40) For convenience, the units for k values and for concentrations will be given In the text as cm3 sec-1 and cm-3 rather than cm3 molecule-1 sec-1 and molecule cm-3, respectively.
The flash photolysis of C O ( N H~)~C O~+ in aqueous solution generates the C03-radical which is characterized by its absorption spectrum with A, , , 600 nm and its second-order decay kinetics. The radical is also generated from Co(NH3)&03+ and, to a lesser extent, from Co(en)zC03+. By monitoring the decay of the C03-absorption as a function of the concentration of added scavenger solutes, values of k(CO3-+ S) have been obtained for some organic and inorganic solutes. In particular, k(C03-+ indole-3-propionic acid) was determined as a function of ionic strength and pH; at pH 7, k is independent of ionic strength while a t pH 12 there is a positive dependence of log k us. &*. From the data it is concluded that a t pH 7 the radical must exist in its acidic form, C03H, with a pK, = 9.6 f 0.3. The dependence of k on pH at constant ionic strength for N-acetyltryptophan and P,P'-dithiodipropionic acid supports the model presented as does the lack of a pH dependence for indole.(24) S. -N. Chen and M. Z. Hoffman, to be submitted for publication.
The rate constants for the reaction of C03-and CO3H radicals, generated in the flash photolysis of aqueous solutions of Co(NH3)4C03+, with tryptophan and its derivatives have been measured as a function of pH. At 0.1 M solution ionic strength, k for indole-3-propionic acid, IV-acetyltryptophan, and N-methyltryptophan follows a "titration" curve that represents the acid-base equilibrium of the radical (pKa = 9.6 ± 0.3) . At 0.003 M ionic strength, the rate constants are the same for the three compounds, independent of pH, and equal to that for indole (4.2 X 108 M~l sec-1) indicating that the mode of reaction is interaction of the radical with the aromatic system and that the intrinsic reactivity of the acidic and basic forms of the radical are the same. Amino-containing compounds show variation of the rate constant according to the pKa of the NH3+ group (tryptamine, tryptophanamide, tryptophan methyl ester, glycyltryptophan, glycylglycyltryptophan, tryptophylglycine, and tryptophan) with the rate constant being greater by almost a factor of 2 for NH3+ than for NH2. A small but significant enhancement of the rate is noted for amino compounds lacking the carboxylate group. A model is proposed wherein the reaction of the radical with the aromatic system is enhanced by interaction with the amino group which results in a transitory cooperative mechanism causing the radical to be in closer proximity to the reactive site.
The 254-nm irradiation of neutral, buffered, solutions of Co(NH3)4C03+ and Co(en)2C03+ produces Coaq2+ with a relatively low quantum yield (0.06 and 0.02, respectively); Co(NH3)6CQ3+ also is photoreduced. The irradiation of Co(NH3)4C03+ also generates Co(NH3)4(OH2)23+ with a quantum yield of 0.11 ± 0.03. The flash photolysis of the three complexes yields a transient intermediate which has 600 nm and which is identified as the C03~radical. The flash photolysis of the bidentate carbonato complexes also reveals a long-lived tail absorption in the 300-nm region which decays via pH-dependent first-order kinetics. The behavior of this intermediate is identical with that of the aquocarbonato species proposed as an intermediate in the acid-catalyzed aquation of the bidentate carbonato complexes. It is proposed that photochemical excitation of these complexes in the intense ligand-to-metal charge-transfer band produces the charge-transfer singlet state (1CT) which generates, via
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