Intrinsic affinities of alkali cations for 15-crown-5 and 18-crown-6:: Bond dissociation energies of gas-phase M+-crown ether complexes

Bond dissociation energies (BDEs) of M+[c-(C2H4O)(5)] and M+[c-(C2H4O)(6)] for M = Na, K, Rb, and Cs are reported. The BDEs are determined experimentally by analysis of the thresholds for collision-induced dissociation of the cation-crown ether complexes by xenon measured by using guided ion,beam mass spectrometry. In all cases, the primary and lowest energy dissociation channel observed experimentally is endothermic loss of the ligand molecule. The cross section thresholds are interpreted to yield 0 and 298 K BDEs after accounting for the effects of multiple ion-molecule collisions, internal energy of the complexes, and unimolecular decay rates. For both 18-crown-6 and 15-crown-5, the BDEs decrease monotonically with increasing cation size. These results indicate that the intrinsic affinity of c-(C2H4O)(5) and c-(C2H4O)(6) for M+ is determined principally by the charge density of the cation not by the ratio of the ionic radius to the cavity size. The BDEs reported here are in fair agreement with recent ab initio calculations at the MP2 level with 6-31+G* basis sets. The experimental values are systematically smaller than the computed values by 8 +/- 2 kJ/mol per metal-oxygen interaction. The existence of less strongly bound isomers in the experimental apparatus for Rb+(15-crown-5) and Cs+(15-crown-5) appears likely, but their absence for Na+ and K+ complexes indicates interesting metal-dependent dynamics to the formation of such isomers.