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Journal ArticleDOI

Spin-catalysis phenomena

05 Feb 1996-International Journal of Quantum Chemistry (John Wiley & Sons, Inc.)-Vol. 57, Iss: 3, pp 519-532
TL;DR: In this paper, a general definition of spin-catalysis was proposed, which can be classified into two main categories: spin-orbit coupling induced and paramagnetic-exchange induced spincatalysis.
Abstract: 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.
Citations
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Journal ArticleDOI
TL;DR: The theory and principles of computational phosphorescence are illustrated by highlighting studies of classical examples like molecular nitrogen and oxygen, benzene, naphthalene and their azaderivatives, porphyrins, as well as by reviewing current research on systems like electrophosphorescent transition metal complexes, nucleobases, and amino acids.
Abstract: Phosphorescence is a phenomenon of delayed luminescence that corresponds to the radiative decay of the molecular triplet state. As a general property of molecules, phosphorescence represents a cornerstone problem of chemical physics due to the spin prohibition of the underlying triplet-singlet emission and because its analysis embraces a deep knowledge of electronic molecular structure. Phosphorescence is the simplest physical process which provides an example of spin-forbidden transformation with a characteristic spin selectivity and magnetic field dependence, being the model also for more complicated chemical reactions and for spin catalysis applications. The bridging of the spin prohibition in phosphorescence is commonly analyzed by perturbation theory, which considers the intensity borrowing from spin-allowed electronic transitions. In this review, we highlight the basic theoretical principles and computational aspects for the estimation of various phosphorescence parameters, like intensity, radiative...

362 citations

Journal ArticleDOI
TL;DR: A simple formalism devised to calculate the condensed-to-atoms Fukui function has been further analyzed within a spin-polarized density-functional theory framework, giving an extended local reactivity description of systems based on the frontier spin-up and spin-down molecular orbitals.
Abstract: A simple formalism devised to calculate the condensed-to-atoms Fukui function [R. R. Contreras, P. Fuentealba, M. Galvan, and P. Perez, Chem. Phys. Lett. 304, 405 (1999)] has been further analyzed within a spin-polarized density-functional theory framework. The model is based on a frozen-core approximation to these local reactivity indices [M. Galvan, A. Vela, and J. L. Gazquez, J. Phys. Chem. 92, 6470 (1988)], giving us an extended local reactivity description of systems based on the frontier spin-up and spin-down molecular orbitals. Degenerate molecular spin orbitals have been explicitly included in our model equations. Computational results for the nitric oxide (NO) and some simple carbene systems are presented in order to test the model. These quantities have been discussed in the context of changes both in charge density and spin density within the context of electron charge transfer or spin-polarization processes.

84 citations

Journal ArticleDOI
TL;DR: Hybrid density functional calculations have been performed for the reductive activation of dioxygen by glucose oxidase, for which recent experiments have shown substantial kinetic O-18 isotope effe... as mentioned in this paper.
Abstract: Hybrid density functional calculations have been performed for the reductive activation of dioxygen by glucose oxidase, for which recent experiments have shown substantial kinetic O-18 isotope effe ...

69 citations

Journal ArticleDOI
Bernd A. Heß1
01 Jan 1997
TL;DR: In this paper, the nature of relativistic effects in the electronic structure of atoms and molecules is explained and an account of theoretical methods for relativism calculations in a quantum-mechanical framework is given.
Abstract: The nature of relativistic effects in the electronic structure of atoms and molecules is explained and an account of theoretical methods for relativistic calculations in a quantum-mechanical framework is given. The focus is on the requirement of reducing the number of degrees of freedom in the theory in order to obtain computationally efficient and still highly accurate approximations. Some applications illustrating the relativistic effects in coinage metal atoms and molecules are discussed.

68 citations

References
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TL;DR: The second-order bulk recombination rate is slowed down by the magnetic field, demonstrating that F pairs behave rather like geminate triplet pairs, which has also been confirmed in many CIDNP investigations as discussed by the authors.
Abstract: ions was studied by Margulis et al.156 These authors used solvent mixtures of water or ethanol with glycerol to obtain very highly viscous solvents. Varying the temperature and the solvent composition, they showed that the MFEs are determined by the value of T/q. MFEs on the free radical yield (determined by flash spectroscopy) or on the permanent bleaching reaction (determined by continuous photolysis) became detectable only at T/q values smaller than 10 K/cP. At room temperature this corresponds to viscosities larger than 20 cP. In the systems investigated by Margulis et (cf. Table 8). The magnetic field causes an increase of free radical yield, which results from a suppression of tripletsinglet transitions by a magnetic field. The MFD curves are of the case 2 type, which is taken as evidence for a contribution of the relaxation mechanism. Remarkably, Margulis et al. also found MFEs on the second-order recombination rate constant in the case of riboflavin semiquinone radicals and benzophenone ketyl radicals. The second-order bulk recombination rate is slowed down by the magnetic field, demonstrating that F pairs behave rather like geminate triplet pairs, which has also been confirmed in many CIDNP investigations. MFEs in the photochemistry of quinoline and isoquinoline derivatives have been reported by Hata et al.318-320 Photochemical hydrogen abstractions by the ring nitrogen from the solvent ethanol are believed to be the primary reactions in the photoreactions of 1isoquinolinecarbonitrile (7) and 4-methylquinoline-2carbonitrile (9). For the reaction of 7 (eq 46) the

1,411 citations

Journal ArticleDOI
TL;DR: In this article, a collisional perturbation of spin-orbital coupling in the π-electron orbitals of naphthalene is interpreted in terms of a collision.
Abstract: A yellow color is produced when two colorless pure liquids, α‐chloronaphthalene and ethyl iodide, are mixed, although no chemical reaction occurs. Spectroscopic examination reveals that the lowest singlet→triplet absorption band of the naphthalene molecule is greatly enhanced in intensity in the presence of the iodide, and that the development of this absorption band is responsible for the color effect. The process is interpreted in terms of a collisional perturbation of spin‐orbital coupling in the π‐electron orbitals of the naphthalene. The significance of this phenomenon in several topics in spectroscopy and chemistry is discussed. In particular, the molecular mechanism of heavy atom quenching of fluorescence by foreign species is resolved. A novel Beer's law anomaly is predicted for intercombinations.

417 citations