MALDI-TOF-MS of Saturated Polyolefins by Coordination of Metal Cations: A Theoretical Study

Density functional calculations were applied to obtain binding energies for metal cation-oligomer complexes of n-alkanes and poly(ethylene glycol)s (PEG). The B3LYP/6-31G* energies for complexing metal cations (Na+, Li+, Co+, Cu+, Zn+, and Zn2+) with straight chain aliphatics (C n H2 n +2, n = 1−12) are in excellent agreement with the limited available experimental and theoretical data. The strength of the complexes increases with an increasing degree of polymerization and with a decreasing size of the metal ion. The weakest calculated complex is Na+−CH4 (7.8 kcal/mol) and the strongest is Co+−dodecane (52.0 kcal/mol). Smaller sized cations, such as Li+, induce more polarized hydrocarbons. Transition metals give stronger complexes than the main group metals because their d-electrons shield the nuclear charge less effectively. M+−methane binding energies, ranging from 12.6 to 23.1 kcal/mol, are also reported for Sc+, Y+, La+, Cu+, Ag+, and Au+. Doubly charged metal ions give much stronger complexes, that is, 67.1 kcal/mol for Zn2+−CH4. Methane binding energies of ca. 20 kcal/mol are obtained when the Be2+, Mg2+, Fe2+, and Zn2+ dications are ligated with a cyclopentadienyl anion. The poly(ethylene glycol)s (HO−[C2H4O] n −H, n = 1−5) bind significantly stronger to the metal cations (Na+ and Cu+) than the aliphatics. The Na+−monomer (C2H6O2) already has a complex strength of 48.5 kcal/mol, while that of the pentamer (C10H22O6), which is the smallest observable Na+−complex by MALDI-TOF-MS, amounts to 88 kcal/mol. The corresponding Cu+ complexes are even stronger with a value of 79.6 kcal/mol for Cu+−C2H6O2. Binding energies of 53.3 and 64.1 kcal/mol are calculated for the respective K+−tetramer and K+−pentamer of which the pentamer is also observed spectroscopically.