E,R,KANTROWITZ, M, Z.HOFFMAN, AND J, F. ENPICOPT
1914
(of course the +ISC might be low) and, again, C H 3 6 (0H)CHv radicals are formed.
Conclusions These results can best be interpreted on the basis of hydrogen-atom abstraction from the amides by triplet excited acetone. It is interesting to note that the site of abstraction by triplet excited acetone, as observed by flash photolysis (this work) and by esr spectroscopy,g is the same as that which has been found7 for OH radicals produced under pulse radiolysis conditions. Hydroxyl radicals are known to be electrophilic in their
reaction with compounds. Furthermore, it would appear that, qualitatively, the rate of abstraction from different amides by triplet acetone follows the same order as the rate’ of abstraction by OH radicals. Should these relationships be found to hold with other sensitizers and H-atom donors, it would be possible to predict the site of abstraction by triplet excited molecules based on the wealth of information available for OH radicals. Acknowledgment. Partial financial support received from the U. S. Army Research Office-Durham is gratefully acknowledged.
Ultraviolet Photochemistry of Acetatopentaamminecobalt(II1) in Aqueous Solution1 by Evan R. Kantrowitz, Morton Z. Hoffman,* Department of Chemistry, Boston Uniuersity, Boston, Massachusetts 09216
and John F. Endicott Department
of
Chemistry, Wayne State University, Detroit, Michkan 48909 (Received February 1 , 1071)
P u b l i c a t h costs borne completely by The Journal of Physical Chemistry
The 254-nm photolysis of Co(NH&O&CHa2* in aqueous solution generates Go2+, COZ, CZH~, and CHa with Qcoi+N 4coz S!( 2 6 ~ $cub) ~ ~ =~ 0.19. The stoichiometry and absence of any photoaquation products indicate that irradiation of the charge-transfer band of the complex results in electron transfer from the acetato ligand to the metal center, Isotopic substitution, radical scavenging, and light intensity studies demonstrate
+
that CHa radicals are released into solution. At 366 nm, on the other hand, irradiation of the ligand-field band with slight charge-transfer overlap yields (NH3)&o(OH2)OzCCHa2+with very small amounts of Go2+, There is no evidence of primary acetate aquation or the oxidation of an ammonia ligand. Introduction those conditions a transient species was detected that could not be identified as Cls- but rather as NHZC1- or Irradiation of pentaammine complexes of Co(II1) NH8C1.4 Product yields and scavenger studies are con[of the general form C O ( N H ~ ) ~ Xin~their + ] intense ligsistent with the formation of the Na radical from Coand-to-metal charge-transfer absorption band results (NHs)sNs2+.6Kinetic and flash results show that a in a number of possible processes, the most predominant radical is implied in the photochemistry of oxalatoof which is photoreduction with the formation of Co2+ and an oxidized ligand free radical from the one-electron transfer. Only in a limited number of cases has (1) This work was supported by Grant GP 11213 from the National the radical been identified and characterized, either by Science Foundation. Aspects of it were presented at the Second Northeast Regional Meeting of the American Chemical Society, direct flash photolytic observation or through the variaProvidence, R. I., Oct 1970. tion of final products in the presence of radical scav(2) V. Balzani and V. Carassiti, “Photochemistry of Coordination engers.2 Penkett and Adamson3 observed Brz- and Compounds,” Academic Press, New York, N. Y.,1970. 1 2 - from the flash photolysis of C O ( P I ” ~ ) ~ B and ~ ~ + (3) S. A. Penkett and A. W. Adamson, J . Amer. Chem. SOC.,87, 2514 (1965). Co(NHa).J2 +, respectively, and concluded that Br and (4! G. Caspari, R. G. Hughes, J. F. Endicott, and M. 2. Hoffman, I atoms were generated in the primary process. On sbzd., 92, 6801 (1970). the other hand, the flash photolysis of C O ( K H ~ ) ~ C ~(5) ~ +J. F.Endicott, M. Z. Hoffman, and L. €3. Beres, J . Phys. Chem., gave no transient unless free C1- was present; under 74, 1021 (1970). The Journal of Physical Chemistry, Vol. 76, No. 19,1971
ULTRAVI~LET' PHOTOCHEMIBTRY OF ACETATOPENTAAMMINECOBALT(III) amine complexes.6 Because of the generally high values of the extinction coefficients of these complexes in the 250-nm region, Co(II1) complexes have potential value as a convenient source of selected radicals for their study in aqueous solution. This technique would be of particular interest if other methods of generation of the radicals involved vacuum ultraviolet photochemistry, radiation chemistry, or thermal reactions where complexities of the system have limited the study of the radicals in the pa&. Spectral evidence has indicated that ammonia aquation is an accompanying process in Co(NH3)sCI2+;' kinetic evidence has demonstrated the same effect upon irradiation of the charge-transfer band of Co(NH3)sNs2+.8 On the other hand, irradiation in the ligandfield (d-d) bands results in ligand substitution, generally aquation, with a small amount of internal oxidation-reduction depending upon the overlap of the charge-transfer and d-d bands. The heterolytic bond fission leading to aquation would be expected to occur a t the more weakly bonded ligand resulting in the formation of C O ( N H ~ ) ~ O Hand ~ * +release of the free ligand. I n this study the ultraviolet photochemistry of Co(NH3)502CCH32+ was examined for the identification and further reactions of the reaction intermediates and for the wavelength dependence of the quantum yields. I n a preliminary review, Adamsonlo reported that the 366-nm photolysis of this complex leads to the formation of Cos+ and the "free acid." More recently," the quantum yield for photoreduction in the 340-380-nm region was given as
