Detachment reaction

Viggiano A A and Paulson J F 1983 Temperature dependence of associative detachment reactions J. Chem. Phys. 79 2241-5... 

To illustrate the specific operations involved, the scheme below shows the first steps and the final detachment reaction of a peptide synthesis starting from the carboxyl terminal. N-Boc-glycine is attached to chloromethylated styrene-divinylbenzene copolymer resin. This polymer swells in organic solvents but is completely insoluble. ) Treatment with HCl in acetic acid removes the fert-butoxycarbonyl (Boc) group as isobutene and carbon dioxide. The resulting amine hydrochloride is neutralized with triethylamine in DMF. 

Collisional Detachment. Reactions of negative ions in flames not containing hydrocarbons have not been widely studied, although OH -ion formation is important in flames containing high electron concentrations. The rate constant k l of the reaction... 

Holroyd (1977) finds that generally the attachment reactions are very fast (fej - 1012-1013 M 1s 1), are relatively insensitive to temperature, and increase with electron mobility. The detachment reactions are sensitive to temperature and the nature of the liquid. Fitted to the Arrhenius equation, these reactions show very large preexponential factors, which allow the endothermic detachment reactions to occur despite high activation energy. Interpreted in terms of the transition state theory and taking the collision frequency as 1013 s 1- these preexponential factors give activation entropies 100 to 200 J/(mole.K), depending on the solute and the solvent. 

Mozumder (1996) has discussed the thermodynamics of electron trapping and solvation, as well as that of reversible attachment-detachment reactions, within the context of the quasi-ballistic model of electron transport. In this model, as in the usual trapping model, the electron reacts with the solute mostly in the quasi-free state, in which it has an overwhelmingly high rate of reaction, even though it resides mostly in the trapped state (Allen and Holroyd, 1974 Allen et ah, 1975 Mozumder, 1995b). Overall equilibrium for the reversible reaction with a solute A is then represented as... 

To obtain the attachment reaction efficiency in the quasi-free state, we denote the specific rates of attachment and detachment in the quasi-free state by kf and kf respectively and modify the scavenging equation (10.10a) by adding a term kfn on the right-hand side, where is the existence probability of the electron in the attached state. From the stationary solution, one gets kf/kf = (kfk ikfkf), or in terms of equilibrium constants, K(qf) = Kr.Kr, where k, and k2 are the rates of overall attachment and detachment reactions, respectively. Furthermore, if one considers the attachment reaction as a scavenging process, then one gets (see Eq. 10.11) = k f fe/(ktf + kft) = fe,f/(l + Ku) and consequently k2 = kfKJ(l + KJ. 

TABLE 10.6 Thermodynamic Functions for Reversible Attachment-Detachment Reactions in Hydrocarbon Liquids... 

Williamson, D.H. Knighton, W.B. Grimsrud, E.P. Effect of Buffer Gas Alterations on the Thermal Electron Attachment and Detachment Reactions of Azu-lene by Pulsed High Pressure Mass Spectrometry. Int. J. Mass Spectrom. 2000, 795/796,481-489. 

New allylic anchors as the HYCRAM (19) (hydroxycrotonylamide) [50] and HY-CRON (21) (hydroxycrotyl-oligoefhylene glycol-n-alkanoyl) [51] linker have been developed, which exhibit excellent properties for the solid-phase synthesis of protected peptides and glycopeptides. A more flexible spacer was inserted in the HY-CRON (21) linker between the anchor and the polymeric support in order to facilitate an efficient access to the Pd(0) complex during the detachment reaction. 

In the first scheme the metal boiling point is less than the oxide boiling point and the model consists of a vaporising droplet of metal surrounded by a detached reaction zone where condensed oxides appear as fine droplets. The reaction rate is said to be controlled by the vapour phase diffusion of metal and atmospheric oxygen into the reaction zone as in Figure 5.6. 

In addition, for the pentanal cluster, all other reaction channels observed start with either Norrish type I, Norrish type II or H detachment reactions, but continue then further, yielding reactions between different pentanal molecular, so-called cross-molecular reactions. Especially, the H detachment step is noteworthy. In the bare pentanal molecule, this pathway was observed in only 1 % of the trajectories. Here, it is involved in most of the trajectory as one of the observed steps of cross-molecular reactions. 

They actually refer to attachment and detachment reactions.19... 

The UV photochemistry of phenol and related systems (such as indole, pyrrole, imidazole) is dominated by a hydrogen detachment reaction which is driven by repulsive 1ira states [33,35 10], For the isolated chromophores, the 1 mr -driven photodissociation has been explored in unprecedented detail by high-resolution photofragment translational spectroscopy [40], The OH (or NH) bond is broken homolytically, resulting in the formation of two radical species, the hydrogen atom and the phenoxy (or indolyl, etc.) radical. Ion pair formation (abstraction of protons) is energetically not feasible for isolated photoacids. 

These three concentration-time equations describe the proton-promoted dissolution reaction in Eq. 4.51 under the assumption that the selenate detachment reaction is already at equilibrium. The case K < < K corresponds to the approximate equality in Eq. 4.52f. Note that the rate of mineral dissolution, d[Mn2+]/dt, will be constant for observation times much smaller than 1/k, . 

Deexcitation-detachment reactions are particularly significant for the disturbed atmosphere, in which anomalously high 1.27 m/ intensities are found, presumably due to O A,) in some aurorae [173-175]. 

For the case of high temperatures found in the vicinity of hot stars, or in cloud regions under the influence of a shock wave, or in ionized nebulae, several authors have suggested (c.f. Jura, 1975, Dalgamo and McCray 1973) a gas phase formation scheme which could be a significant source for molecular hydrogen, Hj. The associative detachment reaction... 

The low effective value for the direct dissociative attachment process was later shown to be due, in part, to the regeneration of electrons via the associative detachment reaction,... 

Several exothermic associative detachment reactions do not occur at measurable rates (k < 10 12 cc./sec.), an example being... 

Associative detachment reactions are important in controlling the electron concentration in the earth s mesosphere [75]. Reactions in which more than one neutral product are formed also occur and are sometimes referred to as reactive detachment [76]. 

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