Active sites electron orbitals

It seems to be realistic to relate catalytic activity to the most stable [111] plane of fee metals. Bond (135) describes the electron structure of the this plane. So-called 2g electron orbitals point toward those interstices where metal atoms in the subsequent overlayer would be accommodated. These orbitals have metallic character. So-called orbitals point toward the next nearest neighbor. These are localized and able to form real covalent bonds. The degree of hybridization of these orbitals is imknown. Knor (136) assumes that only orbitals would stick out of the plane, but they are almost completely hybridized. He assumes that the /2g electrons are parts of the electron gas of the metal. The and sites are by no means equivalent. 

The transition metal atoms or Ions on the surface of the active sites of the catalyst form weak bonds with the reactant molecules. It Is thought that the presence of unpaired d electrons or unfilled d orbitals allows Intermediate complexes to form. The effect of this is to weaken the covalent bonds Inside the reactant molecules and, as these reactant molecules are now held in a favourable position, they are more susceptible to attack by molecules of the other reactant. The overall effect is that an alternative reaction pathway with a lower activation energy is provided and so the rate of the reaction Is Increased. 

Steric effects. These effects result from the repulsion between valence electrons in orbitals on atoms which are in close proximity bnt not bonded to each other, or by shielding an active site from a reactant or solvent. 

Side reactions specific to one component play an important role in the reforming of a mixture. For example, aromatics are more prone to coking upon reforming, so their presence in a mixture can lower syngas yields over time due to catalyst deactivation. Also, the catalyst surface-component interactions may play an important role in the reforming of a mixture. For example, aromatics have an abundance of 71-electrons, so they may occupy active sites for a longer duration, due to 71-complexation between d-orbitals of the metal and 7i-elec-trons. Hence there will not be enough reactive sites available for the desired reaction to occur. 

The absorption spectra of blue copper proteins typically include one major peak and two other peaks of varying size in the range 10,000-30,000 cm-1 (164-166). MCD spectroscopy has proved useful in assigning these peaks. The electronic excitations of the active site can be classed as either d—>d or LMCT transitions. The d- fd transitions will involve excited states where the electron hole remains on the Cu atom while the LMCT transitions will move the hole to the ligands, in particular the sulfur atoms of the Met and Cys groups. Thus the d- d transitions would be expected to be more strongly influenced by spin-orbit coupling and this should be reflected in the relative size of the Cj/Dj ratios of the bands in their MCD spectra. 

The reaction follows the consensus mechanism for aliphatic —H activation by oxyl-ferryl compounds (35) in which the first step is H-atom abstraction via TS1 to give a hydroxo-Fe(III) complex with a C-centered alkyl radical, labeled IN. This is followed by a rebound step via TS2 to give the final product, ethanol and the ferrous active site. Overall, this is a two-electron oxidation process where the bonding orbital serves as the electron donor and the H-atom abstraction is rate limiting. 

Studies of the sticking probability of oxygen on silicon surfaces revealed a change from 10 5 to 1 with increasing density of surface steps (52a). Crystals with a large density of steps may be prepared by cleavage, and their chemistry can be readily studied. Ibach has associated the increased activity of disso-ciatively chemisorbed oxygen to the presence of electron orbitals that became available on silicon atoms at low coordination number step sites on surfaces. 

Additional information has been obtained from single crystal, polarized optical and ESR spectroscopic studies924 on poplar plastocyanin, which have allowed a correlation of the electronic structure of the blue copper active site with its geometric structure. In summary, the three dominant absorption bands at 13 350, 16 490 and 17 870 cm-1 were assigned to CysS- Cu (d 2-,2 charge-transfer transitions. The methionine makes only a small contribution, due to the long Cu—S(Met) bond (2.9 A) and the poor overlap of the methionine sulfur orbitals with the dx y orbital of copper. Histidine-Cu charge transfer contributes to the weaker absorptions at 21 390 and... 

Spectroscopies are also used to experimentally probe transient species along a reaction coordinate, where often the sample has been rapidly freeze quenched to trap intermediates. An important theme in bioinorganic chemistry is that active sites often exhibit unique spectroscopic features, compared to small model complexes with the same metal ion.8 These unusual spectroscopic features reflect novel geometric and electronic structures available to the metal ion in the protein environment. These unique spectral features are low-energy intense absorption bands and unusual spin Hamiltonian parameters. We have shown that these reflect highly covalent sites (i.e., where the metal d-orbitals have significant ligand character) that can activate the metal site for reactivity.9... 

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