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  • 1
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    Biological Mass Spectrometry 24 (1989), S. 539-545 
    ISSN: 0030-493X
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio molecular orbital theory using basis sets up to 6-311G* *, with electron correlation incorporated via configuration interaction calculations with single and double substitutions, has been used to study the structures and energies of the C3H2 monocation and dication. In agreement with recent experimental observations, we find evidence for stable cyclic and linear isomers of [C3H2]+ ·. The cyclic structure (, a) represents the global minimum on the [C3H2]+ · potential energy surface. The linear isomer (, b) lies somewhat higher in energy, 53 kJ mol-1 above a. The calculated heat of formation for [HCCCH]+ · (1369 kJ mol-1) is in good agreement with a recent experimental value (1377 kJ mol-1). For the [C3H2]2+ dication, the lowest energy isomer corresponds to the linear [HCCCH]2+ singlet (h). Other singlet and triplet isomers are found not to be competitive in energy. The [HCCCH]2+ dication (h) is calculated to be thermodynamically stable with respect to deprotonation and with respect to C—C cleavage into CCH+ + CH+. The predicted stability is consistent with the frequent observation of [C3H2]2+ in mass spectrometric experiments. Comparison of our calculated ionization energies for the process [C3H2]+ · → [C3H2]2+ with the Qmin values derived from charge-stripping experiments suggests that the ionization is accompanied by a significant change in structure.
    Additional Material: 3 Tab.
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  • 2
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Journal of the American Chemical Society 110 (1988), S. 2375-2378 
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Journal of the American Chemical Society 117 (1995), S. 6789-6790 
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The potential energy curve describing the fragmentation of a diatomic dication AB2+ is considered as arising from an avoided crossing between an attractive diabatic curve (correlating with A2+ + B) and a repulsive diabatic curve (correlating with A+ + B+). The simplest avoided-crossing (AC) model neglects diabatic coupling and polarization and leads to useful predictions of the transition structure bond length (rTS) and the kinetic energy released (T) in fragmentations of dicationic systems in which the difference (Δ1) between the ionization energies of A+ and B is small. When Δ1 is not small, it is necessary to include diabatic coupling and polarization in the treatment. The resultant ACDCP (avoided crossing with diabatic coupling and polarization) model provides very satisfactory estimates of rTS and T for both small and large Δ1. Its implementation requires only atomic ionization energy and polarizability data and comes at virtually no computational cost. Both the AC and ACDCP models are readily generalized to fragmentations of more highly charged cations.
    Additional Material: 3 Ill.
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  • 6
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 237-248 
    ISSN: 0947-6539
    Keywords: ab initio calculations ; allenes ; ketenes ; matrix isolation ; rearrangements ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Alkoxyvinylketenes4are generated by flash vacuum thermolysis (FVT) or photolysis of 3-alkoxycyclobutenones3. The thermal interconversion of4and allene carboxylic acid esters5under FVT conditions is demonstrated by Ar matrix FTIR spectroscopy. In addition, ethoxy-vinylketene4bundergoes thermal elimination of ethene with formation ofs-cis-ands-trans-acetylketene(8). An analogous aminovinylketene-to-allenecarbox-amide conversion is observed on FVT of 3-dimethylaminocyclobutenone3c. A facile 1, 3-chlorine migration in 2, 3-buta-dienoyl chloride(5d)is also reported. Consistent with the experimental observations, 1, 3-methoxy, 1, 3-chloro, and 1, 3-dimethylamino migrations in vinylketene are calculated (G2(MP2, SVP) level) to have moderate barriers of 169, 157, and 129 kJ mol-1, respectively, significantly less than the corresponding 1, 3-H shift barrier (273 kJ mol-1). The stabilization of the four-center transition structures is rationalized in terms of the donor-acceptor interaction between the lone pair electrons of the migrating donor substituent and the vacant central carbon p orbital of the ketene LUMO. The predicted migratory aptitude in the series of substituted vinylketenes, R-C(=CH2)-CH=C=O, is in the order N(CH3)2〉SCH3〉SH〉Cl〉NH2〉OCH3〉OH〉F〉H〉CH3, and correlates well with the electron-donating ability of the R group.
    Additional Material: 12 Ill.
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  • 7
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Additional Material: 1 Ill.
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  • 8
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: A simple and efficient procedure of calculating molecular volume (VM) based on the Monte Carlo method is presented. The volume of a molecule is defined by the volume occupied by the 0.001-au electron density envelope. We have employed this method to compute the molecular volumes (VM) of a large selection of organic molecules and compare them with the corresponding molar volumes (Vm) measured in the liquid state. A strong correlation is found to exist between the VM and Vm values (VM/Vm ≈ 0.75). Using this linear relationship, the calculated molecular volume may provide an estimate of the cavity-volume parameter in solvent-effect calculations. As a chemical application of molecular volume, we have investigated the conformational equilibrium of 1,2-difluoroethane in the liquid state using the self-consistent reaction field theory. © 1995 by John Wiley & Sons, Inc.
    Additional Material: 1 Ill.
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  • 9
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A compact formalism for the second and third derivatives of the Hartree–Fock energy in the presence of an Onsager solvent reaction field is presented. All three standard algorithms (MO, AO, and direct) are extended to include the reaction field in a unified way. Predictions of the infrared spectrum of formaldehyde in a variety of solvents and of solvent-induced shifts in carbonyl stretching frequencies are presented along with the results of new measurements. As for the gas-phase case, analytical second derivatives are far more efficient than numerical ones. The reaction field provides very good predictions of solvent effects at negligible computational cost.
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 1572-9001
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Ab initio molecular orbital theory with triple-zeta-valence plus polarization basis sets and with electron correlation incorporated at the fourth-order Møller-Plesset level has been used to study the tetrahydridoonium dications, OH4 2+, SH4 2+, and SeH4 2+. The tetrahydridoselenonium dication SeH4 2+ is predicted to have a tetrahedral (T d )structure, similar to OH4 2+ and SH4 2+, with short bonds to hydrogen (1.483 Å). Although deprotonation of SeH4 2+ is thermodynamically favored Cby 104 kJ mol−1), such a reaction is inhibited by a large barrier (240kJmol−1]. Thus, SeH4 2+ lies in a deep potential well and as an isolated species should have a long lifetime in the gas phase. The estimated heat of formation, ΔH° f , for SeH4 2+ is very high (2483 kJ mol−1], as is the case for OH4 2+ and SH4 2+. Of the group IV onium dications (OH4 2+, SH4 2+, and SeH4 2+), SeH4 2+ displays the greatest kinetic and thermodynamic stability toward proton loss. Substantial solvent stabilization is required in order to generate SeH4 2+ in solution.
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