Absolute alkali metal ion binding affinities of several azines determined by threshold collision-induced dissociation and ab initio theory

Threshold collision-induced dissociation of M+ (azine) with xenon is studied using guided ion beam mass spectrometry. M+ include the following alkali metal ions: Li+, Na+, and K+. The azines studied include pyridine, pyridazine, pyrimidine, pyrazine, and 1,3,5-triazine. In all cases, the primary product formed corresponds to endothermic loss of the intact azine and the only other product observed is the result of ligand exchange processes to form MXe+. The cross-section thresholds are interpreted to yield 0 and 298 K bond energies for M+-azine after accounting for the effects of multiple ion-molecule collisions, internal energy of the reactant ions, and dissociation lifetimes. Ab initio calculations at the MP2(full)/6-311+G(2d,2p)//MP2(full)/6-31G* level of theory are used to determine the structures of these complexes and provide molecular constants necessary for the thermodynamic analysis of the experimental data. We find that all of the complexes are very nearly planar, the pi complexes being significantly less stable. Calculated Mi-azine bond dissociation energies compare favorably to the experimentally determined bond energies for these systems. (C) 2000 Elsevier Science B.V.