Dissociation reaction paths

The earliest claims centered on complexes of the types [PdX(Et4-dien)F+ and [PtX(dien)]. The anomalous reactivity of certain nucleophiles, including hydroxide, and the observation of a two-term rate law for anation reactions (X = H2O) led to the postulate that when associative ligand replacement was very slow, then a dissociative reaction path could be seen to compete. Because of this, these sterically hindered molecules were often termed pseudooctahedral. This early evidence has been reviewed (6, 12), and it is now known that these apparent anomalies can be interpreted instead in terms of the usual associative pathways. For example, the original detection of a two-term rate law for the anation of [Pt(OH2)(dien)] is probably a consequence of examining the reaction as it approached equilibrium (142). 

Figure 3.28. Most favorable dissociation reaction paths for CO on Rh stepped...

As schematically shown in Fig. 18, a small barrier observed for the dN = 0 (polarizational) internal dissociation reaction path, can easily be avoided on real (110)-rutile surface during a small dN > 0 fluctuation in the cluster number of electrons. Such an electron population displacement may result from the influence of the cluster environment (the surface remainder or the catalyst support) thus facilitating the energetically more favourable 1 - 4 - 3 path of the water dissociation. 

Structure promoters can act in various ways. In the aromatization of alkanes on Pt catalysts, nonselective dissociative reaction paths that lead to gas and coke formation can be suppressed by alloying with tin. This is attributed to the ensemble effect, which is also responsible for the action of alkali and alkaline earth metal hydroxides on Rh catalysts in the synthesis of methanol from CO/H2 and the hydroformylation of ethylene. It was found that by means of the ensemble effect the promoters block active sites and thus suppress the dissociation of CO. Both reactions require small surface ensembles. As a result, methanol production and insertion of CO into the al-kene are both positively influenced. 

The reaction path shows how Xe and Clj react with electrons initially to form Xe cations. These react with Clj or Cl- to give electronically excited-state molecules XeCl, which emit light to return to ground-state XeCI. The latter are not stable and immediately dissociate to give xenon and chlorine. In such gas lasers, translational motion of the excited-state XeCl gives rise to some Doppler shifting in the laser light, so the emission line is not as sharp as it is in solid-state lasers. 

Following a similar procedure, we locate and verify the transition structure connecting cis hydroxycarbene and the two dissociated species. Here is the transition structure and the two structures at the end of the reaction path computed by the IRC calculation ... 

The influence of the presence of sulfur adatoms on the adsorption and decomposition of methanol and other alcohols on metal surfaces is in general twofold. It involves reduction of the adsorption rate and the adsorptive capacity of the surface as well as significant modification of the decomposition reaction path. For example, on Ni(100) methanol is adsorbed dissociatively at temperatures as low as -100K and decomposes to CO and hydrogen at temperatures higher than 300 K. As shown in Fig. 2.38 preadsorption of sulfur on Ni(100) inhibits the complete decomposition of adsorbed methanol and favors the production of HCHO in a narrow range of sulfur coverage (between 0.2 and 0.5). 

Figure 12 (from the chapter Exploring Multiple Reaction Paths to a Single Product Channel ). Two-dimensional cut through the potential surface for fragmentation of the transition state [OH CH3 ] complex as a function of the CF bond length and the FCO angle. All other coordinates are optimized at each point of this PES. Pathway 1 is the direct dissociation, while pathway 2 leads to the hydrogen-bonded [CH3OH F ] structure. The letter symbols correspond to configurations shown in Fig. 11. Reprinted from [63] with permission from the American Association for the Advancement of Science.

The influence of the Ni atoms becomes clear from a comparison of the actual reaction path, which consists of physical adsorption and subsequent dissociative chemisorption, with the theoretical alternative reaction path, consisting of dissociation of H2 followed by the formation of two Ni-H bonds. H2 is a very stable molecule and, as a consequence, the potential energy of the dissociated H-atoms is very high. In moving to the adsorbed state, Ni-... 

Because the cis-decalin molecule extends its two methine carbon-hydrogen bonds on the same side in contrast to frans-decalin, the carbon-hydrogen bond dissociation of adsorbed decalin would be advantageous to the cis-isomer on the catalyst surface (Figure 13.17). A possible reaction path by octalin to naphthalene in dehydrogeno-aromatization of decalin will be favored to the cis-isomer, since its alkyl intermediate provides the second hydrogen atom from the methine group to the surface active site easily. 

The half-order of the rate with respect to [02] and the two-term rate law were taken as evidence for a chain mechanism which involves one-electron transfer steps and proceeds via two different reaction paths. The formation of the dimer f(RS)2Cu(p-O2)Cu(RS)2] complex in the initiation phase is the core of the model, as asymmetric dissociation of this species produces two chain carriers. Earlier literature results were contested by rejecting the feasibility of a free-radical mechanism which would imply a redox shuttle between Cu(II) and Cu(I). It was assumed that the substrate remains bonded to the metal center throughout the whole process and the free thiyl radical, RS, does not form during the reaction. It was argued that if free RS radicals formed they would certainly be involved in an almost diffusion-controlled reaction with dioxygen, and the intermediate peroxo species would open alternative reaction paths to generate products other than cystine. This would clearly contradict the noted high selectivity of the autoxidation reaction. 

Kim, H. J. and Hynes, J. T. A theoretical model for SNI ionic dissociation in solution. 2. Nonequilibriurn solvation reaction path and reaction rate, JAm.Chem.Soc., 114(1992), 10528-10537... 

The first step of the reaction path in TPR on Mo(l 12)-(1 x2)-0 without extra oxygen is the dissociation of methanol to form CH30(a) + H(a) and the recombinativc desorption of the adsorbed hydrogen, which occur above 300 K and at 380 K, respectively. 

Example More extensive substitution at the oxirane system brings additional dissociation pathways for the molecular ions. Nevertheless, one of the main reaction paths of molecular ions of glycidols gives rise to enol radical ions by loss of a aldehyde (R = H) or ketone molecule. [218] The reaction mechanism can be rationalized by the assumption of a distonic intermediate (Scheme 6.78) ... 

Nucleophilic substitution on methyl / -nitrobenzenesulfonate in CH2CI2 has been studied with a series of chloride salts with different structures and solvations BU4NCI, PPNCl [bis(triphenylphosphoranylidene)ammonium chloride], KCl complexed by 18-crown-6 or Kryptofix 2,2,2, and for comparison PPNBr. ° Rate constants and activation parameters are in accordance with an S 2 mechanism. The results were treated by the Acree equation. There are two reaction paths the first, involving the chloride ion, has the same rate for all the salts, whereas the second slower path, involving the ion pair, has a rate related to the dissociation constant of the salt. 

---