380 



E. LUMRY AND J. D. SPIKES 



constant percentage inhibition is the same in deuterium oxide as in 

 water, as shown in Fig. 6. Such a situation is incompatible with any 

 mechanism whereby water enters directly into the activated complex 

 for the limiting elementary reaction of kr>, since such a reaction would 

 manifest itself in a change of activation energy accompanied by a 



2.0 — 



0.5 



.0034 



.0036 



Fig. 6. Arrhenius plots of the dark reaction rate parameter ko as measured in 

 water and deuterium oxide over the temperature range indicated. Rates were 

 measured potentiometrically (2) at eight light intensities. The values for the ex- 

 perimental energies of activation as determined by the method of least squares 

 from the above data were: in water, 12.6 kcal.; in deuterium oxide, 12.5 kcal. 



negligible change in activation entropy. We must, in general, assume 

 that ki, consists of some combination of true rate constants for ele- 

 mentary steps multiplied by the concentrations of unknown "hidden" 

 reactants. The existence of an unchanged activation energy demon- 

 strates that water cannot be such a hidden reactant, and therefore it 

 is reasonable to conclude that water is not involved as such in any 



