Dissociative neutral

In radiolysis, a significant proportion of excited states is produced by ion neutralization. Generally speaking, much more is known about the kinetics of the process than about the nature of the excited states produced. In inert gases at pressures of a few torr or more, the positive ion X+ converts to the diatomic ion X2+ very rapidly. On neutralization, dissociation occurs with production of X. Apparently there is no repulsive He2 state crossing the He2+ potential curve near the minimum. Thus, without He2+ in a vibrationally excited state, dissociative neutralization does not occur instead, neutralization is accompanied by a col-lisional radiative process. Luminescences from both He and He2 are known to occur via such a mechanism (Brocklehurst, 1968). 

Under the Born-Oppenheimer approximation, two major methods exist to determine the electronic structure of molecules The valence bond (VB) and the molecular orbital (MO) methods (Atkins, 1986). In the valence bond method, the chemical bond is assumed to be an electron pair at the onset. Thus, bonds are viewed to be distinct atom-atom interactions, and upon dissociation molecules always lead to neutral species. In contrast, in the MO method the individual electrons are assumed to occupy an orbital that spreads the entire nuclear framework, and upon dissociation, neutral and ionic species form with equal probabilities. Consequently, the charge correlation, or the avoidance of one electron by others based on electrostatic repulsion, is overestimated by the VB method and is underestimated by the MO method (Atkins, 1986). The MO method turned out to be easier to apply to complex systems, and with the advent of computers it became a powerful computational tool in chemistry. Consequently, we shall concentrate on the MO method for the remainder of this section. 

The use of the equilibrium condition for a fast step can greatly simplify mathematics, as will be seen in various examples in later chapters. Prominent among the fast steps to which the approximation can be applied are dissociation reactions in the gas phase and ionic reactions such as electrolytic dissociation, neutralization, and complex formation, as well as loss, addition, and exchange of... 

In contrast to the aforementioned systems, relatively low translational energy thresholds for fragmentation are observed in the scattering of H2, N2+, and O2+ on Ni(lll), O2+ on Ag(lll), ° H2+ on Cu(lll), and OCS+ on Ag(lll). In each of these systems, a dissociative neutralizar tion mechanism has been assigned. Dissociative neutralization occurs when a surface electron transfers to an incident cation, whereupon a repulsive... 

Of particular interest in soil science is ion exchange accompanied by reactions such as dissociation, neutralization, hydrolysis, and complex formation. Such reactions affect both equilibria and kinetics. 

In the quadruple-barrier case one needs to distinguish whether dissociation/ neutralization or propagation are the rate-limiting steps. Furthermore, a parameter a is needed describing the ratio of the forward reaction rate constants of the anion and the cation propagation, i.e. 

Dissociative Neutral (Dm). Dm transformations proceed via initial dissociation of an L-type ligand to form a neutral five-coordinate Pd adduct, which is followed by C-X bond-forming reductive elimination. 

On the basis of this data, the authors proposed that C-O coupling from 11 proceeds via a dissociative neutral (I>N)/carbocation mechanism. The proposed pathway involves pre-equiUhrium dissociation of the pyridine ligand to generate the... 

In a special extension of this technique, the mass-analyzed primary ions are neutralized to form a beam of dissociating neutrals, whose products are reionized to form an NR mass spectrum (McLafferty et al. 1980a Wesdemiotis and McLafferty 1987 Holmes 1989 McLafferty 1990). Such dissociation of the... 

Random rearrangements are the basic reason why the spectra of many isomeric hydrocarbons (Equation 8.1), such as 1- and 2-butene, are similar. The activation energy for isomerization of 1-C4H8 is 1.0 eV, versus 1.6 eV for its lowest-encrgy dissociation, so that extensive equilibration 1-C4H8 2-C4Hy lakes place before dissociation. Neutralization-reionization mass... 

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