Two questions arise from the CID spectra of M⁺PMMAn:

        1. Why is the loss of M⁺ more abundant for the larger alkali ions? (i.e. Why do the bare Rb⁺ and Cs⁺ peaks dominate the CID spectra while Na⁺ and Li⁺ do not?)

        2. Why are there only small m/z fragments in the CID spectra that originate from each end of the oligomer? (i.e. Only A₁, A₂ and B₁, B₂ fragments are observed. Why not A₇ or B₅?)

    The first question appears to be related to how well the PMMA oligomer can wrap around the metal cation. The lowest-energy structures of the PMMA 9-mer cationized by Li⁺, K⁺, and Cs⁺ are shown at right. All three metal cations bind to carbonyl oxygens near both ends of the oligomer, but the smaller Li⁺ ion is almost enveloped by the PMMA oligomer while the larger K⁺ and Cs⁺ are significantly more exposed.
    The larger metal cations also have significantly smaller M⁺-PMMA binding energies (see table below left). Hence, once energized by collisions with the bath gas in the CID experiments, it is more energetically feasible to lose Cs⁺ than it is Li⁺.
    The second question appears to be related to the U-shaped conformations that the M⁺PMMAn oligomers fold into. Upon collisional activation, the M⁺PMMA oligomer can simply "open up", leaving M⁺ attached to either end of the oligomer. From this configuration, the M⁺ ion can either dissociate (more likely for larger metal cations) or induce a backbone cleavage nearby (generating A_x and B_y fragments). This scheme is illustrated in the figure below at right.
    It was originally proposed that the small A_x and B_y fragments arose from sequential depolymerization of the PMMA oligomer similar to poly(styrene) oligomers. However, this would be an energetically uphill battle. Molecular mechanics calculations on the formation of M⁺-A_x and M⁺-B_y fragments indicate that M⁺ is bound much stronger to larger A and B fragments. For example, the binding energy for Na⁺-A₁ is 26 kcal/mol but the Na⁺-A₅ binding energy is 87 kcal/mol.

    Since only small m/z fragments (A₁, A₂ and B₁, B₂) are observed in the CID spectra, the M⁺ cations must preferentially induce cleavage nearby. Also, once fragmentation occurs, the products must permanently separate, otherwise the M⁺ cation would attach to the larger neutral fragment (for energetic reasons) and larger A_x and B_y fragments would be observed. This is possible if the M⁺PMMAn oligomer is U-shaped. The initial "opening up" step would yield a backbone that is highly excited. When a small fragment is cleaved, it would have to leave at high velocity in order to conserve angular momentum.