Hydrides potential barrier

Examine the transition state for the hydride shift. Calculate the barrier from the more stable initial carbocation. Is the process more facile than typical thermal rearrangements of neutral molecules (.05 to. 08 au or approximately 30-50 kcal/mol) Is the barrier so small (<.02 au or approximately 12 kcal/mol) that it would be impossible to stop the rearrangement even at very low temperature Where is the positive charge in the transition state Examine atomic charges and the electrostatic potential map to tell. Is the name hydride shift appropriate If not, propose a more appropriate name. 

The next step is the hydride transfer which occurs in the same way as before. The calculated barrier is only 3.0 kcal/mol. After the minor proton motion as described in Section III.A, there is again an electron and proton release. The calculated electron affinity is now 90.7 kcal/mol corresponding to a redox potential of —0.26 V. The proton affinity of the product is 289.9 kcal/mol. The entire catalytic cycle for the case with the protonated His77 is shown in Fig. 9. 

Since these hydrides are thermodynamically stable in the metal, the passive oxide can only be considered as a transport barrier, not as an absolute barrier. Various electrochemical techniques including EIS and photoelectrochemical measurements have been used to identify the mechanism by which the Ti02 may be rendered permeable to hydrogen, and to identify the conditions under which absorption is observable (31). These determinations show that H absorption into the Ti02 (and hence potentially into the metal) occurs under reducing conditions when redox transformations (Ti1 —> Tim) in the oxide commence. However, the key measurement, if H absorption is to be coupled to passive corrosion, is that of the absorption efficiency. 

Bulky H should not diffuse or show marked oscillational movement as indicated by magnetic resonance studies. The hydridic model actually provides a reasonable explanation for the mean amplitude of H vibrations (ca. 0.2 A.) and is noncommittal about diffusion. Conceivably, the barrier to diffusion comprising an Is2- configuration about the proton is in effect lowered by the distance of the barrier from the mean position of the nucleus. If the movement of hydrogen is quasitautomeric—for example, in keto-enol tautomerism—one may consider that it moves from one potential well to another as a proton. 

---