 kcal/mole above the first minimum. For the 1-vinyl-6-methyl-2-pyridone, the torsion angles φ 0 and 180° correspond to energy maxima; the energy minima occur at φ 35° and 140°. In the harmonic oscillator approximation, the intervals of torsional vibrations of the vinyl group in these comopunds have been estimated. Data from the AM-1 calculation are in qualitative agreement with the available1H and13C NMR data on the structure of 1-vinyl-2-pyridone, 1-vinyl-5-methyl-2-pyridone, and 1-vinyl-4-pyridone.