Internal rotation in relatively complicated systems: benzoic, nicotinic, isonicotinic and picolinic acids and their thio derivatives
Abstract
The internal rotation of an OH group around a single CO bond has been investigated for benzoic, nicotinic and isonicotinic acids and two conformers of picolinic acid. The rotation (torsion) angle was varied from 0°(syn conformer) to 180°(anti conformer) and total energy was calculated at the MNDO level with complete geometry optimization. The height of the potential barrier for internal rotation was calculated with reference to the energy of the syn conformer which was scaled to zero energy. For comparison, the internal rotation of an SH group around a single CS bond was investigated for the molecules thiobenzoic acid, thionicotinic acid and thioisonicotinic acid. Some molecules are ‘free-rotators’ while others are ‘rigid’. The height of the rotational barrier was dependent on the ‘rotamer’, OH or SH, as well as on the specific isomer of the framework.
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