Intrinsic affinities of alkali metal cations for diaza-18-crown-6: Effects of alkali metal cation size and donor atoms on the binding energies

Abstract

Threshold collision-induced dissociation of alkali metal cation-diaza-18-crown-6 complexes, M+(da18C6), with xenon is studied using guided ion beam tandem mass spectrometry techniques. The alkali metal cations examined here include: Na+, K+, Rb+, and Cs+. In all cases, M+ is the only product observed, corresponding to endothermic loss of the intact da18C6 ligand. The cross section thresholds are analyzed to extract zero and 298K M+da18C6 bond dissociation energies (BDEs) after properly accounting for the effects of multiple ion-neutral collisions, the kinetic and internal energy distributions of the reactants, and the lifetimes for dissociation. Density functional theory calculations at the B3LYP/def2-TZVPPD and B3LYP/6-31+G* levels of theory are used to determine the structures of da18C6 and the M+(da18C6) complexes and provide molecular constants necessary for the thermodynamic analysis of the experimental data. Theoretical BDEs are determined from single point energy calculations at the B3LYP and MP2(full) levels of theory using the def2-TZVPPD and 6-311+G(2d,2p) basis sets using the B3LYP/def2-TZVPPD and B3LYP/6-31+G* optimized geometries. The agreement between B3LYP/def2-TZVPPD theory and experiment is excellent for all four M+(da18C6) complexes. The M+da18C6 BDEs decrease as the size of the alkali metal cation increases, consistent with the electrostatic nature of the binding in these complexes. The M+(da18C6) structures and BDEs are compared to those previously reported for the analogous complexes of 18-crown-6 and hexaaza-18-crown-6, to examine the effects of the donor atoms (N versus O) on the structure and strength of binding.