Study on Reaction of Water Dimer Radical Cation with Benzene and its Derivatives Using Mass Spectrometry

Study of chemical properties of water radical cations is of great significance for elucidating related reactions process. In this paper, the reaction characteristics of (H₂O)₂^+• prepared online with several benzene derivatives were investigated. The results showed that, when (H₂O)₂^+• reacted with aromatic compounds without electron-withdrawing groups, such as benzene (C₆H₆, 78 u), anisole (C₇H₈O, 108 u) and 2,4-dimethylaniline (C₈H₁₁N, 121 u), the corresponding phenol product ((C₆H₅OH) ^+•, m/z 94), (C₇H₇O−OH)^+•, m/z 124) and (C₈H₁₀NOH+H)⁺, m/z 138)) were observed, accompanied with the production of electron transfer (ET) products (C₆H₆) ^+•, (C₇H₈O) ^+•) or proton transfer (PT) product ((C₈H₁₁N+H)⁺). Isotope labeling experiments proved that OH in corresponding phenol products was originated from (H₂O)₂^+•. Interestingly, when (H₂O)₂^+• reacted with substituted benzenes bearing strong electron-withdrawing group, such as benzonitrile (C₆H₅CN, 103 u) and nitrobenzene (C₆H₅NO₂, 123 u), the main products were the radical substituted adducts (C₆H₅CN+ H₂O)^+• (m/z 121) and (C₆H₅NO₂+H₂O)^+• (m/z 141), respectively, without phenol products or substituted benzene radical cation were noticed. It was obvious that the reaction between (H₂O)₂^+• and the substrate occurred through four competing processes: (a) electron transfer (ET) reaction, in which aromatic compounds release an electron to (H₂O)₂^+• to from an aromatic radical cation; (b) proton transfer (PT) reaction in which aromatic compounds got a proton derived from the dissociation of (H₂O)₂^+•; (c) dissociative electron transfer (DET) reaction, accompanied with the electron transfer process, C−H bond cleavage and C−OH bond formation; (d) radical substitution reaction in which a radical cation adduct consisting of aromatic molecule and H₂O generates. It was speculated that the diverse reactivity of (H₂O)₂^+• was owe to its two interchange structures ((H₂O)H⁺−^•OH and H₂O∴OH₂^+•) generates. It was proposed that the electron withdrawing groups on the aromatic ring would benefit for the existence of two-center and three-electron structure of (H₂O)₂^+• and induced the radical substitution reaction. In contrary, compounds without electron withdrawing groups or bearing electron donating groups on the aromatic ring would contribute to the presence of proton transfer structure of (H₂O)₂^+• (containing H⁺ and ^•OH) and facilitate the C−OH bond formation. This study will help to elucidate the chemical reaction processes involving benzene or substituted benzenes and (H₂O)₂^+•.