^aInstitut für Organische Chemie, Technische Universität Berlin, Berlin (Germany)
^bDepartment of Chemistry and Biochemistry, University of Notre Dame, Indiana (USA)
^cInstitude de Chimie Physique, Université de Fribourg, Fribourg (Switzerland) .

Abstract: The ion chemistry of anti-o,o'-dibenzene (1) was examined in the gaseous and the condensed phase. From a series of comparative ion cyclotron resonance (IRC) mass spectrometry experiments which involved the interaction of Cu⁺ with 1, benzene, or mixture of both, it was demonstrated that 1 can be brought into the gas phase as an intact molecule under the experimental condition employed. The molecular ions, formally 1^.+ and 1^.-, were investigated with four-sector mass spectrometer in metastable-ion decay, collisional activation, charge reversal, and neutralisation-reionization experiments. Surprisingly, the expected retrocyclization to yield two benzene molecules was not dominant for the long lived molecular ions; however, other fragmentations, such as methyl and hydrogen losses, prevailed. In contrast, matrix ionization of 1 in freon (77 K) by g-irradiation or in Ar, (12 K) by X-irradiation leads to quantitative retrocyclization to the cationic dimer of benzene 2^.+. Theoretical modelling of the potential energy surfaces of the retrocyclization shows that only a small, if any, activation barrier is to be expected for this process. In another series of experiments, metal complexes of 1 where investigated. 1/Cr⁺ was formed in ion source and examined by metastable ion decay and collisional activation experiments, which revealed predominant losses of neutral benzene. Nevertheless, comparison with the bis-ligated [(C₆H₆)₂Cr]⁺ complex provided evidence for the existence of an intact 1/Cr⁺ under these experimental conditions. No evidence for the existence of 1/Fe⁺ was obtained, which suggest that iron mediates the rapid retrocyclisation of 1/Fe⁺ into bis-ligated benzene complex[(C₆H₆)₂Fe]⁺.