Making and Breaking Bonds

The reduced sites donate electrons to the reactant oxygen molecule and to intermediates formed, this electron transfer being coupled with bond breaking and making involved in the ORR process. 

Alternative mechanisms consistent with the observations would require much higher energy transition states - transition states in which multiple bond breakings and makings occur simultaneously, or even transition states of impossible geometries. 

Figure 10. Network of bond-breaking and -making patterns explored by the reactivity functions leading to the correct prediction of product 3 from...

In our MD simulations [13,14] of HOD in D2O we found that the dynamics of an intact hydrogen bond is underdamped, that the relationship between the OH stretch frequency and the length of the hydrogen bond in which the OH is involved is characterized by considerable dispersion, and that the time scale for spectral diffusion on the order of a picosecond is related to hydrogen bond breaking and making. Related conclusions have been reached by Lawrence and Skinner [16], and more recently by Fecko et al. [6],... 

The A and A S 1 values are similar reaction is 1 —> 1 and so the entropy change is not associated with a change in the number of molecules. The activated complex resembles the product and so bond breaking and making is well developed in the activated complex. On isomerization there is release of the strain in the three-membered ring. 

Quaternary stereocentres in /3-keto esters have been deracemized in an enantio-convergent decarboxylative allylation process, catalysed by palladium(II).202 One catalyst is involved in both C-C bond-breaking and -making steps. 

To this point we have considered only equilibria, not rates. Proton-transfer reactions onto a bound hydroxide and off a bound water are rapid. For the slower reactions involving bond breaking and making to Ptn we consider two limiting situations. First, we assume that the Ptn complexes are in equilibrium with 4 mM ambient chloride of the cell nucleus so that the equi-... 

To devise analogues of such effects of conformational changes in chemical models is difficult. Almost invariably studies have been directed at elucidating the mechanism of bond-breaking and -making whilst ignoring possible conformational or strain effects. This is necessary in our present state of knowledge of enzymes and of solution chemistry, but for a complete description such effects must be taken into account. 

Reduction of [Cu( [ISJaneSe) affords the corresponding Cu(I) complex [Cu( [18]aneSe) ], the single-crystal X-ray structure of which confirms (Fig. 86) a distorted tetrahedral stereochemistry at the metal center, Cu-S = 2.245(2), 2.253(2), 2.358(2), 2.360(2) A 111). The two remaining S-donors are not bound to the Cud) center. The interconversion of Cu(II) and Cud) within the coordination sphere of [18]aneS6 occurs, therefore, via rapid Cu-S bond-breaking and -making between six- and four-coordinate complexes (111) related redox/stereochemical interconversion between [Cu( [15]aneSs) has been observed 76). 

From the variable temperature H-NMR spectrum Saunders found = 11.8 kcalmol" and A = 12.3 s for the degenerate rearrangement of bullvalene. The remarkably small probability and entropy barriers to the rearrangements that involve both carbon-carbon bond breaking and making were ascribed both to extremely favourable geometry and to weakness of the bond . These values were amended slightly... 

The AH for your transition will involve bond breaking and making as well as geometry change. Table 3.5 and crystal field theory can be used to estimate enthalpy changes associated with geometry changes. 

---