The β-Distonic Ion from the Reaction of Pyridine Radical Cation and Ethene: A Demonstration of High-Pressure Trapping in Fourier Transform Mass Spectrometry

The adduct ion formed in the reaction of pyridine radical cation and ethene exhibits different chemical reactivities and fragmentation behavior than the isomeric ethylpyridine radical cations. The most characteristic features of the adduct ion are the collisionally activated loss of C₂H₄ to give back the pyridine radical cation and the second ionization to produce a doubly charged ion. A characteristic reaction is with dioxygen to form a new adduct, which may have the structure of a distonic peroxy radical cation. The reactive site for attachment of C2H₄ is the nitrogen atom of the pyridine, giving rise to the β-distonic ion adduct, C₅H₅N⁺−CH₂C·H₂, which can be differentiated from the isomeric α-distonic ion, C₅Η₅Ν⁺−C·ΗCΗ₃. Formation of the β-distonic ion requires collisional stabilization, and this species cannot be directly observed under conventional Fourier transform mass spectrometry (FTMS) conditions. Incorporation of a new radiofrequency (RF)-only-mode event in FTMS allows the adduct to be formed and then to be characterized under the high-pressure conditions of two of these RF-only-mode events.