Mass spectrometry of pyridine compounds-II: Hydrogen exchange in the molecular ions of isonicotinic-and nicotinic acid as a function of internal energy

From the mass spectra of site‐specifically deuterated analogues of isonicotinic acid it appears that the molecular ion eliminates hydroxyl and water after an exchange between the hydroxylic and β‐hydrogens. The percentage of exchange in these reactions depends on the internal energy of the molecular ion and is shown to be 53 to 57% in the ion source, 92 to 97% in the first and ∽ 100% in the second field free regions. Furthermore, the isotope effect i, operative in the loss of water, increases with decreasing internal energy of the molecular ion, being 1.6, 2.0 and 2.3 in the ion source, first‐ and second field free regions, respectively. In the molecular ions, losing successively hydroxyl and carbon monoxide as deduced from diffuse peaks in the first‐and second field free regions, a substantially lower percentage of exchange (ca. 20%) is found, which is due to the higher internal energy of these molecular ions. In the molecular ion of nicotinic acid only one of the ortho hydrogens (α) is involved in the exchange of hydrogen. The percentage of exchange for loss of hydroxyl in the ion source is 66%. Molecular ions, which successively eliminate hydroxyl and carbon monoxide, show a 45% exchange of hydrogen as calculated from diffuse peaks in the first‐ and second field free regions.