RICHARD C. JARNAGIN
786
kcal./mole). However, reaction 2 is rather more complicated than (7), since it involves both carbonate replacement and H + transfer, precluding direct comparisons between them. E” = EO AH6 for both Nd and N6, making E6 30 kcal./ mole for both ammines, in acceptable agreement with Bronsted’s data for the corresponding reaction of the nitrato salt.25 For tn2, E’ lies between E2
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and E? AHl, depending on how small K1 is. Similarly, E” lies between Ea and Ea A€I; for tnz. A H l is not known but is probably quite small, so that E2 S E3 S 25 kcal./mole in this case. Support of this research by the U. S. Atomic Energy Commission through Contract AT(30-1)-15% is gratefully acknowledged. BUFFALO, N. Y .
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DEPARTMENT O F CHE>lISTRY, YALE
UNIVERSITY]
Investigation of the Catalytic Mechanism of Catalase and Other Ferric Compounds with Doubly O18-labeled Hydrogen Peroxide
c. JARNAGIN’
B Y RICHARD
AND JUI
€1. 1v.YSNG
RECEIVED AUGUST 19, 1957 A method for determining the origin of molecular oxygen in the catalytic decomposition of hydrogen peroxide is developed and applied t o the catalytic decomposition of hydrogen peroxide by catalase, polyamine-Fe(II1) chelates and ferric hydroxide. I n all cases studied, the oxygen molecule originates from the intact 0-0 bond in hydrogen peroxide.
One method of approach to the study of catalysis is via the synthesis and investigation of relatively simple model molecules of known structure. By this means Wang2 discovered the remarkably high catalytic efficiency of triethylenetetramine-Fe(II1) chelate for the decomposition of hydrogen peroxide. The postulated mechanism of catalysis involves the metathetical displacement of a bound OH- by OOH-, followed by the displacement of another bound OH- by the second oxygen atom of the same OOH- as illustrated below.
riched water vapor. If the isotopic atom-fraction of OIs in the resulting HzOz is X , the mole-fraction of HOl80IxHshould be approximately equal to X 2 . (Here it is assumed that the isotope-effect due to the difference in zero point energies of light and heavy 0-atoms in I - 1 2 0 2 is negligible.) When this HzOz is catalytically decomposed, the mole-fraction of 018018 in the oxygen gaq liberated should also be X 2 no matter whether the decomposition takes place through mechanism (A) or mechanism (B), because the isotopic distribution in this HrO2 is in accordance with natural probabilities. However, when \Le/OH I 013 + + each mole of this 0IX-enrichedhydrogen peroxide is +OHmixed with q moles of ordinary hydrogen peroxide, I I1 the isotopic atom-fraction of OI8 and the tnoleSince the 0-0bond in compound I1 is under strain, fraction of H01a018Hin the mixed H202 become 4) and [X’ q(0.0020)2]/ it becomes more reactive and reacts subsequently [X q ( @ . O O Z O ) ] / ( l q ) , respectively, where 0.0020 is the isotopic with a second OOH- to produce 0 2 and regenerate (1 atom-fraction of Olx in ordinary HzO?. This last the catalyst. There are two alternative mechanisms for this isotopic distribution is unnatural, for had this last reaction step to take place. These are depicted H202 been made directly from enriched water vapor with atom-fraction of Olx equal to [X below as mechanisms ( 4 ) and (B), respectively. q(0.0020)]/(1 q ) , then the mole-fraction of H018018Hin the resulting Hz02 would be [X q(0.0020)]2/(1 a)?. Now if this 018-enriched H202 with unnatural isotopic distribution is catalytically decomposed by, say, triethylenetetramine-Fe(III), the mole-fraction of 018018 in the liberated 0 2 will be different for mechanisms (A) It is of particular interest to find out whether the and (B), respectively. hlechanism (B) involves a catalytic decomposition of hydrogen peroxide by reshuffling of the 0-atoms, ;.e., restoration to the triethylenetetramine-Fe(II1) chelate takes place natural distribution of the isotopic species, and hence predicts a mole-fraction of [X q(0.0020)]2/ through mechanism (A) or (B). 4)’ for O’eO1bin the liberated 0 2 gas. But a In general even for cases in which the detailed (1 mechanism is unknown, it is still of interest to know mechanism such as (-A), which produces all the 0 2 2 liberated came from the intact 0-0bonds, leaves the original isowhether the two 0-atoms in each 0 from the same HzO2 molecule or from two separate topic distribution undisturbed, and hence predicts the mole-fraction of 0 * 8 0 1 8 to be [X? q(O.OOZO)?]/ H202 molecules. (1 4 ) . Principle of the Method If X = 0.10 and q = 1.0, we have [ X 2 q Suppose O18-enrichedhydrogen peroxide is made (0.0020)2]/(1 q ) = 0.0050 and [X q(0.0020)]2/ directly by an electric discharge through OlX-en- (1 q ) ? = 0.00%. Consequently a mass-spectrometric analysis of the 0 2 produced by the cata(1) National Science Foundation Predoctoral Fellow, 1947-1958. lytic decomposition of hydrogen peroxide could (2) J. H. Wang, TaIs J O U R N A L , 7 7 , 4715 (1955).
[/: \OOH]+
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