Sigma versus Pi interactions in alkali metal ion binding to azoles: Threshold collision-induced dissociation and ab initio theory studies

Threshold collision-induced dissociation of M+(azole) with xenon is studied by guided ion beam mass spectrometry. M+ include the following alkali metal ions: Li+, Na+, and K+. The azoles studied include pyrrole, 1-methylpyrrole, pyrazole, 1-methylpyrazole, and 1-methylimidazole. In all cases, endothermic loss of the intact neutral azole is observed as the primary reaction pathway. Minor production of M+Xe formed by ligand exchange is also observed. The cross-section thresholds are interpreted to yield 0 and 298 K binding energies for M+-azole after accounting for the effects of multiple ion-neutral collisions, internal energy of the reactant ions, and dissociation lifetimes. Ab initio calculations at 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. Single-point calculations at the MP2(full)/6-311+G(2d,2p) level using the MP2(full)/6-31G* optimized geometries are used to obtain theoretical bond dissociation energies. Zero-point energy and basis set superposition error corrections are also included. Excellent agreement between theory and experiment is found for the Na+ and K+ systems, whereas the theoretical bond dissociation energies to the Li+ systems are systematically low. The calculated and measured bond dissociation energies are compared among the systems examined here and to previous values from the literature, to determine the influence that the metal ion, the nature of the binding interaction (pi vs sigma binding), and the dipole moment, quadrupole moment, and polarizability of the ligand have on the strength of the binding in these complexes.