Catalysts spin states

Fig. 4. Structures of the various spin states of a [Ru2] catalyst intermediate. For the closed-shell species no Ru-oxo structure exists but all structure optimizations converge to the superoxo state.

Oxidation of Mercaptans by Ov. Mercaptans are autoxidized in the presence of 02 in alkaline medium. In general, the oxidation is slow in the absence of catalyst because of unfavorable spin state symmetries that result from differences in the electronic configuration of the reactants (54). However, the reaction proceeds rapidly in the presence of traces of metal ions or transition metal phthalocyanines (55—58). The catalyst tends to alter the electronic structure of either the reductant and/or 02 so as to surmount the activation energy barrier imposed on the reaction by spin-state symmetry restriction. The coupled oxidation system in the presence of catalyst can be represented by ... 

All different spin states have been calculated in the ground state and the calculations indicate that the potential structure of the active Jacobsen catalyst is the Mnv(0)(ligand)Cl t complex shown in Fig. 17.104... 

Applied to VPO catalysts, spin-echo mapping not only provides information on the oxidation state of different vanadium nuclei, but can also differentiate different phases with the same oxidation state. The development of a NMR technique to probe such materials has been extremely valuable, as their often poorly crystalline nature prevents characterization through X-ray diffraction. Additionally, variable-temperature spin-echo mapping has been shown capable of determining magnetic characteristics of materials, such as their Weiss temperature [34, 37]. 

Iron salts or iron exchanged MMT deactivate H2O2 with the generation of O2, and show low alkane oxidation activity (< 0.5%). Complexation of iron by organic ligands has very specific effects on the spin state (II, III) and its electrophilicity and allows non-Fenton-type chemistry [13,14]. Mononuclear ferrous N,N -bis(2-pyridinecarboxamide)-l,2/3-R complexes in Na-Y zeolite (with R= alkane or benzene) are oxo-type catalysts [13]. Here a different reactivity pattern is expected for the dinuclear ferric complexes [14]. 

Very often the catalyst (unsaturated TM complex) has a low-spin ground state with a high-spin state being very close in energy [23, 36, 37]. This situation is very useful for catalysis it is not necessary to produce a spin-flip and a low barrier is achieved. The metal surface is an ideal catalyst in this sense, since any spin multiplicity can be realized at the local state of the surface, perturbed in the course of the chemisorption process. In other words, a metal cluster which simulates the surface in the course of the chemisorption process has a number of quasi-degenerate state with different spin-multiplicities. 

An important advantage of the VB approach to catalysis is that it is conceptually closely related to the traditional way in which chemists rationalize chemical structure and reactivity [6, 7, 45]. It is of considerable interest to construct diabatic electronic states representing specific resonance structures of a catalytic system. In this way the importance of the triplet excited s-tate of organic substrates becomes immediately evident, and it is then easy to understand that only the low spin state of the catalyst is reactive. 

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