2-Pyridinecarboxylic Acid

The crystal structure of the title compound, C₆H₅NO₂, has been redetermined to confirm that the space group C2/c is correct for the averaged structure in time and space. A zigzag chain is formed by the hydrogen bonds, N-H...N and O-H...O, and these protons are disordered around the twofold axis or the center of symmetry. The solid state high resolution ¹⁵N-CP/MAS NMR spectrum clearly revealed the two separate signals of protonated and nonprotonated N atoms. It was concluded that the intermolecular N-H...N hydrogen bond is asymmetric and hydrogen atom is trapped at one of the two nitrogen atoms. However, there is a crystallographic twofold axis through the mid point of the N...N bond. These facts indicate that the neutral molecules and zwitter ions are arranged alternately at local domains in the NMR time scale.

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