Charge transfer, bonding complexes

Scheme 10.5 Tentative mechanism for cytochrome P450-cata-lyzed epoxidation of a double bond. The reactive iron-oxo species VII (see Scheme 10.4) reacts with the olefin to give a charge transfer (CT) complex. This complex then resolves into the epoxide either through a radical or through a cationic intermediate.

These iridium(IV) complexes have UV-visible spectra dominated by intense absorptions around 500 nm (X = Cl) and 700 nm (X = Br) assignable to 7tx —> Ir(t2g) ligand-to-metal charge-transfer bonds. 

The quantitative effects of steric encumbrance on the electron-transfer kinetics reinforce the notion that the inner-sphere character of the contact ion pair D+, A- is critical to the electron-transfer paradigm in Scheme 1. Charge-transfer bonding as established in the encounter complex (see above) is doubtless an important consideration in the quantitative treatment of the energetics. None the less, the successful application of the electron-transfer paradigm to the... 

Table 5.15. A comparison of neutral and charged H-bond complexes B- -HA from Sections 5.2.1 and 5.2.2 (ordered by H-bond strength), showing net H-bond energy A hb, leading cr stabilization AEn fr2 net charge transfer 0b->-ah, and NRT bond orders bA—h and b-h...

Photochemical ET reactions can be classified in at least three categories (which can co-exist), namely (i) simple homolysis of bonds of neutral molecules to give radicals of low redox reactivity (ii) excitation of a species D to produce an excited state D which initiates a second-order ET reaction involving another component of acceptor type, A, with formation of the radical pair D + A (iii) direct excitation of a charge transfer (CT) complex formed between two reaction components D and A to form the same radical pair D + A -. The first case is obviously an ideal situation if it can be realized, but this is seldom the case. The incursion or predominance of situations (ii) and/or (iii) in almost any system is possible, and precautions must be taken to avoid these complications. Much can be done by controlling the wavelength of the light source, but it is also possible to affect the chemistry in a predictable manner. 

Because of the high electron density in the aromatic ring, toluene behaves as a base both in the formation of charge transfer r complexes and in the formation of sigma complexes. When only n-electrons are involved, toluene behaves much like benzene and xylene. When o-bonds and complexes arc involved, toluene reacts much faster than benzene and much slower than xylenes. 

The electronic absorption spectroscopy of charge transfer (CT) complexes of donor molecules of n-, n- and cr-type (DX) with jt- and rr-acceptors (A = TCNE, I2 etc.) allows one to study the influence of the X substituents bonded to a donor centre, D, on the energies of charge transfer bands, hvcr129- The hvcr and Ip parameters are connected by a linear dependence given in equation 19. 

When charge-transfer bonds are obscured by those of the original donors and acceptors, one may find of value a difference method [1] (e.g., Forster s tandem method [70]). Four cuvets of equal path length are used, two containing the charge-transfer complex solutions in series in the indicator beam of a double-beam spectrophotometer, and two cuvets, one with the unreacted donor and the other with unreacted acceptor solution, also in series in the reference beam. A difference spectrum is thus obtained which, however, needs special care in its interpretation. 

Weak Chemical Coordination/Complexation/ Charge-Transfer Bonding 2-50... 

The intensities and the positions of absorption bands may also be influenced by solvent molecules. Electrostatic dipole interactions, and specific interactions that lead to formation of complexes based on hydrogen bonds and of charge-transfer type complexes, result in changes in the spectra of chemical compounds. 

The addition of halogens to carbon-carbon double bonds is interpreted as a stepwise addition which is initiated by a species containing a positively polarized halogen in the present case the chlorine molecule which becomes polarized in close proximity to the jr-electron cloud of ethylene. The initial step resnlts in the formation of a T-shape charge-transfer intermolecnlar complex which has been identified both theoretically [29-31] and experimentally [32,33] in the gas phase. 

As shown in Scheme 18.1, the reaction mechanism for the photocatalytic direct decomposition of NO over the isolated tetrahedral titanium oxide species can be proposed, that is, two NO molecules are able to adsorb onto these oxide species as weak ligands to form reaction precursors. Under UV irradiation, the charge-transfer excited complexes of the oxides [Ti +-0 ] are formed. Within their lifetimes, the electron transfers from the Ti site, on which the photo-formed electrons are trapped, into the anti-ir -bonding orbital of the NO molecule, and the electron transfers simultaneously from the n-bonding orbital of another NO molecule into the 0 site, where... 

The porphyrins play important roles in the nature, due to their sp>ecial absorption, emission, charge transfer and complexing properties as a result of their characteristic ring structure of conjugated double bonds (Rest et al.,1982). 

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