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_nH_2n+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.