Reduction of Ag I 1 (NH 3 ) 2 + to Ag 0 1 (NH 3 ) 2 in Solution. Redox Potential and Spectral Study Isabelle Texier, Samy Re ´ mita, Pierre Archirel, and Mehran Mostafavi* Laboratoire de Physico-Chimie des Rayonnements, CNRS URA 75, UniVersite ´ Paris-Sud, Centre d’Orsay, Ba ˆ t. 350, 91405 Orsay Cedex, France ReceiVed: December 1, 1995; In Final Form: February 26, 1996 X Pulse radiolysis is used to determine the absorption spectra of the transient species formed during the decay of the hydrated electron in an aqueous solution of Ag I 1 (NH 3 ) 2 + . The absorption spectrum attributed to Ag 0 1 (NH 3 ) 2 presents three peaks at 345, 385, and 435 nm. A theoretical estimation of the redox potential of the couple Ag I 1 (NH 3 ) 2 + /Ag 0 1 (NH 3 ) 2 yields the value -2.4 V NHE . This value is consistent with the fact that Ag I 1 (NH 3 ) 2 + is not directly reduced by (CH 3 ) 2 C ˙ O - and shows that the redox potential of the silver monomer couple is lowered by ammonia ligands. Introduction The study of the size-dependent ionization potential of metal aggregates in solution started in the 1970s. 1-4 Most of the experiments were carried out on short-lived silver cluster systems. 5-9 These few results showed the drastic influence of the nuclearity n on the ionization potential (IP) of the smallest clusters. The role of the hydration energy is so important, mostly at low n, that the general trend is an increase of the IP with n relative to vacuum. 10 In particular, the IP of metal atoms in solution is very low, and the reduction of isolated metal cations in solution as silver ions requires very strong reducing species. As the metal cations M + are often complexed by ligands (L), it is interesting to understand the influence of ligands on the redox potential of the monomer couple M I L/M 0 L which concerns the first step of the nucleation. The first estimation of such a redox potential has been carried out recently in the case of silver monomer couple complexed by two cyanides. 11 The redox potential of the Ag + /Ag 0 couple in aqueous solution (-1.74 V NHE ) is significantly decreased by the cyanide ligands down to -2.6 V NHE . 11 This estimation has also been confirmed by the experiment. 12 On the other hand, spectra composed of two or three bands have already been observed in the case of the transient Ag 0 complexed by ligands such as EDTA and cyanide. 13 The presence of additional absorption bands bears witness to the interaction between the silver atom and these ligands. In the present work, we study the effect of the ligand ammonia. First of all, we observe the product of the reaction of Ag I 1 (NH 3 ) 2 + with the hydrated electron by pulse radiolysis. Then we evaluate the redox potential of the couple Ag I 1 (NH 3 ) 2 + / Ag 0 1 (NH 3 ) 2 and eventually we experimentally check the reactiv- ity of 2-propanol radical as reducing agent toward Ag I 1 (NH 3 ) 2 + . Experimental Section All the reagents were pure chemicals: AgClO 4 from Aldrich, 2-propanol, ammonia, and acetone from Prolabo. The aqueous solutions of silver ions were prepared in the dark with an excess of NH 3 for complexing all silver ions at pH 11. The solutions were also degassed in vacuum. The irradiation source was a 137 Cs γ facility of 10 14 Bq with a dose rate of 1 kGy h -1 . Electron pulses (3 ns duration) were delivered by a Febetron 706 accelerator (600 keV electron energy) to samples contained in a quartz Suprasil cell with a thin entrance window (0.2 mm) for the beam and an optical path length of 1 cm. The cell was degassed by a nitrogen flow. The solution was changed after each pulse. OH • and H • radicals produced during radiation are scavenged by 2-propanol (0.2 mol L -1 ) to form (CH 3 ) 2 C ˙ OH radicals. Concentrations of hydrated electrons and alcohol radicals which form during pulse amount to 5 × 10 -5 and 6 × 10 -5 mol L -1 , respectively. In the presence of acetone and 2-propanol all radicals produced under irradiation are scavenged to form (CH 3 ) 2 C ˙ OH or (CH 3 ) 2 C ˙ O - depending on the pH. Absorption of transient species was analyzed by means of a classical xenon lamp, monochromator, and photomultiplier setup connected with a transient digitizer. Splitting of the beam makes it possible to record the signals simultaneously at two different wavelengths. 14 Results 1. Spectral Study. Pulse radiolysis of an aqueous solution of Ag I 1 (NH 3 ) 2 + is used to determine the absorption spectrum of the transient species formed during the decay of the hydrated electron. The absorption spectrum obtained 125 ns after the pulse (Figure 1a) presents three peaks at 345, 385, and 435 nm, the later being the least intense. Figure 2 shows the kinetic evolution of the optical density (OD) at these three wavelengths and at 600 nm where the hydrated electron absorbs. The absorbance at 345, 385, and 435 nm is due partly to the hydrated electron and partly to the product of the reduction of Ag I 1 (NH 3 ) 2 + by e - hyd . As the absorbance increases after the pulse, the extinction coefficient of the product is larger than that of the hydrated electron. The hydrated electron decays quickly in an aqueous solution of Ag I 1 (NH 3 ) 2 + , with a constant k(Ag I 1 (NH 3 ) 2 + + e - hyd ) ) 3.2 × 10 10 M -1 s -1 . 15 This decay is correlated with the increase of the optical densities at 345, 385, and 435 nm. The kinetic evolution is similar at these three wavelengths, and the maximum absorption is reached 125 ns after the pulse. Therefore, we ascribe these three absorption peaks to the same transient species formed during the decay of the hydrated electron. The spectrum shown in Figure 1a is quite different from that reported by Pukies et al. 15 which displays a very flat band in the range 250-400 nm and which has been attributed to the silver atom without ammonia, produced by the following reaction: X Abstract published in AdVance ACS Abstracts, June 15, 1996. Ag I 1 (NH 3 ) 2 + + e - hyd f Ag 0 + 2NH 3 (1) 12472 J. Phys. Chem. 1996, 100, 12472-12476 S0022-3654(95)03565-9 CCC: $12.00 © 1996 American Chemical Society