Site of protonation of benzonitrile hydrogen interchange in the protonated species

The site of protonation of gaseous benzonitrile in reactions with H ₃ ⁺ , CH₃OH ₂ ⁺ , and CH₃CNH⁺ as protonating agents has been examined by using tandem mass spectrometry in combination with deuterium and ¹³C labeling. Metastable and collision-induced dissociation studies of C₆X₅CNX⁺ (X = H or D) show that proton attachment occurs on the CN group. The metastably decomposing C₆X₅CNX⁺ leads only to C₆X₅ ⁺ + XCN. This reaction proceeds via a mechanism involving H⁺ (D⁺) transfer from the CN group to the phenyl ring in which H/D exchange occurs. The efficiency of the CN-to-ring H+ (DC ) transfer increases in the order para < meta < ortho position. Evidence for incomplete H/D atom randomization in C₆X₅CNX⁺ prior to XCN loss has been obtained. Both processes, CN-to-ring H/D exchange and H/D exchange at the phenyl ring, are affected by the internal energy of the C₆X₅CNX⁺ ions. The results have been interpreted in terms of the internal energy distribution of the ions fragmenting within the metastable time window. Collision-induced dissociation of C₆X₅CNX⁺ causes the energy-enriched ions to decompose by direct bond cleavage into C₆X ₅ ⁺ + XNC.