Mass spectrometry of alkylbenzenes and related compounds. Part II. Gas phase ion chemistry of protonated alkylbenzenes (alkylbenzenium ions)

Mass spectrometry and ion chemistry of alkylbenzenes, today, is much more than the gas-phase reactions of their radical cations and their prominent fragment ions, such as C7H7+ and C7Hs+'. Indeed, the gas-phase chemistry of gaseous ions originating mainly from the electron impact (EI) ionization of alkylbenzenes has been investigated extensively for more than 35 years and has revealed to us numerous amazing insights in elementary chemical processes. Many of these aspects, covered in Part I of this review (l), are under continuous investigation. Nevertheless, this has been only half of the story. Mass spectrometry of alkylbenzenes also comprises the gas-phase chemistry of protonated alkylbenzenes. Interest in protonated alkylbenzenes developed with the introduction of chemical ionization (CI) mass spectrometry (2-6). Munson and Field (7) were the first to apply this method to alkylbenzenes. However, the much closer relevance of evenelectron, protonated arene ions to electrophilic aromatic substitution (8,9) in solution and synthetic organic chemistry in general has made the gas-phase ion chemistry of protonated alkylbenzenes an independently important topic of mass spectrometric research. At the same time it was found that protonated alkylbenzenes can be generated in superacidic media (10) and studied by nuclear magnetic resonance spectrometry as well as by other spectroscopic techniques (11-17). Furthermore, computational approaches were widely applied to estimate the intrinsic properties of these ions (18). Later, besides simple protonation, ion-molecule reactions were used to generate transient or collisionally stabilized adduct ions from electrophiles and aromatic substrates. In additions to CI mass spectrometry, newer and complementary techniques were applied, among them, ion cyclotron resonance [ICR (19-21)] and high-pressure chemical ionization mass spectrometry [HPCI, (22)], triple-stage quadrupole mass spectrometry [TQMS (23)], and radiolytic and nuclear fission