Halocarbon acceptors

In contrast to the dihalogens, there are only a few spectral studies of complex formation of halocarbon acceptors in solution. Indeed, the appearance of new absorption bands is observed in the tetrabromomethane solutions with diazabicyclooctene [49,50] and with halide anions [5]. The formation of tetrachloromethane complexes with aromatic donors has been suggested without definitive spectral characterization [51,52]. Moreover, recent spectral measurements of the intermolecular interactions of CBr4 or CHBr3 with alkyl-, amino- and methoxy-substituted benzenes and polycyclic aromatic donors reveal the appearance of new absorption bands only in the case of the strongest donors, viz. Act = 380 nm with tetramethyl-p-phenylendiamine (TMPD) and Act = 300 nm with 9,10-dimethoxy-l,4 5,8-... 

Both theoretical and experimental data (in the solid, liquid, and gas phases) prove that the tendency of halocarbons to work as XB donors decreases in the order I > Br > Cl [66-68]. Clearly, polarizability and not electronegativity plays a key role. 3-Halo-cyanoacetylene works as self-complementary module and the N X distance is beautifully consistent with the scale reported above, being 2.932, 2.978 and 2.984 A in the iodo, bromo and chloro derivatives, respectively [69,70]. The same trend is observed when a phenyl, rather than a triple bond, spaces the donor and acceptor sites. The N Br distance in 4-bromobenzonitrile is longer than in the 4-iodo derivative [71,72] and no XB is present in the chloro and fluoro analogues, wherein molecules are pinned by N H and X- H short contacts [73]. PFCs have a very poor tendency, if any, to work as XB donors [74-77] and no crystal engineering can be based on such tectons. However, F2 is a quite strong XB donor and several adducts have been described in the gas phase [11,18] (see also the chapter by Legon in this volume). 

In summary, the triplet (do po) excited states of the d -d metal dimers [Ir(p-pz)(C0D)]2 and Pt2(pop)4 " undergo a variety of photochemical reactions. Electron transfer to one-electron quenchers such as pyridinium cations or halocarbons readily occurs with acceptors that have reduction potentials as negative as -2.0 V. With the latter reagents, net two-electron, photoinduced electron transfer yields d -d oxidative addition products. Additionally, the triplet (da pa) excited state of Pt2(pop)4 apparently is able to react by extracting a hydrogen atom from a C-H bond of an organic substrate. 

By definition enzymes that are able to oxidize chloride, bromide, and iodide are called chloroperoxidases and those able to oxidize bromide and iodide, bromoperoxidases. If a nucleophilic acceptor (RH) is present, a reaction will occur with HOX and halogenated compounds see Halocarbons Halocarbon Complexes) are produced (equation 2). 

The cation [ Fe( i3-CO)Cp 4] can also be produced by irradiating 67 (M = Fe, E = CO) in the presence of halocarbons. Although the mechanism of the photooxidation process is not well understood, the charge-acceptor nature of the solvent was thought to be important (160). 

It should be noted that similar "wavelength effects" have been observed for the photooxidation of ferrocene In halocarbon-contalning solvents (Section III-B-5). In these cases, however, the reaction is initiated by the excitation of ferrocene-solvent donor-acceptor complexes. As clear evidence against a similar mechanism was not presented for the ferrocene photooxidation involving N2O, this appears to remain a possibility. 

Trichloroacetate rapidly reacts with the solvated electrons produced by laser flash photolysis of natural organic matter isolated from the Suwannee River, and thus quenches the absorption of the electrons at 720 nm. The ibsorption is also quenched by the addition of other good electron acceptors, including oxygen, protons, or nitrous oxide. In natural waters, halocarbon concentrations are typically very low, and the dominant scavenger of solvated electrons is oxygen. 

Transient absorption spectra of some "satellite ions" closely resemble the spectra of olefin radical cations. In cyclohexane, a band centered at 270 nm (at 2 ns [22]) is observed from 250 ps [25] after the ionization event (this band overlaps with the strong 240 nm band of cyclohexyl radicals [22]). The scavenging behavior and the decay kinetics of the UV-absorbing species suggest that they are normally-diffusing radical cations [25]. In the first few nanoseconds after the ionization event, the VIS absorbance is dominated by solvent excited states [22,57]. When the thermalized electrons are rapidly scavenged using a suitable electron acceptor (halocarbons or N2O), this... 

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