Dissociative recombination

The dissociation of a molecule in solution and the approach to an equilibrium distribution of molecules and radicals has been treated by Berg [278]. His detailed and careful analysis uses the diffusion equation exclusively to describe microscopic motion. During molecular dissociation on a microscopic scale (i.e. involving only a few molecules), molecules dissociate, recombine, dissociate etc. many times. The global rate of dissociation is much less than that of an individual molecule, indeed smaller by a factor of (1 + kACijAiiRD), that is an average number of times the molecule dissociates and recombines. For reactions which do not go to completion... 

Most of these reactions have been measured and their rates are thus reasonably well known (Fehsenfeld et al. 1967). In comparing the two reaction schemes, it is interesting to note that HjO does recombine dissociatively with an electron to form H2O and OH, but that the analoguous reaction sequence with seems to take place. The branching ratio of the H30 dissociative recombination is not accurately known. In this connection, it is also important to note that reactions of H with atomic carbon seem to be endothermic (Burt et al. 1970 and references therein), and thus the carbon chemistry in dense molecular clouds can not start this way. 

Sensitivities toward A TP of the Recombined Dissociation Products of Thrombosthenin and Actomyosin from Rabbit Muscle... 

ATPase Activities of the Recombined Dissociation Product of Thrombosthenin and... 

Up until the late 1980 s measured activation energies for the reverse reaction were combined with assumed values of the forward reaction to give Second Law heats of formation for R [74]. However, the values of AfH (R) were found to be incompatible with those measured by addition (or recombination)/dissociation equilibria [71, 75]. The reasons for this were rationalized by a series of experiments performed by Gutman and coworkers [76, 77] and others [78, 79] who directly measured the rate... 

Let us introduce the rate coefficients of forward and backward reactions kb,r- For exchange reaction (4) (r = ex), and recombination-dissociation reaction (5) (r = diss) they have the form ... 

The Influence of Hindered Rotations on Recombination/Dissociation Kinetics... 

Part IV again is a theoretical one, in which different approaches for the calculation of state-specific and thermal rate data are described. The article by A.F. Wagner presents a new approach to describe the influence of hindered rotations on recombination/dissociation kinetics in the framework of transition state theory. In the papers by D.C. Clary and G. Nyman an approximate quantum mechanical method is described and used to calculate thermal rate coefficients for gas phase reactions of interest in atmospheric chemistry which involve polyatomic molecules. Finally, different approaches to describe vibrational relaxation of diatoms in thermal collisions are discussed by E,E, Nikitin,... 

When investigating time parameters it was shown, that a storage time in Xe of spectral purity was 10 mcs, while a restoring time was 10 ms with subsequent decrease in the case of addition of small amounts of air (less than 1%). As basic processes influencing time parameters, both dissociative recombination and three-particle adhesion of electrons to oxygen molecules have been considered. 

When a molecule adsorbs to a surface, it can remain intact or it may dissociate. Dissociative chemisorption is conmion for many types of molecules, particularly if all of the electrons in the molecule are tied up so that there are no electrons available for bonding to the surface without dissociation. Often, a molecule will dissociate upon adsorption, and then recombine and desorb intact when the sample is heated. In this case, dissociative chemisorption can be detected with TPD by employing isotopically labelled molecules. If mixing occurs during the adsorption/desorption sequence, it indicates that the mitial adsorption was dissociative. 

CHXNH. Many of the reactions involve radiative association. Dissociative electron-ion recombination then yields neutrals such as CH (methane), C2H OH (ethanol) and CH CN (acetonitrile) [158]. It is often joked... 

Schroeder J and Troe J 1993 Soivent effects in the dynamics of dissociation, recombination and isomerization reactions Activated Barrier Crossing ed G R Fieming and P Hanggi (Singapore Worid Scientific) p 206... 

Marcus R A 1952 Unimolecular dissociations and free radical recombination reactions J. Chem. Rhys. 20 359-64... 

Dissociative recombination and associative ionization are represented by the forward and backward directions of... 

With M = He, experimeuts were carried out between 255 K aud 273 K with a few millibar NO2 at total pressures between 300 mbar aud 200 bar. Temperature jumps on the order of 1 K were effected by pulsed irradiation (< 1 pS) with a CO2 laser at 9.2- 9.6pm aud with SiF or perfluorocyclobutaue as primary IR absorbers (< 1 mbar). Under these conditions, the dissociation of N2O4 occurs within the irradiated volume on a time scale of a few hundred microseconds. NO2 aud N2O4 were monitored simultaneously by recording the time-dependent UV absorption signal at 420 run aud 253 run, respectively. The recombination rate constant can be obtained from the effective first-order relaxation time, A derivation analogous to (equation (B2.5.9). equation (B2.5.10). equation (B2.5.11) and equation (B2.5.12)) yield... 

Retardation of the reaction rate by the addition of dimethyl sulfide is in accord with this mechanism. Borane—amine complexes and the dibromoborane—dimethyl sulfide complex react similarly (43). Dimeric diaLkylboranes initially dissociate (at rate to the monomers subsequentiy reacting with an olefin at rate (44). For highly reactive olefins > k - (recombination) and the reaction is first-order in the dimer. For slowly reacting olefins k - > and the reaction shows 0.5 order in the dimer. 

Zircon is synthesized by heating a mixture of zirconium oxide and silicon oxide to 1500°C for several hours (163). The corresponding hafnium silicate, hafnon, has been synthesized also. Zircon can be dissociated into the respective oxides by heating above 1540°C and rapidly quenching to prevent recombination. Commercially, this is done bypassing closely sized zircon through a streaming arc plasma (38). 

DispEcement. In many of the appHcations of chelating agents, the overall effect appears to be a displacement reaction, although the mechanism probably comprises dissociations and recombinations. The basis for many analytical titrations is the displacement of hydrogen ions by a metal, and the displacement of metal by hydrogen ions or other metal ions is a step in metal recovery processes. Some analytical pM indicators function by changing color as one chelant is displaced from its metal by another. 

A well-known example of non-prototropic tautomerism is that of azolides (acylotropy). The acyl group migrates between the different heteroatoms and the most stable isomer (annular or functional) is obtained after equilibration. In indazoles both isomers are formed, but 2-acyl derivatives readily isomerize to the 1-substituted isomer. The first order kinetics of this isomerization have been studied by NMR spectroscopy (74TL4421). The same publication described an experiment (Scheme 8) that demonstrated the intermolecular character of the process, which has been called a dissociation-recombination process. 

All of the atomic species which may be produced by photon decomposition are present in plasma as well as the ionized states. The number of possible reactions is therefore also increased. As an example, die plasma decomposition of silane, SiH4, leads to the formation of the species, SiH3, SiHa, H, SiH, SiH3+ and H2+. Recombination reactions may occur between the ionized states and electrons to produce dissociated molecules either direcdy, or tlrrough the intermediate formation of excited state molecules. 

---