Gas-phase dissociation

Figure 6.10 A plot of dissociation enthalpy versus substitution pattern for the gas-phase dissociation of alkyl chlorides to yield carbocations. More highly substituted alkyl halides dissociate more easily than less highly substituted ones.

An example application of transition state theory is to simple gas-phase dissociations of a diatomic molecules such as CO... 

The gas phase photofragmentation of transition metal cluster complexes is discussed. The information available for the gas phase dissociation of... 

By inspection alone, we can guess that AY will be negative for bimolecular reactions, since two components associate to form one (the TS). The value of AY is positive for unimolecular processes, such as gas-phase dissociation. 

Table 24 Arrhenius parameters for the gas-phase dissociation of O-protonated 1-aryl-1-methoxyethanes...

O-protonated 1-aryl-l-methoxyethanes Arrhenius parameters for gas-phase dissociation, 260t... 

The cross sections for ESD processes on most surfaces are usually much smaller than cross sections for comparable gas phase processes involving electron-induced dissociation and dissociative ionization . This may be a consequence of the fact that many fragments remain adsorbed on the surface and/or that non-radiative processes such as those described in Sect. 2.1.1 cause the molecule to de-excite before it dissociates. For 100 eV electrons, typical cross sections for gas-phase dissociation are 10 cm (see Ref. 150). For most adsorbates, cross sections lie in range of 10 to 10 cm. A few examples of higher cross sections for adsorbed layers are known, and many examples of smaller cross sections exist. 

Adsorption from the gas-phase Desorption to the gas-phase Dissociation of molecules at the surface Reactions between adsorbed molecules Reactions between gas and adsorbed molecules. 

The proposed mechanism for this overall reaction involves a gas-phase dissociation step GeCl4(g) GeCl2(g)+Cl2(g) (3.108)... 

The surface decomposition process of AP propellants is net exothermic, with heat absorbed at the AP and fuel surfaces by endothermic pyrolysis but with more heat liberated in the gas phase close to the AP crystals. In detail, decomposition at the AP surface consists of endothermic pressure-independent solid-to-gas phase dissociative sublimation, followed by exothermic pressure-dependent gas-phase oxidation. The over-all reaction for the AP decomposition is pressure dependent. 

R+ X", becomes the ground configuration. What this means, of course, is that dissociation of RX in solution is expected to yield ions (due to the R- -X/R+ X avoided crossing) while gas-phase dissociation leads to radicals (since at all geometries the dominant configuration is R X). 

An interesting consequence of the R—X avoided crossing in solution is that its very existence in certain systems may actually depend on the magnitude of solvation. Since in the gas phase dissociation tends to be homolytic and in solution heterolytic, it is conceivable that in certain cases the solvent may determine whether homolysis or heterolysis takes place. 

Staley and coworkers6 tyave provided direct measurement of HSAB effects in gas-phase dissociation energies between transition metals (where the principle has... 

As a final attempt to understand the origin of the unexpectedly low slope, we split AG into four different parts, i.e., the quantum chemical gas-phase dissociation, the COSMO-interaction energies of the neutral and the ionic species and the chemical potential difference from COSMO-RS. On the basis of these reasonably independent descriptors, we performed a multilinear regression of pKa yielding an almost identical regression coefficient and rms error as before with slopes ranging from 0.55 to 0.62 for the four contributions. Thus all contributions—although very different in nature—show the same unphysical slope with respect to the pKa. 

Computational methods have also been used frequently to estimate the thermodynamic stabilities of superelectrophiles. These calculations have often involved the estimation of barriers to gas phase dissociation or deprotonation, and the proton affinities of conventional electrophilic intermediates. Other useful studies have calculated the heats of reactions for isodesmic processes. An interesting example of these calculations comes from a study of the protoacetyl dication (Cf COH2"1- ).42 In calculations at the 6-31G //4-31G level of theoiy, the protoacetyl dication (83) is estimated to react with methane by hydride abstraction with a very favorable... 

Of importance for understanding kinetics and oxidation mechanism of various substrates with the participation of hydrogen peroxide are the conditions of H202 dissociation into intermediates. As already mentioned in early studies on heterogeneous gas-phase dissociation of H202 [3], the reaction is rather sensitive to the type of reactor walls and depends on their preparative treatment, which makes obtaining reproductive data more difficult. 

The general conclusion from Eqs. (9)—(14) is that the molecular heat of chemisorption ( AB rapidly decreases as the gas-phase dissociation (total bond) energy DAB increases. The values of QAB are smaller than QA(QB), typically by a factor of 5-10 but sometimes even 15-20. For this reason, the periodic changes in QAB for molecules such as CO, NH3, NO, H20, C2H4, and C2H2 are expected to be small and potentially irregular, unlike the large and systematic variations in QA observed for the relevant multiply bonded adatoms A. 

The BOC-MP method provides reasonably accurate estimates of the heats of chemisorption Q and the dissociation and recombination barriers AE for various molecules and molecular fragments. Combined with the knowledge of the molecular total bond (gas-phase dissociation) energies, this allows one to construct potential energy profiles of surface reactions. 

---