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Resonance Theory of Catalytic Action of Transition-Metal Complexes: Isomerization of Quadricyclane to Norbornadiene Catalyzed by Metal Porphyrins

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Abstract

The theory of catalytic activity of transition-metal compounds is a fascinating problem especially if a comparison of different catalysts is necessary. The isomerization of quadricyclane (QC) to norbornadiene (NB) catalyzed by transition-metal porphyrins is a challenge and incidentally a suitable benchmark for various theories of catalysis. We analyze this process in detail using a valence bond-like scheme adjusted for the description of reaction centers containing transition-metal atoms. A qualitative explanation of contrasting catalytic behavior of Mn-phthalocyanine and Co-tetraphenylporphyrin is obtained from the analysis of the spectra of local many electron states of free catalysts and their complexes with the reactant/product. This picture is supported by the numerical analysis of potential energy profiles for the QC to NB isomerization in the presence of a catalyst performed in the effective Hamiltonian approximation. This exemplary reaction is put in a more general perspective of theories of catalytic activity of transition-metal complexes and in relation with oxygenation reactions. V C 2013 Wiley Periodicals, Inc.

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... [11,12] These results indicate that a thermally activated pathway is possible, though there are changes in the energy landscape when the molecule is bound in the junction vs. the solution pathway. [30] Finally, we examined the switching process in situ by holding a molecular junction and stepping the bias to 250 mV for 1000 ms (Figure 2 e) at T 0 = 78 K. In this case, the NB-to-QC transition occurred probabilistically with 44.5 % of the junctions switching within the hold time. ...
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... + g state by itself does not imply whatever reactivity since no concrete reaction is specified, as seen from the triplet O 2 ground state. This problem is discussed in our recent publication[52].Downloaded by [Andrei Tchougréeff] at 15:29 16 March 2016 ...
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Band Shapes of Electronic Spectrad–d, Charge Transfer, Infrared, and Raman SpectraX-Ray and Ultraviolet Photoelectron Spectra; EXAFSMagnetic PropertiesGamma-Resonance SpectroscopyElectron Charge and Spin Density Distribution in Diffraction Methods Summary NotesExercises and ProblemsReferences
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Introduction Two Archetypal Valence Bond Diagrams The Valence Bond State Correlation Diagram Model and Its General Outlook on Reactivity Construction of Valence Bond State Correlation Diagrams for Elementary Processes Barrier Expressions Based on the Valence Bond State Correlation Diagram Model Making Qualitative Reactivity Predictions with the Valence Bond State Correlation Diagram Valence Bond Configuration Mixing Diagrams: General Features Valence Bond Configuration Mixing Diagram with Ionic Intermediate Curves Valence Bond Configuration Mixing Diagram with Intermediates Nascent from “Foreign States” Valence Bond State Correlation Diagram: A General Model for Electronic Delocalization in Clusters Valence Bond State Correlation Diagram: Application to Photochemical Reactivity A Summary References Exercises Answers
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Basic Concepts. General Density Matrix Theory. Coupled Systems. Irreducible Components of the Density Matrix. Radiation from Polarized Atoms: Quantum Beats. Some Applications. The Role of Orientation and Alignment in Molecular Processes. Quantum Theory of Relaxation. Appendix A: The Direct Product. Appendix B: State Multipoles for Coupled Systems. Appendix C: Formulas from Angular Momentum Theory. Appendix D: The Efficiency of a Measuring Device. Appendix E: The Scattering and Transition Operators. Index.
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The results of studies on intramolecular interconversions in systems of norbornadiene quadricyclane and their derivatives are classified and discussed. The mechanisms of the forward photoreaction and reverse thermal process in relation to the nature of the substituents and carbocycles, type of sensitiser and catalyst, and properties of the medium are discussed in detail and classified. An analysis is made of the influence of these factors on the spectroscopic, kinetic, and thermodynamic properties of this system as an accumulator and converter of light energy. Methods of improving its characteristics — light absorption, the quantum yield of the photoreaction, and stability of the photoproduct — have been established. The bibliography includes 188 references.
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Data on the valence isomerisation of norbornadiene and its derivatives into the corresponding quadricyclanes published between 1990 and 2001 are considered and described systematically. The applicability of this reaction for the storage of solar energy is discussed. The bibliography includes 112 references.
Article
Momentum Distributions (MDs), obtained using high-resolution electron momentum spectroscopy (HREMS), are reported for norbornadiene's 18 valence orbitals. Corresponding theoretical results, using generalized gradient approximation density functional theory (DFT) together with TZVP, DZVP, and DZVP2 basis functions and a plane wave impulse approximation to describe the ionization process, are also detailed. This work represents the first comprehensive HREMS/DFT investigation into the complete valence electronic structure of norbornadiene (NBD), with significant results being obtained. In particular, an exacting comparison between our experimental and theoretical MDs enables us to define the "optimum" basis for NBD, from those we studied. This "optimum" basis is then used to extract a wide range of NBD's important molecular property information, which are subsequently compared with the results of independent measurements and calculations. Agreement between our results and those from independent measurements was generally very good, highlighting the utility of HREMS in a priori basis set evaluation.
Article
Electron transfer catalysis is an efficient method for the catalysis of symmetry-forbidden or slow pericyclic reactions. Accurate quantum mechanical calculations are an important tool for gaining insights into the mechanistic details of these fast reactions involving radical cations. The current “state of the art” of computational studies of pericyclic reactions of radical cations is reviewed. In particular, four parent reaction types are discussed: (i) the ring-opening of the cyclobutane radical cation; (ii) the [2+2] cycloreversion of the cyclobutane radical cation; (iii) the radical cation Diels−Alder reaction of 1,3-butadiene and ethylene; and (iv) the [1,3] methylene shift in the vinylcyclopropane radical cation. The transfer of these findings to chemically more relevant substituted systems is also briefly discussed. The potential energy hypersurfaces obtained are very flat and have activation barriers that are significantly lower than the ones for the corresponding neutral reactions, which is in agreement with the large rate acceleration observed experimentally. Many of the located radical cation structures closely resemble their biradical counterparts in the neutral, stepwise pathways. The reactions generally follow a lower symmetry pathway, due to Jahn−Teller distortions induced by the unpaired electron. Finally, the results from computationally efficient B3LYP/6-31G* calculations are found to be in good agreement with those from highly correlated MO calculations.
Article
A unified description of chemical reactivity is made possible with valence bond (VB) diagrams, such as those shown for the a simple case (only reactants and products must be considered) and a more complex system (an intermediate state also plays an important role; RC = reaction coordinate). Analysis of reactivity and mechanistic problems in organic and organometallic chemistry exemplifies the generality of the VB paradigm: in situ DNA repair, C−F and C−H bond activation, SRN2c mechanism, stepwise versus concerted cycloaddition, and a lot more.