Evidence for the Participation of Excited Electronic States in the Unimolecular Dissociation Reactions of Ionic Complexes between NO and Aromatic Compounds

Abstract

The nitrosation of aromatic molecules A (where A=benzonitrile, benzene, pyridine, furan and thiophene) has been investigated by tandem mass spectrometry. The unimolecular dissociation of metastable ionic complexes generated by NO chemical ionization of benzonitrile changes as a function of ion source pressure. As the NO concentration in the source increases, the metastable ion (MI) mass spectrum goes from being dominated by NO+ to one dominated by ionized benzonitrile and its fragmentation products. This can only be explained by the formation of an excited-state complex in the source [formed when NO+ (a 3Σ+) binds to benzonitrile] that can dissociate to form high internal energy ionized benzonitrile. A similar result is observed when A=pyridine. When A=benzene, the MI mass spectrum is dominated by ionized benzene, a result that is consistent with the adiabatic dissociation of the ground singlet-state of the complex, provided that electron transfer between the two members of the complex is slow compared with the microsecond timescale of the experiments. If fast exchange does occur, as is suggested by theoretical calculations of the complex, then the lowest energy triplet-state must be co-produced in the ion source. The MI mass spectra of the complexes involving furan and thiophene are consistent with the dissociation of the lowest energy singlet-state.