Chimica di specie fluorurate neutre e ioniche di interesse fondamentale, ambientale e applicativo
Author(s)
Giordani, Maria
Date Issued
April 22, 2008
Type
Doctoral Thesis
Abstract
The results obtained in my doctorate on the investigation of the neutral and ionic fluorinated species were discussed in the present paper. In particular, as part of continuing interest by my group in the chemistry of fluorinated species, I studied the properties of the nitrogen trifluoride, neutral and ionic germanium fluorides and perfluoalkanesulfonamides. The structure, stability and termochemistry of these compounds are of considerable interest not only for fundamental reasons, but also for their industrial role and potential environmental impact. Nitrogen trifluoride is one of the perfluorocompounds most extensively used in semiconductor technology and it has features of greenhouse gases. The interaction of NF3 with monoatomic ions has been theoretically investigated over the years and we decided to extend the studies to the adducts of NF3 with Be+ and Mg+. Our results suggest that the Be+-(NF3) and Mg+-(NF3) complexes could be actually observed as stable species in the gas phase, and support the proposal that techniques such as Mg+ ion attachment mass spectrometry could be used to quantify the emission of NF3. The ion chemistry occurring in germane/fluorocompound gaseous mixtures is still unexplored and mixtures containing GeH4 and SiF4 or SiF6 have been used to deposit hydrogenated and fluorinated Si-Ge and S-Ge alloys by plasma techniques. We studied GeH4/NF3, GeH4/SF6 and GeH4/SiF4 gaseous mixtures by ion trap mass spectrometry and ab initio calculations. Our results indicate that the GeHn+ (n = 0-3) show instead a distinct preference for the fluorine atoms of MFn (MFn = NF3, SF6 and SiF4), and form exclusively ionic products such as GeF+ and F-GeF2+. The fluorinating ability of MFn is also strictly dependent on the strength of the M-F bond and strongly decreases in the order NF3 >> SF6 > SiF4. The reactivity of GeHn+ results in the order Ge+ >> GeH2+ > GeH+ >> GeH3+. The properties of neutral germanium fluorides GeFn (n = 1-5) have been intensively investigated for their role in the fine processing of semiconductors. We decided to undertake the computational investigation of the instead little explored GeFn+ (n = 1-3). The obtained results on the structures, harmonic frequencies, enthalpies of formation and dissociation energies of these species were discussed in connection with previous experimental and theoretical data. Stimulated by a previous study on cationic hybrides of group XIV, which disclosed a periodic structural switch from covalent structures to ion-molecule complexes, we performed ab initio calculations on the geometries, harmonic frequencies, and relative stabilities of the (Si,H,F)+, (Ge,H,F)+, (Si,F3)+, (GeF3)+ and (A,HnF3-n+ (A = Si, Ge, Sn, Pb; n = 0-2) isomeric ions. Our results indicate that, for the fluorine-substituted cations, ion-molecule complexes such as FGe+-(H2) and FGe+-(HF), as well as Ge+-(HF), are indeed more stable that their corresponding covalent structures. In addition, for tin- and lead-containing analogues, the energy differences increase. From a general point of view, these findings suggest that the covalent structure of even the simplest AH2-X+- or AHX2+-substituted cations of group XIV cannot be taken as granted. The reactivity of gaseous GeFn+ is still essentially unexplored, and we decided to investigate by ion-trap mass spectrometry (ITMS) and ab initio calculations, the reaction between GeF+ and H2O. Similar to the previously investigated reaction between SiF+ and H2O, this process involves the initial formation of the FGe+-OH2 intermediate, its isomerisation to HOGe+-FH, and the eventual dissociation into Ge-OH+ and HF. Compared with SiF+, the reaction between GeF+ and H2O is apparently more efficient. This suggest that the GeFn+ cations (n = 1-3) could undergo ion-molecule reactions similar to the variegated processes already ascertained for the cationic silicon fluorides. Perfluoroalkanesulfonates (PFSAs, CF3(CF2)nSO3-) and perfluoroalkanecarboxylates (PFCAs, CF3(CF2)n,CO2-) extensively used in number of applications, have been recently detected as persistent contaminants of wildlife. They could be formed by the atmospheric degradation of precursors such as the fluorotelomer alcohols CnF2n+1C2H4OH (n = 6, 8, 10) and the N-substituted perfluoroalkanesulfonamides. These processes probably involve the oxidation of the primary N-radicals. The atmospheric fate of the amine radicals R1-N-R2 has however received only limited attention, and, in particular, neither experimental nor theoretical studies have been reported on the structure and stability of the products arising from the oxidation of the sulfonamide radicals R1-SO2-N-R2. We therefore decided to undertake a theoretical investigation on the Cl-initiated oxidation of the model compound CF3SO2-NH-CH2CH3. Our calculations describe in particular the thermodynamic aspects of the possible Cl-initiated oxidation reactions of the N-ethyl-perfluoroalkanesulfonamides so to obtain a first indication on the structure and stability of the observed experimentally products.
Additional information
Dottorato di ricerca in Scienze ambientali
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