Joseph D. Anderson, Eric S. Paulsen, D. Dearden
May 15, 2003
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2
Influential Citations
70
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Journal
International Journal of Mass Spectrometry
Abstract
Abstract We have experimentally bracketed the gas phase Li+, Na+, K+, Rb+, and Cs+ affinities of dibenzo-18-crown-6 in the order 15- crown -5 (15 C 5) dibenzo -18- crown -6 ( DB 18 C 6) crown -6 (18 C 6) . Comparison with published experimental threshold collision-induced dissociation measurements yields enthalpies of binding at 298 K (in kJ mol−1) as follows: DB18C6·Na+, −299±26; DB18C6·K+, −221±19; DB18C6·Rb+, −154±38; DB18C6·Cs+, −136±35 (experimental values for the bracketing ligands binding Li+ are not available). Thus, dibenzo substitution of the crown makes it a weaker ligand toward the alkali metal ions, in contrast to dicyclohexano substitution, which was previously shown to make the crown a stronger ligand toward all the alkali metal ions. We have also calculated the enthalpies of binding between these metal ions and DB18C6 using both the LANL2DZ basis set and a modified version of the 6-31+G∗ basis set wherein diffuse functions are removed from carbon atoms and the K+, Rb+, and Cs+ basis functions are those of the Hay–Wadt (n+1) effective core potential set. Full geometry optimizations were carried out at the HF/6-31+G∗ and HF/LANL2DZ levels of theory, and energies were also calculated for the HF/6-31+G∗ geometries at the MP2(full)/6-31+G∗ and B3LYP/6-31+G∗ levels. The computed enthalpies of binding for the K+, Rb+, and Cs+ complexes of DB18C6 fall between computed values in the literature for 15C5 and 18C6 complexes at both HF/6-31+G∗ and MP2(full)/6-31+G∗ levels, in good agreement with our bracketing experiments. The values at both levels of theory for the DB18C6 complexes of Li+ and Na+ are larger than those in the literature for both 15C5 and 18C6; the reasons for this discrepancy are not clear.