Influence of s and d Orbital Occupation on the Binding of Metal Ions to Imidazole

Abstract

Threshold collision-induced dissociation of M+(imidazole) with Xe is studied using guided ion beam mass spectrometry techniques. The metal ions, M+, studied include: Na+, Mg+, Al+, Ca+, Sc+, Ti+, V+, Cr+, Mn+, Fe+, Co+, Ni+, Cu+, and Zn+. For the systems involving Na+, Mg+, Al+, Ca+, and the late transition metals, Cr+ through Cu+, the primary product corresponds to endothermic loss of the intact imidazole molecule. For the Zn+ system, this process also occurs but forms Zn + imidazole+ as the dominant product and Zn+ + imidazole as a minor dissociation pathway. For the complexes to the early transition metal ions, Sc+, Ti+, and V+, loss of the intact imidazole ligand competes with endothermic elimination of HCN to form M+(C2H3N). The energy-dependent collision-induced dissociation cross sections for M+(imidazole) are modeled to yield threshold energies that are directly related to 0 and 298 K bond dissociation energies (BDEs) for M+−imidazole after accounting for the effects of multiple ion-neutral collisions, kinetic and internal energy distributions of the reactants, and lifetimes for dissociation. Density functional theory calculations at the B3LYP/6-31G* level of theory are performed to determine the structures of these and the H+, Li+, and K+ complexes and to provide molecular constants needed for the thermochemical analysis of experimental data. Theoretical BDEs are determined from single point energy calculations with an extended basis set, B3LYP/6-311+G(2d,2p), using the B3LYP/6-31G* optimized geometries. Excellent agreement between this level of theory and experiment is found for the Mn+, Fe+, Co+, Ni+, Cu+, and Zn+ systems examined here and for Li+ and K+ examined in earlier work. Although the agreement between theory and experiment is not as good for the other systems, the periodic trends in the BDEs are nearly parallel. Calculations at several other levels of theory are also performed to determine the level of theory required to obtain accurate energetics for these systems. The measured BDEs are compared to those previously measured for ammonia and adenine and to results for alkali metal ions bound to imidazole. The periodic trends are found to parallel those previously measured for these nitrogen donor ligands, suggesting that the binding in these complexes is very similar. The activated dissociation pathway observed in the complexes to the early transition metals also parallels that observed in complexes to adenine.