Gas‐phase reactions of the 4‐dehydroanilinium ion and its isomers

Collisional activation of protonated 2‐, 3‐ and 4‐iodoanilines in a dual‐cell Fourier transform ion cyclotron resonance spectrometer results in the loss of an iodine atom to yield product ions of m/z 93. Ion–molecule reactions and energy‐resolved collision‐activated dissociation experiments demonstrate that these three ions are structurally distinct, and that they are distonic ions. The ions were identified as the 2‐, 3‐ and 4‐dehydroanilinium ions, the high‐energy distonic isomers of ionized aniline. The reactivity of these charged phenyl radicals is largely analogous to that of the phenyl radical. For example, the 4‐dehydroanilinium ion abstracts an iodine atom from isopropyl iodide, a thiomethyl group from dimethyl disulfide, and adds to cyclohexene. Comparison of the collision‐activated dissociation product abundances of the ion formed upon CH3S abstraction to that of a reference ion suggests that bond formation occurs at the formal radical site upon CH3S abstraction. Hence, the reaction probably occurs via a simple radical abstraction mechanism similar to that of the phenyl radical. However, some neutral reagents react with the charge site of the 4‐dehydroanilinium ion, and yet others undergo a two‐step reaction sequence involving the charge site in addition to the radical site. For example, certain basic molecules induce isomerization of the 4‐dehydroanilinium ion to the significantly more stable ionized aniline by deprotonating the charge site and then donating a hydrogen atom to the radical site.