Alkali Metal Cation Interactions with 15-Crown‑5 in the Gas Phase: Revisited

Quantitative interactions of the alkali metal cations with the cyclic 15-crown-5 polyether ligand (15C5) are studied. In this work, Rb⁺(15C5) and Cs⁺(15C5) complexes are formed using electrospray ionization and studied using threshold collision-induced dissociation with xenon in a guided ion beam tandem mass spectrometer. The energy-dependent cross sections thus obtained are interpreted to yield bond dissociation energies (BDEs) using an analysis that includes consideration of unimolecular decay rates, internal energy of the reactant ions, and multiple ion–neutral collisions. 0 K BDEs of 175.0 ± 9.7 and 159.4 ± 9.6 kJ/mol, respectively, are determined and exceed those previously measured [J. Am. Chem. Soc. 1999, 121, 417−423] by 68 and 57 kJ/mol, respectively, consistent with the hypothesis proposed there that excited conformers had been studied. Because the analysis techniques have advanced since this early study, we also reanalyze the published data for the Na⁺(15C5) and K⁺(15C5) systems to ensure a self-consistent interpretation of all four systems. Revised BDEs for these systems are 296.1 ± 15.5 and 215.6 ± 10.6 kJ/mol, respectively, which are within experimental uncertainty of the previously reported values. In addition, quantum chemical calculations are conducted at the B3LYP/def2-TZVPPD level of theory with theoretical BDEs in reasonable agreement with experiment. Computations are also used to explore features of the potential energy surfaces for isomerization of the M⁺(15C5) complexes