Electron capture, femtosecond electron transfer and theory: A study of noncovalent crown ether 1,n-diammonium alkane complexes

Complexes of doubly protonated 1,n-diaminoalkanes with one or two molecules of 18-crown-6-ether undergo consecutive and competitive dissociations upon electron capture from a free thermal electron and femtosecond collisional electron transfer from Na and Cs atoms. The electron capture dissociation (ECD) and electron capture-induced dissociation (ECID) mass spectra show very different products and product ion intensities. In ECD, the reduced precursor ions dissociate primarily by loss of an ammonium hydrogen and the crown ether ligand. In ECID, ions from many more dissociation channels are observed and depend on whether collisions occur with Na or Cs atoms. ECID induces highly endothermic CC bond cleavages along the diaminoalkane chain, which are not observed with ECD. Adduction of one or two crown ethers to diaminoalkanes results in different electron capture cross-sections that follow different trends for ECD and ECID. Electron structure calculations at the B3-PMP2/6-311++G(2d,p) level of theory were used to determine structures of ions and ion radicals and the energetics for protonation, electron transfer, and ion dissociations for most species studied experimentally. The calculations revealed that the crown ether ligand substantially affected the recombination energy of the diaminoalkane ion and the electronic states accessed by electron attachment.