Transient anion

Chitosan has been recently found to be soluble in alkaline media, viz. NH4HCO3 solutions, where it assumes the ammonium carbamate form Chit-NHC02 NH4, i.e., a transient anionic form that keeps it soluble at pH 9.6, while reversibly masking the polycationic nature of chitosan. Because ammonium carbamates and NH4HCO3 decompose thermally and liberate CO2, NH3 and water, this alkaline system is suitable for producing chitosan microspheres by spray-drying (Table 1) [206]. 

Earlier, in Sect. 8.3.1, a generalized mechanistic scheme for the reduction of simple alkyl halides was presented. What distinguishes aryl halides (ArX) from alkyl halides (RX) is the finite lifetime of the initially electrogenerated anion radical (ArX ). Thus, although ArX exhibits the same kinds of reactions as RX, a key difference is that the transient anion radical (ArX ) can undergo a homogeneous electron-transfer reaction with the aryl radical (Ar) (Eq. 4) ... 

Dissociative electron attachment (DEA) occurs when the molecular transient anion state is dissociative in the Franck-Condon (FC) region, the localization time is of the order of or larger than the time required for dissociation along a particular nuclear coordinate, and one of the resulting fragments has positive electron affinity. In this case, a stable atomic or molecular anion is formed along with one or more neutral species. Dissociative electron attachment usually occurs via the formation of core-excited resonances since these possess sufficiently long lifetimes to allow for dissociation of the anion before autoionization. 

Equation (6) defines most of the intrinsic characteristics of the DEA process. For further information on the mechanism of transient anion formation and its effects in isolated electron-atom and electron-molecule systems, the reader is referred to the review articles by Schulz [17] and others [7,19,20]. Information on resonance scattering from single layer and submonolayer of molecules physisorbed or chemisorbed on conductive surfaces can be found in the review by Palmer et al. [21-23]. The present article provides information essentially on resonances in atoms and molecules condensed onto a dielectric surface or forming a dielectric thin film. 

This striking result can be qualitatively understood as related to CB DOS-influenced changes in the 02 anion lifetime [118]. For a diatomic molecule with R as the internuclear coordinate, a transient anion state is described in the fixed nuclei limit [123,124] by an energy and i -dependent complex potential Vo i R,E ) = Fd(2 ) + A( i)—l/2 T( i), where Va R) = a R) + is the potential energy curve of the discrete state, Vg(R) is the... 

The hydroxyquinoline (39-2) provides the starting material for a quinolone that incorporates a hydrazine function. Reaction of (39-2) with 2,4-dintrophenyl O-hydroxylamine ether (41-1) in the presence of potassium carbonate leads to a scission of the weak N-O hydroxylamine bond by the transient anion from the quinolone the excellent leaving character of 2,4-dinitrophenoxide adds the driving force for the overall reaction, resulting in alkylation on nitrogen to form the hydrazine (41-2). The primary amine is then converted to the formamide (41-3) by reaction with the mixed acetic-formic anhydride. Alkylation of that intermediate with methyl iodide followed by removal of the formamide affords the monomethylated derivative (41-4). Chlorine at the 7 position is then displaced by A-methylpiperazine and the product saponified. There is thus obtained amifloxacin (41-6) [48]. 

The photoelectrons are trapped at the pigment surface by adsorbed nitro compound to give a transient anion radical (PNBX ) which subsequently fragments to initiator species (63,101). 

The use of heteroaryl vinyl sulfides and vinyl dithiocarbamates (166) as hetero-Michael addition acceptors has been described. Combined chelating and electron-withdrawing effects were postulated to stabilize the transient anionic species and allow smooth Michael-induced ring closure to produce C-glycosides (167).191... 

Both nucleophilic and electrophilic reactions are known, and the reaction sequence can be directed by a suitable choice for a heteroligand [41,52], As suggested by Scheme 6.11a, the ability of the heteroligand to direct electrophilic or nucleophilic attack by the peroxocomplex can provide an important tool in oxidative reactions, where selectivity of action is required. A second mode of electrophilic reaction chemistry is available through attack of sulfur electrons at the vanadium center to give a transient anion/cation radical pair via formation of V(TV) and S-+ (Scheme 6.1 lb). 

Formation of the Au-S bond in hexanethiol adsorption leads to a localized a resonance which broadens and shifts when the hexanethiol molecules, initially lying flat at low coverages, realign vertically at higher coverages. Extensive studies of the model system C61 v>/Cu(111) have shown the presence of a transient anionic resonance, and similar studies are now being carried out on other systems. 

It seems as if this problem can be solved by considering mechanisms that occur before the electron has time to fully relax. The idea being that transient anions associated with the lowest unoccupied molecular orbitals will excite vibrational modes. These ideas are outlined in a new paper by Kumar and Sevilla that looks at C5-05 bond dissociation in 5 -dTMP. They report that on the vertical potential energy surface, the B3LYP/6-31G(d) calculated barrier height for C5 -05 bond dissociation is ca. 9 kcal/mole which is lower than the adiabatic value for this same process [112],... 

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