Absolute alkali metal ion binding affinities of several azoles determined by threshold collision-induced dissociation

Collision-induced dissociation of M+(azole) with xenon is studied using guided ion beam mass spectrometry. M+ include the following alkali metal ions: Li+, Na+ and K+. The azoles studied include imidazole, 1,2,4-triazole, 1,2,3-triazole, and tetrazole. In all cases, the primary product formed corresponds to endothermic loss of the intact azole. The cross-section thresholds are interpreted to yield 0 and 298 K bond energies for M+(azole) after accounting for the effects of multiple ion-molecule collisions, internal energy of the reactant ions, and dissociation lifetimes. The possibility that the triazoles and tetrazole tautomerize during the formation of the complexes and during their dissociation is explicitly considered. Ab initio calculations at the 6-31G** level are used to help assign the observed thresholds to specific dissociation processes. We find that the kinetically favored (thermodynamically less stable) complex is formed in all cases, indicating that tautomerization barriers in the presence of the metal ions are appreciable. (C) 1999 Elsevier Science B.V.