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Positive and negative gas-phase ion chemistry of chlorofluorocarbons in air at atmospheric pressure

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Abstract

This paper presents a report on the ionization/dissociation of some representative chlorofluoro-carbons (CFCs) induced by corona discharges in air at atmospheric pressure. Both positive and negative ions formed from Freons 1,1,1-trichlorotrifluoroethane (CFC 113a), 1,1,2-trichlorotrifluoroethane (CFC 113), and 1,1,1,2-tetrachlorodifluoroethane (CFC 112a) were analyzed using an atmospheric pressure chemical ionization mass spectrometry (APCI-MS) instrument. Energy-resolved mass spectra were obtained by modulating the kinetic energy of the ions via adjustment of the sampling cone potential (Vcone). Positive ion spectra of the CFCs (M) at low Vcone show no signals due to either M or MH+ but only those due to species [M − Cl]+ and CX3+ (X = Cl, F), likely formed via C–Cl and C–C bond cleavages following ionization via charge exchange. Charge localization in the products of C–C bond cleavage in M is driven by the stability of the neutral fragment. At low Vcone the hydrates [M − Cl]+(H2O) are also observed. In the case of 1,1,2,-trichlorotrifluoroethane, [M − F]+ species also form as a result of ion-molecule reactions. As Vcone is increased collision-induced dissociation of [M − Cl]+ and [M − F]+, i.e., the perhalogenated cations C2X5+ (X = Cl, F), takes place via carbene elimination. In some cases such elimination is preceded or accompanied by rearrangements involving transfer of halogen from one carbon to the other. Evidence is also presented for the occurrence of a condensation reaction of C2Cl3F2+ with water to form a C2Cl2F2HO+ species via elimination of HCl. Negative ion spectra are dominated by Cl and its ion-neutral complexes with M and with water. Additional components of the plasma include ion-neutral complexes O3(M), the molecular anion M (observed only with 1,1,2-trichlorotrifluoroethane), and an interesting species corresponding to [M − Cl + O]. The origin and structure of these [M − Cl + O] species are discussed in terms of available thermochemical and reactivity data and current mechanistic views concerning reaction of O2 with halogenated compounds. The observation of both positive and negative ions containing oxygen is of special relevance to development of new processes for the treatment of volatile organic compounds (VOCs) based on oxidative decomposition induced by corona discharges in air at room temperature and pressure. Copyright © 2002 John Wiley & Sons, Ltd.

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