Periodic trends in the binding of metal ions to pyridine studied by threshold collision-induced dissociation and density functional theory

Threshold collision-induced dissociation of M+(pyridine) with xenon is studied using guided ion beam mass spectrometry. M+ include the following metal ions: Mg+, Al+, Sc+: Ti+, V+, Cr+, Mn+, Fe+, Co+, Ni+, Cu+, and Zn+. In all cases, the primary product corresponds to endothermic loss of the intact pyridine molecule, with minor production of MXe+ formed by ligand exchange. In the Cr+(pyridine) system, an additional minor reaction pathway involving a ring-opening reaction with small neutral molecule loss is also observed. The cross-section thresholds are interpreted to yield 0 and 298 K bond dissociation energies for M+-pyridine after accounting for the effects of multiple ion-molecule collisions, internal energy of the reactant ions, and dissociation lifetimes. Density functional calculations at the B3LYP/6-31G(d,p) 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 at this level of theory are unreliable as even the trends in the calculated values do not parallel measured values. Measured M+-pyridine bond dissociation energies compare favorably to the four values previously determined by kinetic method or photodissociation experiments.