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  • 1
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 2
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Molecular engineering 6 (1996), S. 261-279 
    ISSN: 1572-8951
    Keywords: Spin catalysis ; radical recombination ; H2−O2 reaction ; singlet-triplet mixing
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Recombination of the triplet state radical pair consisting of two hydrogen atoms catalysed by molecular oxygen is considered as a simulating example of a paramagnetic-exchange catalytic process. Intermolecular exchange interaction in the collision complex between the H2 and O2 molecules is calculatedab initio in STO-6G and 6–31 G* basis sets with complete active space configuration interaction. Calculations are done at a fixed O−H distance (≃3 Å), scanning the H−H bond length from 0.6 till 12 Å at the linear geometry of collision. The mixture of the triplet (‘T’)3∑ u + and singlet (‘S’)1∑ g + states of the hydrogen moiety is possible because both states have the same triplet symmetry in the ‘collision’ complex with O2 (3∑ g − ). A strong mixture of the ‘S’ (1∑ g + , H2 +3∑ g − , O2) and ‘T’) and ‘T’ (3∑ u + , H2 +3∑ g − , O2) states is actually obtained even at large H−H distances. The quintet and singlet states5,1(3∑ u + , H2 +3∑ g − , O2) are also considered for comparison of the exchange potentials. Atr(H−H)≥4.4 Å the ‘S-T’ splitting is approximately constant (12 cm-1 in the STO-6G basis set; 55.5 cm-1 in the 6–31 G* basis set) and is determined by the exchange interaction between O2 and the nearest hydrogen atom in the O−O...H fragment. The paramagnetic catalyst can accelerate radical recombination through the ‘triplet-singlet’ nonadiabatic transition to the lowest ‘S’ reactive state when the radical encounter takes place in the vicinity of the catalyst. Though we do not consider the radical dynamics in a real solvent, which modulates the exchange potentials and the ‘T-S’ transitions, the nature of this mechanism of spin catalysis is obvious. The electric polarization and charge transfer are important in the analysis of the exchange interaction and radical recombination potentials for all multiplets. In accordance with the concept of spin catalysis, the electronic spin-uncoupling mechanism, induced by O2 perturbation, has the same nature as other known catalytic processes of paramagnetic-exchange type.
    Type of Medium: Electronic Resource
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  • 4
    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: Ab initio configuration interaction (CI) calculations have been performed for the O2 + H2 complex in a trapezoidlike collision arrangement with C2v symmetry. The potential energy surfaces of the four lowest states of this van der Waals complex (arising from the X3 Σg-, a1 Δg, and b1 Σg+ states of the oxygen moiety), as well as the collision-induced b1 Σg+ - a1 Δg electric dipole transition moment (Mb-a), have been analyzed for different CI expansions, using as a reference determinant the restricted open-shell Hartree-Fock (ROHF) function for the ground state of the complex H2(X1 Σg+) + O2(X3 Σg-). The geometry optimized at the ROHF/6-311G** level was refined by a partial optimization at the CI level scanning the intermolecular distance. The equilibrium distances for the X, a, and b states have been found to be a slightly different in the region 3.02-2.98 Å. The larger binding energy of the b1 Σg+ state (2.96 kJ/mol) in comparison with the a1 Δg (2.1 kJ/mol) and ground X3 Σg- states (1.35 kJ/mol) presumably could be explained as resulting from charge-transfer interactions. A good convergence of the calculated transition moment Mb-a for the larger CI expansions (approximately 50,000 configuration-state functions) has been obtained. The calculated collision-induced intensity of the b1 Σg+-a1 Δg and a1 Δg-X3 Σg- transitions in molecular oxygen are in reasonable agreement with recent experimental data for several foreign gases. © 1994 John Wiley & Sons, Inc.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    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: On the grounds of results from quantum chemical calculations, we introduce spin-catalysis as a new concept, and classify different types of chemical reactions according to this concept. We propose a general definition of spin-catalysis as phenomena in which chemical reactions are promoted by substances which assist in overcoming spin-prohibition or in which the activation barrier is lowered through spin uncoupling induced by a paramagnetic catalyst. A number of known phenomena fall into this definition and can be classified according to two main categories: (1) spin-orbit coupling induced and (2) paramagnetic-exchange induced spin-catalysis. Other types of spin-catalysis are also discussed: (3) processes with participation of substances which assist in (a) photochemical and (b) thermal or electrochemical generation of active particles by energy and - by electron transfer [radicals, diradicals, O2(1Δg), etc.], reacting further without spin-prohibition: (4) processes induced by an external magnetic field. Processes (3) and (4) are quite general and well known; their inclusion in the spin-catalysis classification does not introduce any new findings for the chemical kinetics, but the general features, which unite them with the important catalytic processes of the first two types, serve as a useful guide in catalysis theory. Few models of spin-catalytic processes have been simulated for the purpose of illuminating the principles of spin-catalysis; the cis-trans isomerization of ethylene catalyzed by molecular oxygen, the external heavy atom effect in ethylene photochemistry, and some others. © 1996 John Wiley & Sons, Inc.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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  • 6
    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 intersystem crossing (ISC) between the lowest triplet and singlet states occurring in the reaction of atomic oxygen with ethylene was studied. The importance of spin-orbit coupling (SOC) in oxirane biradicals (ĊR′R″ - CRR* - Ȯ) is stressed through calculations where the spin-orbit matrix elements over the full Breit-Pauli SOC operator has been obtained in the singlet-triplet crossing region. The calculations are performed with a multiconfigurational linear response approach, in which the spin-orbit couplings are obtained from triplet response functions using differently correlated singlet-reference-state wave functions. Computational results confirm earlier semiempirical predictions of the spin-orbit coupling as an important mechanism behind the ring opening of oxiranes and addition of oxygen O(3P) atoms to alkenes. © 1995 John Wiley & Sons, Inc.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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