Infrared and electronic spectroscopy of benzene-ammonia cluster radical cations [C₆H₆(NH₃)_1,2]⁺: Observation of isolated and microsolvated σ-complexes

We report infrared (IR) and electronic spectra of benzene-ammonia cluster radical cations [C₆H₆(NH₃)_n] ⁺ (n = 1 and 2) in the gas phase to explore cluster structures and chemical reactivity of the simplest aromatic radical cation with base (nucleophile) molecules. The electronic spectra in the visible region indicate that these cluster cations no longer have the benzene cation chromophore as a result of an intracluster reaction. Analyses of the IR spectra, on the basis quantum chemical calculations and the vibration-internal rotation analysis, reveal that both [C₆H₆(NH₃)_1,2] ⁺ form σ-complex structures, in which the ammonia moiety is covalently bonded to the benzene moiety due to the intracluster nucleophilic addition. For [C₆H₆(NH₃)₂] ⁺, it is also shown that the second ammonia molecule solvates the σ-complex core via a N-H⋯N hydrogen bond. Such σ-complex structures are generally supposed to be a key intermediate of aromatic substitution reactions. The observed mass spectra and energetics calculations, however, show that [C₆H₆(NH₃)_n] ⁺ systems are inert for aromatic substitutions. The present experimental observations indicate the inherent stability of these σ-complex structures, even though they do not show the aromatic substitution reactivity.