Density functional theory insertion reactions

Elementary reaction mechanisms for nitrous oxide (N20) dissociation were studied on Fc"( i-0)( i-0H)Fc" + exchanged in ZSM-5, using density functional theory (DFT). The effect of the cluster size on the energetics and on the reaction routes of N20 dissociation were investigated over di-iron core inserted inside two different Z cluster (Z ) and (Z oh)- The results show that while the relative stability changes with the cluster termination, the height of the energetic barriers are similar. 

Theoretical considerations leading to a density functional theory (DFT) formulation of the reaction field (RF) approach to solvent effects are discussed. The first model is based upon isolelectronic processes that take place at the nucleus of the host system. The energy variations are derived from the nuclear transition state (ZTS) model. The solvation energy is expressed in terms of the electrostatic potential at the nucleus of a pseudo atom having a fractional nuclear charge. This procedure avoids the introduction of arbitrary ionic radii in the calculation of insertion energy, since all integrations involved are performed over [O.ooJ The quality of the approximations made are discussed within the frame of the Kohn-Sham formulation of density functional theory. 

Imhof et al. [22] studied the reaction mechanism of the [2+2+1] cycloaddition reactions of diimines, CO, and ethylene catalyzed by iron carbonyl complexes on the basis of density functional theory (Scheme 4). The catalytic reaction does not start when CO dissociates from 10 followed by the addition of ethylene, but instead the associative pathway to 11 is proposed. In addition, it can be concluded that the insertion of CO in 11 takes place into a C-Fe bond but not... 

In the original paper [2], it was proposed that the results shown arise from a halophilic attack of the silylene nucleophile on the halogen atom of the reactants. On the other hand, Su has studied the Lewis type reaction of several silylenes with density functional theory [3, 4]. His basic idea is that the reaction may proceed in three steps. In the first, the halocarbon is expected to form a weak complex with the silylene. In the next, the complex is transferred into the insertion product. In the last step, the attack of a second silylene molecule leads to the formation of the final disilane product. 

Fang, W.-H., Phillips, D. L., Wang, D., Li, Y.-L. A Density Functional Theory Investigation of the Simmons-Smith Cyclopropanation Reaction Examination of the Insertion Reaction of Zinc into the C-l Bond of CH2I2 and Subsequent Cyclopropanation Reactions. J. Org. Chem. 2002, 67, 154-160. 

Based on a density functional theory (DFT) study of the model reaction of (CO)5Cr=C(OH)CH=CH2 with HC=CH, Torrent et recently proposed that an association type mechanism according to which insertion of the alkyne into the Cr=C bond followed by CO dissociation may be energetically more favorable than the mechanism of Scheme 1. However, their claims have been rejected because they are contradicted by the above described kinetic results. ... 

Substrate probes have aided mechanistic understanding of the key C— H activation step in the MMOH reaction cycle. Chiral alkanes and radical-clock substrate probes " " were used to discriminate between radical recoil/rebound and nonsynchronous concerted insertion pathways. A short lifetime (< 150 fs) estimated for the putative radical species derived from cyclopropane-based radical-clock substrates favors the latter process,whereas partial racemization of chiral ethane substrate is consistent with the former scenario. A unifying model was proposed, in which both recoil/rebound and concerted reaction channels are available for a bound radical intermediate and the partitioning between each trajectory is dependent on the substrate. Formation of carboca-tion-derived products from certain probes implicates yet another route involving a formal OH+ insertion.Participation of multiple species capable of oxygen transfer is an emerging mechanistic view in both heme and nonheme systems, as exemplified by the studies of cP450s and their synthetic models.Scheme 3 depicts various density functional theory (DFT) models of MMOHq and their computed reaction pathways, which are reviewed in detail elsewhere. 

In an attempt to rationalize the experimental observations, a number of theoretical studies have been performed to probe the mode of propagation and chain transfer. As with oxidation state considerations for the active catalyst, uncertainty also exists about the precise electronic structure of the iron species. In an initial full ah initio study on the diisopropylphenyl Fe(II) catalysts derived from la, Gould and coworkers [132] determined the key structures operating for the first monomer insertion and showed that intermediates along the reaction coordinate have low spin (S = 0) configurations. Ziegler et al. have carried out density functional theory... 

Oxidative addition of fluorinated aromatics to give metal(aryl) fluoro complexes has been observed frequently for complexes of group 10 metals, but rarely otherwise. The reactions include the insertion of an Ni(PEt3)2 unit into a carbon-fluorine bond in hexafluorobenzene (Scheme 5). The very slow conversion (4 weeks) was initially reported by Fahey and Mahan full spectroscopic and crystallographic characterization of the product was performed by Perutz et al " Mechanistic studies and density functional theory (DFT) calculations give strong evidence for precoordination of the aromatic substrate followed by a concerted oxidative addition. " " Oxidative addition has also been... 

Dinuclear complexes of trans- and cis-geometry (trans-complexes are more thermodynamically stable) have the ArZ-Pd-ZAr structural unit, which is needed to operate in the catalytic cycle. The observed rate constant of the addition reaction of PhsSes to alkyne was linearly dependent on the concentration of dinuclear complexes [Pd(SePh)4(PPh3)2] [108]. Theoretical study at the density functional theory level has shown that alkyne insertion in the dinuclear complex preferentially involves the terminal ZAr group rather than the bridging ZAr group coordinated in mode [112]. A deficiency of the phosphine ligand under catalytic conditions resulted in rapid... 

This means that the number of active catalyst centers was small at the beginning of the polymerization reaction but increased during the run which readily explains the activity profile shown in Figure 7.5. These experimental observations could be explained by a substantial difference in the ethylene insertion barrier into the Cr-methyl bond compared to Cr-propyl (or Cr-polymer) bonds. Once the ethylene has been inserted into the first Cr-Me complex, the subsequent insertions are facilitated so that the Cr-Me complexes serve as a reservoir for more active catalyst species. This interpretation was supported by comprehensive density functional theory (DFT) calculations carried out in cooperation with Koppel and coworkers [28]. 

In chapter 2, Profs. Contreras, Perez and Aizman present the density functional (DF) theory in the framework of the reaction field (RF) approach to solvent effects. In spite of the fact that the electrostatic potentials for cations and anions display quite a different functional dependence with the radial variable, they show that it is possible in both cases to build up an unified procedure consistent with the Bom model of ion solvation. The proposed procedure avoids the introduction of arbitrary ionic radii in the calculation of insertion energy. Especially interesting is the introduction of local indices in the solvation energy expression, the effect of the polarizable medium is directly expressed in terms of the natural reactivity indices of DF theory. The paper provides the theoretical basis for the treatment of chemical reactivity in solution. 

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