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+ (D+) 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 + XCN.