Poly (ethylene terephthalate) oligomers cationized by alkali ions: Structures, energetics, and their effect on mass spectra and the matrix-assisted laser desorption/ionization process

In this article the folding dynamics and energetics for a set of poly(ethylene terephthalate) (PET) oligomers cationized by various alkali ions are studied: M⁺PETn for n = 2 to 7 and M = Li, Na, and K. Experimental cross sections were determined for matrix-assisted laser desorption/ionization (MALDI) generated ions using the ion mobility based ion chromatography method. Very good agreement was obtained with cross sections generated by the AMBER 4.0 suite of molecular dynamics programs. For n = 2 and 4 the benzene rings of the oligomers π stack with the metal ion coordinated to both terminal hydroxyl oxygen atoms and several of the nearby carbonyl oxygen atoms. For n = 3, two isomers are both observed and predicted by theory: an open form where the third PET monomer attaches to the dimer and extends into space and a closed form where the third PET moiety bends back and coordinates its hydroxyl oxygen with the metal ion. For Na⁺PET3, equilibrium is observed between 100 and 180 K with an Arrhenius analysis yielding an open to closed form isomerization barrier of 1.6 kcal/mol. For this same system the two isomeric forms are frozen out at 80 K and coupling the observed isomeric distribution with an RRKM analysis indicates the closed form is more stable by 0.5 kcal/mol. For K⁺PET3 the barrier to isomerization is too low to observe (<1.0 kcal/mol), whereas for Li⁺PET3 a temperature independent isomer distribution is observed (80 to 55°K). Using methods developed for determining isomerization barriers in carbon clusters it was possible to obtain an open to closed form isomerization barrier of 7 ± 2 kcal/mol for Li⁺PET3. In this system, the open and closed form isomer populations were observed to be strong functions of the laser power in the MALDI source. This allowed a detailed description of the formation mechanism to be formulated and indicated alkali ion attachment to the polymer during expansion of the plume emanating from the surface. Finally, the mass spectrum of a PET oligomer sample has been shown to strongly depend on the cationizing alkali metal ion. It is qualitatively shown that M⁺-PETn binding energies and structures are responsible.