Reaction rate optical factor

For nonabsorbing substrates, a factor can be defined that takes into account the effect of all the optical processes in the reaction rate. This factor has been called reaction rate optical factor (RROF) (Arancibia-Bulnes and Cuevas, 2004)... 

Figure 8 Reaction rate optical factor as a function of catalyst concentration for a parabolic trough solar photocatalytic reactor. Adapted from Arancibia-Bulnes and Cuevas 2004, with permission from Elsevier.

In an attempt to discern the factor(s) most responsible for ordered solvent induced alterations of reaction rates and specificities, we have investigated the influence of cholesteric liquid-crystalline and other optically active media upon the induction or loss of optical activity in the atropisomers of 1,1 -blnaphthyl (BN, equation 1). We find that optical induction is negligible from thermal (ground-state) lsomerizations (usually <0.1%) but is larger for excited-state lsomerizations conducted in cholesteric mesophases (up to 1.1%). The factors responsible appear to be the geometry and polarizability of the 15N triplet state and rather specific solvent-solute interactions in ordered... 

The quantum yield of conversion of 4-chlorophenol was not measured with high accuracy ( 0.4 0.1 ). At low conversion rate (<0.1), the relative error on the conversion of 4-chlorophenol determined by HPLC measurement is about 25%. Moreover, the optical density at 254 nm increeised much with the progress of the reaction ( about a factor of 5 for 15% of transformation ). At conversion rates higher than 0.1, secondary reactions are not negligible trtien the solution was Irradiated at 296 nm. Nevertheless, the initial quantum yield of formation of Cl ( 0.25 0.05 ) appeared to be significantly lower than the quantum yield of conversion of 4-chlorophenol (37). 

Monolayers. The response timescale factor involves how rapidly the sample species penetrates the indicator phase volume as well as the chemical reaction rate of the optical-response-producing reaction. Clearly the indicator phase volume should be small to offer any chance of a reasonably fast response a monomolecular layer is the limiting case in this regard. Because of sensitivity considerations, planar waveguides would appear to be the most suitable geometry for use with reagent monolayers a sensitivity enhancement for absorbances by monolayers of 10 has been reported (54). 

Experimental verification of these effects can be found in one study of the substitution of optically active 2-methyl-2-butene (mbn ) in the confound PtCl2(mbn )(4-X-pyridine) by non-optically active 2-methyl-2-butene (mbn), els-l.2-dlchloroethvlene (dee), and 2,3-dimethyl-2-butene (dmb) (23). For a given amine ligand, the rate of reaction was in the order mbn > dee > dmb, with the reaction rates decreasing by a factor of nearly 200 in going from... 

Using optical absorption techniques, Phelps and Molnar also verified (51) apd were able to deduce from their data a specific reaction rate at 300°K of 2 X 10 cm sec A theoretical calculation of this specific reaction rate (or cross section) has been carried out by Von Roos, but his result is smaller than the experimental value by a factor of 10". ... 

In case of optically active alkyl halides, partial racemisadon takes place. Reaction rate is determined mainly by electronic factors. 

In the simplest case, where (+)-AH and (-)AD are isotopically pure, a = [a]H[AH]0 and a2 = [a]D[AD]0 where a is the specific rotation of the AH and AD isotopomers, respectively, and [AH]0 and [AD]0 are the concentrations of the substrates in g ml-1 at time t = 0. When the substrate is neither isotopically nor enantiomerically pure, corrections must be made in calculating fli and a2 (Bergson et al., 1977). It is important to note that the pre-exponential factors, a and a2, which contain the information about the starting conditions, can be determined with high accuracy. The extreme, ae (the maximum or minimum value of the optical rotation in the optical rotation versus time plot) and the corresponding reaction time, te, are functions of the rate constant ratio (5 = kHlkD) (65) and the difference between the rate constants (66), respectively. 

A recently proposed semiclassical model, in which an electronic transmission coefficient and a nuclear tunneling factor are introduced as corrections to the classical activated-complex expression, is described. The nuclear tunneling corrections are shown to be important only at low temperatures or when the electron transfer is very exothermic. By contrast, corrections for nonadiabaticity may be significant for most outer-sphere reactions of metal complexes. The rate constants for the Fe(H20)6 +-Fe(H20)6 +> Ru(NH3)62+-Ru(NH3)63+ and Ru(bpy)32+-Ru(bpy)33+ electron exchange reactions predicted by the semiclassical model are in very good agreement with the observed values. The implications of the model for optically-induced electron transfer in mixed-valence systems are noted. 

---