Substituent Effects in the Binding of Alkali Metal Ions to Pyridines Studied by Threshold Collision-Induced Dissociation and ab Initio Theory:  The Aminopyridines

Threshold collision-induced dissociation of M + ( x -aminopyridine) with xenon is studied using guided ion beam mass spectrometry. M + include the following alkali metal ions: Li +, Na +, and K + . All three structural isomers are examined, x = ortho, meta, and para. In all cases, the primary product corresponds to endothermic loss of the intact x -aminopyridine molecule, with minor production of MXe + formed by ligand exchange. The cross section thresholds are interpreted to yield zero and 298 K bond dissociation energies for M + − x -aminopyridine 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)/6-31G* 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. Theoretical bond dissociation energies are determined from single point calculations at the MP2(full)/6-311+G(2d,2p) level using the MP2(full)/6-31G* optimized geometries. Excellent agreement between theory and experiment is found for the Na + and K + systems, whereas the theoretical bond dissociation energies to Li + systems are systematically low but still within experimental error. The measured bond energies are compared among the systems examined here, and to the analogous methyl-substituted systems examined in a previous study, to determine the influence of the position and the nature of the substituent on the binding and factors that control the strength of such binding.