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The Journal of Physical Chemistry, Vol. 82, No. 5, 1978
Primary Processes Studied by Pulse Radiolysis of Liquid Ammonia. 1. Oxidizing Radical Scavenging and Identification of Ultraviolet Transient Absorption Spectrum J. Belloni," F. Bllllau, P. Cordier, J. A. Delalre, and M. 0. Delcourt Laboratoire de Physico-Chimie des Rayonnements, associ.5 au CNRS, Universit.5 de Paris-Sud, Centre d' Orsay, 9 1405 Orsay, France (Received July 27, 1977) Publication costs assisted by Laboratorie de Physic0 Chimie des Rayonnements, Orsay
The UV transient spectrum induced by pulse radiolysis of liquid ammonia from -50 to 0 "C has been examined. In addition to the UV absorption of solvated electrons, two other bands are observed at -50 "C, with maxima at 260 and 350 nm. They disappear when EtO- or earn-are added as scavengers, and the resulting transient spectra obtained in such systems indicate that the bands belong to NH2and NH radicals. Extinction coefficients of NH2 (260 nm) and NH (350 nm) are respectively 1.3 x lo3and 2.0 X lo4M-l cm-l. Cu2+ions scavenge reducing and oxidizing transients, to give Cu' and Cu3+ions, respectively, the spectra of which are given.
Introduction Several investigators1r2 have focused attention on different problems in the radiolysis of liquid ammonia: the yield of solvated electrons, the effect of scavengers on electron decay, and the spectra of transients. In ow previous paper1 the transient absorption spectrum obtained a t the end of a short pulse (3 ns) of electrons in liquid ammonia was reported. Apart from the IR-visible band of the solvated electrons, two UV bands peaking near 250 and 320 nm were recorded, but only plausible assignments were made. Moreover, it has been shown recently3i4that a UV absorption was associated with the IR-visible absorption of the ammoniated electron. The purpose of the present work was to identify these UV transients unambiguously by studying the products of the reaction with several scavengers, in particular with specific oxidizing radical scavengers such as stable ammoniated electrons. Experimental Section The radiation source was a 706 Febetron delivering a 3-11s electron pulse. The experimental dose, measured from the initial optical absorption of the hydrated electron, was about 5 X 1017 eV mL-l. As our experiments bear essentially on the UV range, care was taken to avoid stray light. A high resolution monochromator (Jobin Yvon HRS) and a photomultiplier tube with an improved UV sensitivity (RTC 56 UVP) were used. Moreover, an optical filter (Corning 7.54) was positioned at the entrance to the monochromator. Since our previous work in liquid ammonia,l a split-beam technique has been introduced which improves the accuracy of the optical detection systemV5In addition, the surface of the silica cells6 currently used is chemically inert to the solvent in contrast to the stainless steel cells previously used. The cell was connected to a vacuum line to which large flasks filled with purified gaseous ammonia at a pressure of 1 bar were attached. Condensation was achieved by cooling the cell with a stream of cold gaseous nitrogen. Overpressure was avoided by keeping the connection between the cell and the flasks open. Some of the cells, which have a narrow electron window (2 X 10 mm2,0.2 mm thick), were sealed off after filling and could withstand pressures up to 10 bar. As a test for purity, the decay time of the solvated electron in the neat medium was measured before each run and, if it was shorter than 3 or 4 ps, the solvent was purified again. The solute, previously put into a side arm, 0022-3654/78/2082-0532$01 .OO/O
was dissolved only after a positive purity test. Cu(C104)2.6H20was from Koch-Light Laboratories. Potassium ethoxide was prepared in vacuo by condensing previously distilled ethanol over a potassium mirror and the excess ethanol was evaporated by pumping. Sodium metal was prepared in the side arm of the cell by the decomposition of sodium azide, NaN3, in vacuo; just before pulsing its concentration in ammonia was measured by optical absorption at 633 nm through the irradiation cell, by using a He-Ne laser beam.
Results 1. Neat Ammonia. The transient spectrum in the pure solvent1 has been reinvestigated under improved experimental conditions: silica cell, double detection, and variable temperature (see Experimental Section). The spectra at -50 and 0 " C are given in Figure 1. At -50 "C, the curve shows two transient absorption bands X and Y as we observedl previously: X peaks at 260 nm, and Y peaks at 350 nm. At 0 "C, the maximum at 350 nm disappears, but band X remains, somewhat more intense than at -50 "C, but with no maximum at X >250 nm. In Figure 1,the UV absorption of the ammoniated electron is reproduced as obtained by Billiau et al.,3 after normalization at 700 nm. After subtracting this absorbance, both bands remain with maxima unchanged at -50 "C. At 0 " C a UV band with a long tail up to 400 nm is obtained. When considering the kinetic behavior of species X and Y, we have to take the underlying absorption of the electron into account, as shown in Figure 2. By making this correction, the pure absorbance of the species is obtained, It can be concluded that species X decays along with the ammoniated electron, in agreement with the results of F a r h a t a ~ i z .After ~ the first 500 ns this decay is second order, with k 4 X 1O1O M-l s d at -50 "C. However species Y exhibits pseudo-first-order decay after 500 ns. These last two observations have already been mentioned in our first paper.l 2. Potassium Ethoxide Solutions. The study of ethoxide solutions offers a means of preventing em- reactions; indeed, it has been found that earn-is then very stable, when formed either by photolysis or by y radiolysis.8 We explored the transient spectra of such solutions in the IR-visible and also in the UV range down to 250 nm, and at different temperatures (-50 to 0 "C). In the IR range, a stable absorbance of earn-is observed. At the lowest concentration (a few millimolar) a slight decrease of earn-lasting for -200 ns is observed before its complete
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0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 5, 1978 533
Pulse Radiolysls of Liquid Ammonia
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Figure 1. End of pulse spectrum in liquid NHBat -50 and 0 O C . Dashed lines are the absorption spectrum of the ammoniated electron at the same temperature normalized for 2700 nm. Ordinates at 0 O C have been calculated on the basis of the same value of Gem-as at -50 O C and egoo nm = 4500 L mol-' cm-'.
Figure 3. Transient absorption spectra in a potassium ethoxide solution ( E 10-3 M) in liquid ammonia at -50 OC: (a) end-of-pulse spectrum; (b) 200 ns after the pulse; (c) 10 ps after the pulse; (d) earn-absorption normalized at 600 nm; (e) absorption of ethoxide ion consumed 2GNH)eEt& (f) difference (b - e). according to (G",
+
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Figure 4. Transient absorption spectra in a sodium-ammonia solution at -50 O C ([Na] = 3.2 X M): (0)end of pulse; (A)t = 200 ns; (0)t = 8 1s; (-) end-of-pulse spectrum of Figure 1. Other curves 8 ps, (----) t a. are calculated (see text): (- .) 200 ns; (.-e-) 4
Figure 2. Decay of optical density at -50 O C and various wavelengths: (dashed lines) after subtraction of underlying earn-decay taken identical with the decay at 800 nm.
nondecay. In the UV range, at the same lowest concentration, the end of pulse spectrum is the same as in the pure solvent (Figure 3, curve a). Then the spectrum rapidly changes into a new one (curve b) in a process which correlates with the slight decrease mentioned above. In saturated solutions (10-1 M) this new spectrum is completely developed during the pulse, without any decrease of em-. Thus, the ethoxide anion EtO- effectively scavenges the species responsible for the earn-decay in pure ammonia. As EtO- itself absorbs below 300 nm (curve e) and is involved in the scavenging reactions, curve b must obviously be corrected in order to obtain the absorbance of the transients. At the same time, bands X and Y are supposed to have vanished. After correction (see Discussion), the spectrum (curve f) shows a UV band with a shoulder at 340 nm which looks like that assigned to the basic form of the hydroxyethyl radical as observed in the pulse radiolysis of basic aqueous solutions of ethanolgJO or basic ethanol.11J2 This assignment is supported by the time evolution of the transient, monitored at 340 nm; its decay occurs by second-order kinetics lasting =lo p s , provided OD, is taken as that of the solvated electron (curve d). It is known8 that the hydroxyethyl radical in its basic form is poorly reactive toward esol; and usually dimerizes and/or disproportionates in other s01vents.l~
--
-+
3. Metal-Ammonia Solutions. The use of the metastable solvated electron as a strong reducing agent in the y radiolysis of liquid ammonia was emphasized early by Cleaver, Collinson, and Dainton.14 By pulse radiolysis, Dye et found, as in y radiolysis, a partial consumption of the scavenger, resulting in a slight bleaching of the IR absorption. The addition of a large amount of solvated electrons to those which are produced radiolytically is expected to enhance the reactions of this species without changing the reaction scheme. The behavior of the UV bands in these systems should provide arguments for their identification. Due to the absorption of these solutions containing earnand also NH2- arising from partial thermal decomposition, the measurements were limited to the range 280-700 nm. Figure 4 shows the evolution of the spectrum after the pulse, and Figure 5, the time dependence of the absorbance at different wavelengths. At the end of the pulse, the spectrum is changed relative to the pure solvent. During the first stage which lasts 600 ns in a 3.2 X M solution, there is a rapid decay of the IR band, leading to an increased transparency after 600 ns (G(-em-) = 0.45) and a correlative evolution of the UV spectrum which transforms into a spectrum with a peak absorption at 330 nm. This band looks like that of NH2(A, 330 nm a t -50 O P ) . Moreover, the absorbance a t ,A, corresponds to that calculated from = 3100 M-l cm-l ,17 and G(NH23 = Gem-+ G(-em-) = 3.45, this identity resulting from the conservation of negative charge. The concomitant decay of em- and increase of NH2- while the
534
Belloni et al.
The Journal of Physical Chemistry, Vol. 82, No. 5, 1978
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Figure 5. Optical density vs. time at different wavelengths, in a 3.2 X M sodium-ammonia solution at -50 O C ; (full lines) calculated
Figure 6. Transient spectra in a 3.3 X M Cu(CIO,), solution at -50 'C; ( 0 )end of pulse (curve a, from Figure 1); (0)t = 100 ns (curve b); (A) t = 1 ps (curve c).
X and Y bands disappear lead to the conclusion that earnreacts with NHz and NH and that these species are responsible for the UV bands. During this first stage ( t < 600 ns), the electron decay and the UV increase are pure pseudo-first-order reactions, which permits calculation of a second-order rate constantla h = (3.5 f 1)X 1O1O M-' s-l corresponding to reaction with both NHz and NH. This value is close to that obtained from the second-order decay of em- observed after 800 ns in pure ammonia. During t h e second stage ( t > 600 ns), the amide band decays slowly (Figure 5), with a half-life of 6 ps, tending toward an asymptotic limit which corresponds to the concentration of stable amide ions. The yield of N H y consumed in this last step is 3.0, which is equivalent to the NH4+ yield.lg This agrees with the balance of charge requirement that the increase of the stable NHz- concentration equals the total consumption of earn-or G(NHy)++ = G(-em-). AS a matter of fact, neutralization is the only possible reaction. The decay is pseudo-first order and, taking account of the global amide content (2 X M) of the partially decomposed solution, we can deduce &Hz- + NH4+ = (7 f 1) X IO8 M-l s-'. 4. Copper Perchlorate Solutions. As we mentioned previously,l divalent copper ions react rapidly with solvated electrons in NH3, according to the reactionz0 earn-t Cu2+4 Cu+ (1) We found hl = 1.5 X 10l1M-l s-l at -50 "C. It also seemed that the X and Y bands were but slightly affected by this scavenger and that in addition to the X and Y bands monovalent copper ions were absorbing at X
