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  • 1
    ISSN: 0947-6539
    Keywords: alkoxy radicals ; Barton reaction ; density functional calculations ; mass spectrometry ; radicals ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reactivity of neutral alkoxy radicals in the absence of any interfering intermolecular interactions is investigated by means of the recently introduced method of neutral and ion decomposition difference (NIDD) spectra. These are obtained from quantitative analysis of the corresponding neutralization-reionization (NR) and charge reversal (CR) mass spectra. The following trends emerge: alkoxy radicals with short (C1—C3) or branched alkyl chains give rise to α-cleavage products, whereas longer-chained alkoxy radicals undergo 1,5-hydrogen migrations from carbon to oxygen, that is, Barton-type chemistry. This facile rearrangement has been studied in detail for n-pentoxy radicals by isotopic labeling experiments and computation at the Becke 3 LYP/6-31 G* level of theory. Further, the NIDD spectra of 3-methylpentoxy radicals permit for the first time the identification of the diastereoselectivity of the gas-phase hydrogen migrations. The results from the NIDD method are compared to those from earlier studies in the condensed phase. This new mass spectrometric approach is suggested as a tool for the examination of intramolecular reactions of free alkoxy radicals which can usefully complement theoretical studies.
    Additional Material: 12 Ill.
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  • 2
    ISSN: 0947-6539
    Keywords: computer chemistry ; electron transfer ; hydrogen peroxide ; mass spectrometry ; water oxide ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Collisional activation, charge reversal, and six different neutralization - reionization mass spectrometric experiments with [H2,O2]·+ radical cations and [H2,O2]·- radical anions were performed in order to probe the predicated existence of neutral water oxide, H2OO, the long sought after tautomer of hydrogen peroxide, HOOH. The experiments together with ab initio calculations indicate that H2OO is a local minimum on the [H2,O2] potential-energy surface, and the elusive molecule seems to be formed as a transient upon neutralization of the corresponding radical cation H2OO·+ in the gas phase.
    Additional Material: 10 Ill.
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  • 3
    ISSN: 0947-6539
    Keywords: ab initio calculations ; dimethyl peroxide ; mass spectrometry ; peroxides ; radical ions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The structure and the unimolecular fragmentations of the metastable dimethyl peroxide radical cation have been investigated by mass spectrometric and isotopic labeling methods as well as high-level ab initio calculations. In line with the theoretical results, neutralization-reionization and charge reversal experiments suggest that ionized dimethyl peroxide bears a CH3OOCH•3 connectivity. In the cation the O-O bond dissociation energy is larger than that of the neutral counterpart; in contrast, the C-O bond strength is slightly and that of the C-H bond significantly reduced upon ionization. These energetic changes upon one-electron oxidation of CH3OOCH3 are also reflected in the NR and CR mass spectra of CH3OOCH•+3. Further, for metastable CH3OOCH•3 two major fragmentation pathways are observed: 1) Loss of a hydrogen atom by cleavage of a C-H bond is associated with a skeletal reorganization, which gives rise to a proton-bound formaldehyde dimer. 2) The expulsion of a CH3O• radical leads to protonated formaldehyde in a surprisingly specific double hydrogen transfer involving a [CH3OH/CH2O]• ion/dipole complex as central intermediate; this complex also accounts for other minor fragmentation channels. The structures of intermediates and transition states are calculated with the BECKE 3LYP density-functional method employing a 6-311++G** basis.
    Additional Material: 6 Ill.
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  • 4
    ISSN: 0947-6539
    Keywords: ab initio calculations ; iron oxide ; iron sulfide ; mass spectrometry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electron-transfer behavior of the binary iron chalcogenides FeO and FeS has been examined by means of mass spectrometry and ab initio calculations using the averaged coupled-pair functional (ACPF) method. The experimental and theoretical results are in good agreement with each other and also with previous studies. The ionization energies (IE) of the diatomic species are found to be IE(FeO) = 8.8±0.2 eV, IE(FeO+) = 17.9±0.4 eV, IE(FeS) = 8.3±0.3 eV, and IE(FeS+) = 16.3±0.5 eV. Two new diatomic dications, FeS2+ and FeO2+, are shown to exist as metastable minima on the corresponding potential-energy surfaces. The data enable an evaluation of the intrinsic gas-phase redox properties of FeS and FeO, and the comparison demonstrates that iron sulfide is more prone to undergo facile reduction and oxidation than iron oxide.
    Additional Material: 3 Ill.
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  • 5
    ISSN: 0947-6539
    Keywords: gas-phase chemistry ; mass spectrometry ; periodic trends ; peroxides ; transition-metal ions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The bimolecular gas-phase reactions of d-block transition-metal cations M+ with dimethyl peroxide were screened by means of Fourier transform ion cyclotron resonance mass spectrometry. The rich chemistry can be classified into four types of reactions: i) Oxygen-atom transfer to generate MO+, ii) elimination of radicals, mostly CH3O·, iii) intramolecular redox reaction of dimethyl peroxide to form CH3OH, CH2O and CO, and iv) charge transfer from the metal cation to produce CH3OOCH3+. Some general trends became apparent from this study. For example, the “early” transition metals almost exclusively induce oxygen transfer to generate MO+, in line with the notoriously high oxophilicities of these metals, and electron transfer is only observed for Zn+ and Hg+. Both the radical loss and the disproportionation reaction emerge from a rovibrationally highly excited insertion intermediate (CH3O)2M+, and for the first-row metals the branching ratio of the competing processes seems to be affected by the M+—OR bond strengths as well as the electronic groundstate configurations of M+. For the 4d and 5d cations Ru+—Ag+ and Pt+—Au+, respectively, products resulting from intramolecular redox reactions dominate; this probably reflects the higher propensity of these metal ions to facilitate β-hydrogen atom shifts.
    Additional Material: 2 Ill.
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  • 6
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1998 (1998), S. 983-987 
    ISSN: 1434-1948
    Keywords: Telluroles ; Neutralization-reionization mass spectrometry ; DFT calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 2,2-Dichloro-1,3-dichalcogena-2-telluroles of sulfur and selenium, C6H4[XTe(Cl)2Y] with X, Y = S, Se, serve as precursors to generate the cation radicals of 1,3-dichalcogena-2-telluroles C6H4[XTeY]+• as well as the corresponding neutral counterparts by means of neutralization-reionization mass spectrometry. These neutral 2-telluroles are intrinsically stable for at least 1 μs in the gas phase. The most abundant fragmentation channel at the neutral and cationic stages is due to loss of atomic tellurium to afford the corresponding benzo-1,2-dichalcogena-cyclobutenes. For the latter, partial isomerization to the related dichalcogena-ortho-quinones is indicated by the mass-spectrometric fragmentation pattern.
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  • 7
    ISSN: 1434-1948
    Keywords: Ab initio calculations ; Mass spectrometry ; Gas-phase chemistry ; Ammonia oxide ; Hydroxylamine ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Mass-spectrometric experiments were combined with ab initio calculations to explore the cationic and neutral [H3,N,O]⋆+/0 potential energy surfaces and relevant anionic species. The calculations predict the existence of three stable cationic and neutral [H3,N,O]⋆+/0 isomers, i.e. ammonia oxide H3NO⋆+/0 (1⋆+/0), hydroxylamine H2NOH⋆+/0 (2⋆+/0) and the imine-water complex HNOH2⋆+/0 (3⋆+/0). Hydroxylamine 2 represents the most stable isomer on the neutral surface (Erel = 0), and the metastable isomers 1 (Erel = 24.8 kcal mol-1) and 3 (Erel = 61.4 kcal mol-1) are separated by barriers of 49.5 kcal mol-1 and 64.2 kcal mol-1, respectively. Adiabatic ionization of 2 (IEa = 9.15 eV) yields 2⋆+, which is 21.4 kcal mol-1 more stable than 1⋆+ and 36.4 kcal mol-1 more stable than 3⋆+. The barriers associated with the isomerizations of the cations are 58.6 kcal mol-1 for 2⋆+ → 1⋆+ and 71.4 kcal mol-1 for 2⋆+ → 3⋆+. Collisional activation (CA) and unimolecular decomposition (MI) experiments allow for a clear distinction of 1⋆+ from 2⋆+. Besides, neutralization/reionization (NR) experiments strongly support the gas-phase existence of the long-sought neutral ammonia oxide.
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  • 8
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 1203-1210 
    ISSN: 1434-1948
    Keywords: Organogermanium ions ; Mass spectrometry ; Ab initio theory ; NBO ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The cation [Ge,C,H]+ has been generated by the electron ionisation of trichloromethylgermane. Collisional activation experiments were used to establish a Ge-C-H connectivity in this species, for which a significant fraction of the ion population was found to survive neutralisation-reionisation mass spectrometry (NRMS) experiments. Thus, the neutral counterpart [Ge,C,H]0 is stable on a microsecond timescale. Becke's 3 parameter hybrid density functional (B3LYP) was used to map the ion and neutral potential-energy surfaces, in conjunction with double-zeta and triple-zeta basis sets. The computational results obtained using the triple-zeta basis sets suggest that, for the cation, the global minimum is the high spin 3Σ GeCH+, with the first Ge-C-H excited state, 1Σ GeCH+, approximately 39 kcal mol-1 less stable. The lowest energy ion structure with H-Ge-C connectivity is bent (3A′′HGeC+,∠H-Ge-C = 126.3) and 69 kcal mol-1 less stable than the global minimum. For the neutral, a doublet (2π) with Ge-C-H connectivity is predicted to be the global minimum. The classical barrier for the neutral 1,2-hydrogen shift reaction on the doublet surface is negligible (0.1 kcal mol-1), while the smallest barrier for the cation is 13.0 kcal mol-1, corresponding to (3A′′) HGeC+ → (3Σ) GeCH+. Natural bond order analysis has been used to establish the order of the metal-carbon bond for selected states of both the neutral and the ion. Neutral and cationic isomers with Ge-C triple bonds were found to be high-energy excited states, with the metal-carbon bonds in the cation and neutral ground states of order 2.0 and 2.5, respectively. The instability of Ge-C triple bonded species is attributed to the energy required for electronic promotion in the metal in order to achieve a hybrid configuration suitable for the formation of such a bond.
    Additional Material: 5 Ill.
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  • 9
    ISSN: 0044-8249
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 1 Tab.
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  • 10
    ISSN: 0044-8249
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 1 Ill.
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