1264 Table I.
Activation Energies of Reactions of Hydrated Electrons ~~
+
Reactant (X)
~
Relative to eaqRelative to eaqNOS- (20") PBP (20") A ( W N O-,~ ~WPB A(AE),,, P , AEeaq-+x, AS*,b k,,, - +x, kcal/ k,,, - +x, kcal/ kcal/ kcal/ cal mole-' M-1 sec-I mole A4-lsec-l mole mole mole deg-I
pH
+
3 4.0 X 10'0 NO?5.5-6 , . [Co(NHa);H ?OF3 5.5-6 5 . 8 X 10'' NOa5.5-6 .. 2-Aminopyrimidine 5.54 1 . 4 x 10'0 p-Bromophenol 5.5-6 1 . 2 X 10" Cyclohexanone 5.5-6 8 X lo9 Phthalate ion 7 4.5 x 109 Benzoate ion 7 3.6 x 109 Pyridine 5.5-6 3 X 10' Benzenesulfonate ion 7 1.15 X 10' Chloroacetate ion 7 1.1 x 109 Benzyl alcohol 5.5-6 1.9X108 Phenylalanine 7 1 . 6 X 10s Acetamide 5.5-6 4 X lo7 Formamide 5.5-6 3 . 8 x lo7 Phenyl acetate ion 7 3 . 1 x 10' Urea 5.5-6 2 . 7 X 10' A(AE)PBP = AEx A(L!?)h.OJ- = A E x - A E x 0 , -
+0.4
2 . 5 X 1Olo 3.4 x 109 4.6X10'' 1.1X10'' 1 . 3 x 10'0
..
,
0.0
... -0.5 +0.4 -0.1 0.0 -0.4 -0.5 -0.6 -0.4 -0.1 -0.4 -0.2 +0.2 +0.3 0.0 -
..
7 . 8 X loB 4.6X1O9 2 . 7 x 109 4.6 X l o 9 8 X 108
0.0 0.0 +0.5 -0.5 -0.2
... -0.4
+0.5 0.0 -0.5 +0.4 ...
, .
1.8X108 1 . 3 5 X 10' 3 . 0 x 107 .. 3 . 3 x 107 , .
-0.5
+0.3 0.0 , . .
-0.4 ...
+0.2 0.0 +0.2 -0.5 -0.3 f0.4 -0.2 +0.2 -0.2 -0.5 -0.1 -0.4 -0.3 0.0 -0.1 $0.2 0.0 0.0
3,2a 3.4 3.2 3.9 3.7 3.0 3.6 3.2 3.6 3.9 3.5 3.8 3.7 3.4 3.5 3.2 3.4 3.4
-2.2 -6.2 -0.7 -3.85 -3.4 -3.7 -4.5 -5.7 -6.3 -6.0 -8.6 -8.5 -12.0 -12.3 -15.1 -15.25 -15.6 -25.1
Specific rate constant (lit.) 2.36 X 10'0~ 3.5-8.1 x 109,d 6.1-7.6XlO"e 8.2-11xlOgf .. .,..
.. , 6.2~1090 3 . 1 x 1090 1-3.7 x 109" 4 x 109' 1.2-3.8 x 1091 1.3X108' 8.8-15 X 10" 1.7 x 107' 4.2XlO" 1.4-5.1 x I O 7 0 3 x 105 1
AEPBP
We use the value of Thomas, et as that of Baxendale, et a/.,Swas derived from measurements at two temperatures only. * Calculated = 3.5 kcal/mole. S. Gordon, E. J. Hart, M. S. Matheson, J. Rabani, and J. K. Thomas, J . Am. Chem. SOC.,85, 1379 (1963). d B. Cercek,private communication, to be published,and ref 5 . E M. Anbar, E. M. Fielden, and E. J. Hart, unpublished, and ref 5. f B. Cercek, private communication, to be published, and ref 3. 0 A. Szutka, J. K. Thomas, S. Gordon, and E. J. Hart, J . Phys. Chem.. 69, 289 (1965). h E. J. Hart, S. Gordon, and J. K. Thomas, J . Phys. Chem., 68,1271 (1964), and B. Cercek, private communication, to be published. * M . Anbar and E. J. Hart, J . Am. Chem. SOC.,86,5633 (1964). i D. M. Brown, F. S. Dainton, J. P. Keene, and D. C . Walker, Proc. Cliem. SOC.,266 (1964); M. Anbar and E. J. Hart, J . Phys. Chem., 69,271 (1965). R . Braams, Radiation Res., 27, 319 (1966). I M. Anbar, E. M . Fielden, and E. J. Hart, unpublished a
for LIE
examples for such cases. The higher values of A E reported for eaqM n f 2 and eaq- C O +reactions ~ in neutral solution are probably due to preequjlibria between aquo complexes at different degrees of hydrolysis. It has been shown1° that the degree of hydrolysis has a significant effect on the rate of reaction of eaq- with transition metal ions. It remains to be demonstrated that these ions have a lower A E in acid solution. Another type of eaq- reaction which might have AE > 3.5 kcal/mole are those which proceed by an atom-transfer mechanism," eaqX + OHHX. H 2 0 is the most likely process to take place by eaqthis mechanism. It has been suggested that eaq reactions involve the incorporation of an electron into the orbitals of the substrate;' thus their rate depends primarily on the electron distribution of the latter. This distribution, which might be changed by electron excitation, is not expected to be affected by temperature up to 100". What should therefore determine the rate of eaq- reactions is the probability of finding an electron vacancy on the substrate molecule; this probability which is represented by the entropy of activation is temperature independent in our range of temperatures. Our findX + X- reactions is equal ings that AE* for all eaqto the energy of activation of diffusion in water corroborated these conclusions. Slow eaqX + X- reactions take place with polyatomic reactants only. These reactions involve a large number of collisions with substrate molecules having an unfavorable electronic configuration. An interaction of eaq- with a reactant molecule in a favorable electronic configuration results in the formation of an activated complex. Once an activated complex
+
+
+
+
M. Anbar The Weizmuiiii Itisrirure o/ Scieiice Rehocotli, Ibruel
+
+
+
(10) M. Anbar and E. J. Hart, J . P h j s . Chem., 69,973 (1965) ( 1 1) R. A. Marcus, see ref 1, p 138.
Journal of the American Chemical Society
has been formed the electron transfer in it is expected to occur within
