Characterization of an exception to the ‘even‐electron rule’ upon low‐energy collision induced decomposition in negative ion electrospray tandem mass spectrometry

Electrospray-generated precursor ions usually follow the 'even-electron rule' and yield 'closed shell' fragment ions. We characterize an exception to the 'even-electron rule.' In negative ion electrospray mass spectrometry (ES-MS), 2-(ethoxymethoxy)-3-hydroxyphenol (2-hydroxyl protected pyrogallol) easily formed a deprotonated molecular ion (M-H)(-) at m/z 183. Upon low-energy collision induced decomposition (CID), the m/z 183 precursor yielded a radical ion at m/z 124 as the base peak. The radical anion at m/z 124 was still the major fragment at all tested collision energies between 0 and 50 eV (E(lab)). Supported by computational studies, the appearance of the radical anion at m/z 124 as the major product ion can be attributed to the combination of a low reverse activation barrier and resonance stabilization of the product ions. Furthermore, our data lead to the proposal of a novel alternative radical formation pathway in the protection group removal of pyrogallol.