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

Abstract

Threshold collision-induced dissociation of M+(x-methylpyridine) with xenon is studied using a newly constructed guided ion beam mass spectrometer, described in detail here. 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-methylpyridine 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-methylpyridine 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* geometries. Excellent agreement between theory and experiment is found for the Na+ and K+ systems, whereas the theoretical BDEs to Li+ systems are systematically low but still within experimental error. Good agreement is found for the single previously measured bond dissociation energy of Li+ to m-methylpyridine.