Lorenza Operti

Investigation of the complex reactions of SiH4 and GeH4 in the ion trap using dynamically programmed scanning

Abstract The rate constants of the gas-phase reactions of primary ions in a 1:1 SiH 4 /GeH 4 mixture have been determined by ion trap mass spectrometry and compared with those obtained for SiH 4 and for GeH 4 alone. The main reaction pathways are hydride or hydrogen transfer, formation of ions containing two silicon or two germanium atoms and ions containing silicon and germanium together. Experimental rate constants have been compared with the calculated collision rate constants and efficiencies have been determined. The formation of mixed ions, important in the radiolytical preparation of materials of interest in photovoltaic technology, is more efficient when SiH + m ( m = 0, 1) ions react with GeH 4 than when GeH + reacts with SiH 4 , and occurs at comparable rates to SiH + 2 and GeH + 2 reacting with GeH 4 and SiH 4 , respectively.

Mechanism of thermal degradation of urea-formaldehyde polycondensates

Abstract The thermal degradation to 500°C of urea-formaldehyde polycondensate occurs in four successive steps. In each step, partial volatilisation takes place while the polymer undergoes chemical modification to give progressively more stable structures. Below 200°C methylene ether bridges are transformed into methylene bridges and branching and crosslinking reactions occur with maximum rates at 125°C and 165°C, respectively. Above 200°C radicals formed by chain scission induce the formation of cyclic structures in the polymer which undergoes extensive fragmentation above 300°C. Water, formaldehyde, carbon monoxide and dioxide, methane, ammonia, monomethylamine and trimethylamine are the gaseous products evolved. By combining data on the chemical modifications and gases evolved in each step, reaction mechanisms are proposed.

Gas phase ion/molecule reactions in monogermane-hydrocarbon mixtures: a comparative Fourier transform mass spectrometry and chemical ionization mass spectrometry study

Abstract The gas phase ion/molecule reactions of GeH 4 with some simple saturated (CH 4 , C 2 H 6 ) and unsaturated hydrocarbons (C 2 H 2 , C 3 H 4 , C 3 H 6 ) have been studied by high pressure mass spectrometry and Fourier transform mass spectrometry. The effects of the nature of the hydrocarbon and of the total pressure and the relative concentrations of the reagent gases on the formation of GeC containing ions are reported. Saturated hydrocarbons give only limited amounts of such species, whereas a variety of GeC containing products are efficiently produced when alkenes or alkynes are added to germane. In these latter cases, GeCH + 3 and GeCH + 5 are among the most abundant products, invariably accompanied by ions containing germanium, carbon and hydrogen in relative yields increasing with the hydrocarbon partial pressure. For all systems, the reaction pattern is presented and discussed in relation to the preparation of amorphous GeC containing materials for photovoltaic applications.

Xenon–Nitrogen Chemistry: Gas‐Phase Generation and Theoretical Investigation of the Xenon–Difluoronitrenium Ion F2NXe+

The xenon-difluoronitrenium ion F(2)N-Xe(+) , a novel xenon-nitrogen species, was obtained in the gas phase by the nucleophilic displacement of HF from protonated NF(3) by Xe. According to Møller-Plesset (MP2) and CCSD(T) theoretical calculations, the enthalpy and Gibbs energy changes (ΔH and ΔG) of this process are predicted to be -3 kcal mol(-1) . The conceivable alternative formation of the inserted isomers FN-XeF(+) is instead endothermic by approximately 40-60 kcal mol(-1) and is not attainable under the employed ion-trap mass spectrometric conditions. F(2)N-Xe(+) is theoretically characterized as a weak electrostatic complex between NF(2)(+) and Xe, with a Xe-N bond length of 2.4-2.5 Å, and a dissociation enthalpy and free energy into its constituting fragments of 15 and 8 kcal mol(-1), respectively. F(2)N-Xe(+) is more fragile than the xenon-nitrenium ions (FO(2)S)(2)NXe(+), F(5)SN(H)Xe(+), and F(5)TeN(H)Xe(+) observed in the condensed phase, but it is still stable enough to be observed in the gas phase. Other otherwise elusive xenon-nitrogen species could be obtained under these experimental conditions.

Gas phase ion-molecule reactions of monogermane with oxygen and ammonia☆

High pressure and Fourier transform mass spectrometry have been used to study the ion-molecule reactions of germanium-containing ions with oxygen, ammonia, and GeH4 itself. The effects of the total pressure and of the ratio between GeH4 and oxygen or ammonia are reported. In self-condensation reactions the most reactive species are Ge+ and GeH2P+, which give dimer ions containing an even number of hydrogen atoms. Formation of GeHnO2+ (n = 0, 1) and GeHnO+ (n = 0−3) ions is observed in GeH4/O2 mixture. The most abundant species is GeHO+, which originates in the reaction of Ge2H2+ with one O2 molecule, as demonstrated by FTMS. High pressure experiments suggest that oxygen-containing ions are also formed by pathways involving monogermanium ions. Analogous behaviour is observed in the GeH4/NH3 mixtures, where GeNHn+ (n = 2, 3, 4, 6) ions are formed in higher abundances than GeHO+ (n = 1−3) ions under similar conditions.

Gas phase ion-molecule reactions of monogermane with carbon oxides and ethylene: Formation of germanium-carbon bonds

Abstract The gas phase ion-molecule reactions of GeH 4 with some carbon-containing compounds (CO, CO 2 , and C 2 H 4 ) have been studied by high pressure mass spectrometry and Fourier transform mass spectrometry. The effects of the total pressure and of the relative concentrations of the reagent gases on the ion pattern are reported. In the presence of CO and CO 2 , GeH n CO + or GeH n CO 2 + , GeH n O + , and GeH n C + ions are observed, all of which show very low abundances. In contrast, condensation processes of GeH 4 with C 2 H 4 give GeC n H m + ( n = 1–4) species, in very high yield for n = 1, 2 but lower for n = 3, 4. For all three systems, reaction mechanisms are suggested and are discussed in relation to the preparation of amorphous materials containing germanium carbides for photovoltaic applications.

GC/MS identification of pyrolysis products from fire retardant brominated epoxy resin

Brominated epoxy resins cured with diaminodiphenyl sulfone are thermally less stable than nonbrominated analogs. This thermal instability is likely to be caused by the formation of hydrogen bromide which destabilizes the epoxy network. As a result of the reaction of HBr with the epoxy resin, bromine-containing aromatics (brominated phenols, alkenyl aryl ether, hydroxyalkyl aryl ethers), as well as nonbromine-containing compounds form high-boiling decomposition products. The mass spectra of the brominated products collected from pyrolysis at 300 C in the open system under helium are presented and discussed.

Gas-phase ion chemistry of silane with ethane and ethyne

Abstract Silane–ethane and silane–ethyne systems have been studied by ion trap mass spectrometry and the variation of the abundances, with reaction time, of the ions containing silicon and carbon together in 5:1, 1:1 and 1:5 mixtures, have been reported. The best ion yield, which increases with the total pressure, is observed for 1:1 silane–ethyne mixture. Reaction mechanisms and rate constants of the first nucleation processes have been determined. In SiH 4 –C 2 H 2 systems, the formation of ions containing new SiC bonds occurs starting mainly from Si m H n + ions in the first steps, whereas the Si m C n H p + cluster species further reacts with high efficiency in processes with ethyne.

Gas phase ion/molecule reactions in phosphine/germane mixtures studied by ion trapping

Abstract Gaseous mixtures of phosphine and germane have been investigated by ion trap mass spectrometry. Reaction pathways together with rate constants of the main reactions are reported. The mechanisms of ion/molecule reactions have been elucidated by single and multiple isolation steps. The GeHn+ (n = 1–3) ions react with phosphine to give GePHn+ (n = 2–4) ions. The GePH4+ ion further reacts with GeH4 to yield Ge2PH6+. The GePHn+ (n = 2–4) mixed ionic family also originates from the P+ phosphine primary ion, as well as from the P2Hn+ (n = 0–3) secondary ions of phosphine reacting with neutral germane and from Ge2H2+ reacting with phosphine. The main reaction pathways of the PHn+ (n = 0–2) ions with GeH4 lead to the formation of the GeH2+ and GeH3+ ionic species. Protonation of phosphine from different ionic precursors is a very common process and yields the stable phosphonium ion, PH4+. Trends in total abundances of secondary GePHn+ (n = 2–4) ions as function of reaction time for different PH3/GeH4 pressure ratios show that excess of germane slightly affects the nucleation of mixed Ge-P ions.

Gas phase ion chemistry in germane/ammonia, methylgermane/ammonia, and methylgermane/phosphine

Abstract Gaseous mixtures of germane or methylgermane with ammonia and methylgermane with phosphine have been studied by ion trap mass spectrometry. Rate constants of reactions of the primary ions and of the most important secondary ion species are reported, together with the calculated collisional rate constants and efficiencies of reaction. The GeH n + (n = 0–3) ions, formed by electron ionization of both GeH 4 and CH 3 GeH 3 , react with ammonia yielding, among others, the GeH n + (n = 2–4) ion family, which, in a successive and slow reaction with NH 3 , only give the unreactive ammonium ion. Also, the CH 3 GeH n + (n = 1, 2) species do not form Ge–N bonds, whereas secondary ions of germane, such as Ge 2 H 2 + , produce species containing germanium and nitrogen together. In the CH 3 GeH 3 /PH 3 mixture a great number of ions are formed with rather high rate constants from primary ions of both reagent molecules and from phosphorus containing secondary ions. GePH n + (n = 2–4) ions further react with methylgermane leading to cluster ions with increasing size such as Ge 2 PH n + and Ge 2 CPH n + . The experimental conditions favoring the chain propagation of ions containing Ge and N, or Ge and P, with or without C, important in the chemical vapor deposition of materials of interest in photovoltaic technology, are discussed.