Conformational studies by ¹H - NMR in a nematic solvent: Methyl isonicotinate, methyl nicotinate and methyl picolinate

The structures of methyl isonicotinate (MI), methyl nicotinate (MN) and methyl picolinate (MP), i.e., 4-, 3-, and 2-pyridinecarboxylic acid methyl esters, were studied by ¹H - NMR at 296 K using nematic liquid-crystal solvent ZLI 1167. Conformational analysis was performed according to the model of Emsley, Luckhurst and Stockley to take account of the correlation between rotation and internal rotation. Only the conformer with a planar skeleton was detected for MI but s-trans and s-cis conformers were found for MN and MP. The relative abundance of the s-trans form was determined to be 64(2) and 68(2)% for MN and MP, respectively, which are in agreement with those determined by gas-phase electron diffraction. The positions of the ring protons of MI in ZLI 1167 agree with those in the gas phase within experimental errors. In the case of MN and MP, however, observed direct coupling constants between the ring protons are in poor agreement with those calculated from the gas-phase structures due to the deformation of the pyridine rings in ZLI 1167.