Ionized Benzonitrile and Its Distonic Isomers in the Gas Phase

Abstract

The dehydrobenzonitrilium distonic radical cations a−c, isomers of benzonitrile radical cation d, were prepared by collisional dehalogenation of protonated halogenobenzonitriles and characterized by tandem mass spectrometry (MS/MS/MS experiments and ion−molecule reactions) making use of a hybrid mass spectrometer of sector−quadrupole−sector configuration. These experimental results were further supported by density functional theory calculations at the B3LYP/6-311++G(3df,2p) level, indicating that although the distonic ions are less stable than ion d by ca. 45−50 kJ mol-1, they are protected against isomerization by relatively large energy barriers. Relative energies of benzonitrile and its isomers as well as their proton affinities and ionization energies were also evaluated. N-protonation in benzonitrile is about 120 kJ mol-1 more exothermic than ring protonation.