Transition state, for decomposition

A model of the potential surface is needed to understand why an SN2 reaction is slow. The double minimum surface shown in Figure 1 can accommodate the experimental results for this system (1-7). Although the central barrier is at a lower energy than that of the reactants, the reaction proceeds slowly because the transition state associated with the central barrier is tight and the sum of states associated with it is smaller than that associated with the loose transition state for decomposition back to reactants. The rate constant for the reaction is given by the rate constant for formation of the complex multiplied by the fraction of complexes that go on to products. This branching fraction is the ratio of the forward step over the sum of the forward and back steps and can be related to the efficiency, which is the reaction rate divided by the collision rate. 

Molecule 5 was found to decompose with a barrier of 40.9 kJ mol and the loss of 237 kJ mol to CO2 and ONNCO as we saw above, this latter was calculated to decompose readily to CO2 and N2 with a barrier of 75 kJmol in a reaction exothermic by 467kJmol (overall exothermicity = 237 -1- 467kJmol = 704kJmol ). Interestingly, a transition state for decomposition to CO2 and N2 could not be found. 

Molecule 10, because of its coimectivity, seems unlikely to decompose to CO2 and N2. We found a transition state for decomposition to CO2 + NO2 + OCNCO, which corresponds to the coimectivity. This reflects a barrier of 36.3 kJ mor and a decomposition energy of 56.5 kJ mol. The radical OCNCO is apparently unknown, although the cation is known [39, 40, 41]. 

Molecule 11 was found to lose CO2 with a barrier of 83.1 kJmol the other fragment(s) we suspect to be 2N2, although an N4 species could not be ruled out (an IRC calculation to throw light on this failed). In ai r case an N4 species is expected to decompose to 2N2 with a very low barrier (Chapter 10). The decomposition energy to CO2 + 2N2 is 887 kJ mol . We could not find a transition state for decomposition to CO + N2O + N2. 

Figure 2. Representation of the transition state for decomposition of diazene N-oxide 7...

Structures 9 and 10 might correspond to transition states for the decomposition of the labile chemical intermediates supposedly involved in the reaction (see also Section V). The use of benzene seems... 

The characteristics of the HERON transition states for both steps in the thermal decomposition of Ai,Ai -diacyl-Ai,A -diaIkoxyhydrazines facilitate the synthesis of esters. Notably, the alkoxyl group migrations in the internal 8 2 displacement of the 1,1-diazene in the first step, and nitrogen in the second step, do not involve a tetrahedral alkoxide intermediate and both Barton and coworkers and Glover and have utilized... 

Analogous methyl azidoformate forms with norbornene a thermal unstable triazoline.251 The decomposition products are 40% aziridine and 55% imide. Furthermore it has been observed that the rate of nitrogen evolution of the triazoline from methyl azidoformate increases threefold when triglyme and 20-fold when dimethyl sulfoxide are substituted for 1,1-diphenylethane as solvents. This fact supports a betaine intermediate in the thermal decomposition reaction. The triazoline from 2,4-dinitrophenyl azide and norbornene could just be isolated, but from picryl azide only the aziridine was obtained.252-254 Nevertheless, the high negative value of the activation entropy (—33.4 eu) indicates a similar cyclic transition state for both reactions. 

Consider the potential energy profile in Figure 12.17a for the iodide ion-catalyzed decomposition of H2O2. What point on the profile represents the potential energy of the transition state for the first step in the reaction What point represents the potential energy of the transition state for the second step What point represents the potential energy of the intermediate products H20(/) + IO aq) ... 

Transition states for rate-limiting elimination of nitrogen on unimolecular thermal decomposition of methyl and ethyl azide have been defined by application of Pulay s SQMFF method.68... 

Ab initio molecular orbital calculations have been carried out by Ignacio and Schlegel on the thermal decomposition of disilane and the fluorinated disilanes Si2H F6 17. Both 1,1-elimination of H2 or HF and silylene extrusion by migration of H and F atoms concerted with Si—Si bond cleavage were considered. The transition states for the extrusion reactions all involved movement of the migrating atom toward the empty p-orbital of the extruded silylene in the insertion which is the retro-extrusion (equation 5). 

The solvent has a strong influence on the rate of decomposition of the arylpentazoles (Table 9). Solvents with lower polarity increase the rate of decomposition. This suggests that the transition state for the decomposition is less polar than the ground state of the pentazole, which was confirmed by quantum-chemical calculations <2003CEJ5511>. The decomposition of phenylpentazoles is not influenced by acids or bases <1957CB2914>. The rate of nitrogen evolution is also independent on the concentration of azide ions at —40 °C. 

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