ИСТИНА

Among other isomers of pyridinecarboxylic acid, namely, nicotinic (vitamin B3) and isonicotinic acids, picolinic acid is of great interest due to its biological properties. According to results of B3LYP/cc-pVTZ and MP2/cc-pVTZ calculations, the most stable conformer of picolinic acid has a planar configuration (Cs total symmetry) with antiperiplanar N-C-C=O and H-O-C=O fragments. The second conformer with a synperiplanar H-O-C=O fragment is predicted to exist in negligible amount of ≈2% at the experimental temperature of 375 K. In comparison to the less stable conformers, the main conformer is stabilized by the OH…N hydrogen bond with re(N…H)=1.949 Å (MP2/cc-pVTZ). The best ab initio structures of the lowest-energy conformers have been estimated at the CCSD(T)(ae)/cc-pwCVTZ level with extrapolation to the complete basis set limit at the MP2 level. Although the substitution structure (rs) of the main conformer is already determined, the semiexperimental equilibrium structure (re ) is still not known. In our study, the r structural parameters are derived from the gas electron diffraction (GED) data taking into account vibrational corrections calculated from the MP2/cc-pVTZ harmonic and anharmonic force constants. The best estimated ab initio (CCSD(T)-based) and semiexperimental equilibrium structures are very close to each other.