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  • 1
    Electronic Resource
    Electronic Resource
    A Study of the Gas-Phase Reactivity of Neutral Alkoxy Radicals by Mass Spectrometry: α-Cleavages and Barton-type Hydrogen MigrationsDedicated to Sir Derek H. R. Barton (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1866-1883 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970031120
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  • 2
    Electronic Resource
    Electronic Resource
    Gas-Phase Experiments Aimed at Probing the Existence of the Elusive Water Oxide MoleculeFirst reported at the 44th Conference on Mass Spectrometry and Allied Topics, Portland (Oregon, USA), 13 May, 1996.Dedicated to Professor Gerhard Ertl on the occasion of his 60th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1235-1242 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021009
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  • 3
    Electronic Resource
    Electronic Resource
    Dimethyl Peroxide Radical Cation: A New Theoretical and Experimental Approach to the C2H6O•+2 Potential Energy Surface (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 626-638 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030420
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  • 4
    Electronic Resource
    Electronic Resource
    Redox Properties of the Diatomic Bare Iron Chalcogenides FeO and FeS in the Gas PhaseDedicated to Professor Helmut Baumgartel on the occasion of his 60th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1230-1234 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021008
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  • 5
    Electronic Resource
    Electronic Resource
    Bimolecular Gas-Phase Reactions of d-Block Transition-Metal Cations With Dimethyl Peroxide: Trends Across the Periodic Table (1995)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 1 (1995), S. 608-613 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19950010907
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  • 6
    Electronic Resource
    Electronic Resource
    Highly Selective Benzylic C—H Bond Activation of Toluene 1 by ‘Bare’ FeO+ in the Gas Phase. Short communication (1992)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 75 (1992), S. 1792-1797 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of FeO+ with toluene in the gas phase occurs at collision rate (kr = 1.36 × 10-9 cm3 molecule-1 s-1), and labeling experiments demonstrate that the total products due to C—H bond activation involve to 〉 92% the benzylic position. In the ‘hydride’ abstraction process (formation of FeOH and C7H7+), the H-atom originates elusively from the benzylic position to generate a benzyl cation, and an intramolecular kinetic isotope effect kH/kD = 1.75 has been obtained. There is no evidence for the existence of isotopically sensitive branching (‘metabolic switching’) in the system studied.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19920750605
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  • 7
    Electronic Resource
    Electronic Resource
    Theory-enforced Re-investigation of the Origin of the Large Metal-Carbon Bond Strength in PdCH2I+ and its reactions with unsaturated hydrocarbonsIn [1], a preliminary account of the reaction of ground-state Pd+ with CH3I is given.Dedicated to Edgar Heilbronner on the occasion of his 75th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1110-1120 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: State-of-the-art ab initio studies demonstrate that the reaction Pd+ + CH3I → PdCH2I+ + H. is endothermic by ca. 20 kcal/mol, which translates into a bond dissociation energy (BDE) of ca. 83 kcal/mol for the Pd+—CH2I bond. This figure is in agreement with an experimental bracket of 68 kcal/mol 〈 BDE(Pd+—CH2I) 〈 92 kcal/mol. Based on these findings, the previously studied Pd+/CH3I system was re-investigated, and double-resonance experiments demonstrate that the formation of PdCH2I+ occurs stepwise via PdCH3+ as a reactive intermediate. Further, ion/molecule reactions of PdCH2I+ with unsaturated hydrocarbons are studied, which reveal the formation of carbon-carbon bonds in the gas phase.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790419
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  • 8
    Electronic Resource
    Electronic Resource
    Substituent Effects on Fe+-Mediated [4+2] Cycloadditions in the Gas Phase (1997)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 80 (1997), S. 1205-1220 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Fe+-mediated [4+2] cycloaddition of dienes with alkynes has been examined by four-sector ion-beam and ion cyclotron resonance mass spectrometry. Prospects and limitations of this reaction were evaluated by investigating several Me-substituted ligands. Me Substitution at C(2) and C(3) of the diene, i.e., 2-methylbuta-1,3-diene, 2,3-dimethylbuta-1,3-diene, hardly disturbs the cycloaddition. Similarly, variation of the alkyne by use of propyne and but-2-yne does not affect the [4+2] cycloaddition step, but allows for H/D exchange processes prior to cyclization. In contrast, Me substituents in the terminal positions of the diene moiety (e.g., penta-1,3-diene, liexa-2,4-diene) induce side reactions, namely double-bond migration followed by [3+2] and [5+2] cycloadditions, up to almost complete suppression of the [4+2] cycloaddition for 2,4-dimethylhexa-2,4-diene. Similarly, alkynes with larger alkyl substituents (pent-1-yne, 3,3-dimethylbut-1-yne) suppress the [4 + 2] cycloaddition route. Stereochemical effects have been observed for the (E)- and (Z)-penta-1,3-diene ligands as well as for (E,E)- and (E,Z)-hexa-2,4-diene. A mechanistic explanation for the different behavior of the stereoisomers in the cyclization reaction is developed. Further, the regiochemical aspects operative in the systems ethoxyacetylene/pentadiene/Fe+ and ethoxyacetylcne/isoprene/Fe+ indicate that substituents avoid proximity.
    Additional Material: 5 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19970800418
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  • 9
    Electronic Resource
    Electronic Resource
    Benzene Oxidation by ‘Bare’ FeO+ in the Gas Phase (1992)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 75 (1992), S. 1281-1287 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Bare FeO+ reacts in the gas phase with benzene at collision rate (k = 1.3 × 10-9 cm3 molecule-1 s-1), giving rise to the formation of Fe(C6H4)+/H2O(5%), Fe(C5H6)+/CO(37%), Fe(C5H5)+/CO/H. (2%), and Fe+/C6H5OH (56%). Neither the reaction rate nor the product distribution are subject to a significant kinetic isotope effect, thus, ruling out several mechanistic variants described in the literature to the operative for ‘analogous’ arene oxidation processes in solution. A mechanism is suggested which is in keeping with the experimental findings, and which also accounts for some remarkable results obtained, when two [Fe, C6,H6H6O]+ isomers are generated and subjected to a neutralization-re-ionization experiment in the gas phase.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19920750429
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  • 10
    Electronic Resource
    Electronic Resource
    Generation of ‘Bare’ FeF+ by C—F Bond Activation in the Gas Phase and Evaluation of Thermochemical Data. Short communication (1992)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 75 (1992), S. 2215-2218 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The title compound FeF+ has been generated in a Fourier-transform ion-cyclotron-resonance mass spectrometer via C—F bond activation by reacting hexafluorobenzene with bare FeO+. Bracketing experiments provide a bond-dissociation energy (BDE) for FeF+ of 86 〈 BDE 〈 101 kcal/mol. High-level CAS-MCSCF pseudopotential ab initio calculation, including 501036 electronic configurations, predict a BDF(Fe+—F) of 100.9 kcal/mol.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19920750706
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