J. Phys. Chem. 1991, 95, 897-901
897
Effect of Ion Pairing on Electron Scavenging and Solvated Electron Yield Enhancement in Methanol: A Comparison between Pulse Radiolysis and Positron Lifetime Spectroscopic Data G.Duplitret and C. D. Jonah* Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439, (Received: May 29, 1990; In Final Form: August 13, 1990) The influence of ion pairing on electron scavenging in nitrate solutions and the effect of amines on the enhancement of the electron yield, two particular processes previously observed in positron annihilation (PA) studies on positronium (Ps)formation in methanol, are examined by using picosecond pulse radiolysis (PR). These studies show that the scavenging of the precursors of the solvated electron (Q-) does not depend on ion pairing while confirming previous results that ion pairing strongly affects the reaction of e;, with LiN03 and NH4N03. The results from positronium annihilation studies agree with the precursor scavenging results for NO< and NH4N03;however, LiN03 is a weaker electron scavenger. No increase in the initial edyield is detected in PR in the presence of amines, whereas the Ps yield is significantly increased. These apparent differences can be explained if one consideres the importance of the trap depth for the electron, whether it has been captured by a solute (LiN03) or it has recombined with the positive ion (amines). In either case, in PR, the electron is lost for the free [,e, yield. In PA on the contrary, if both types of traps are shallow enough, the electron can still be picked off by the positron to form Ps.
Introduction In the past decade, several experiments have been aimed at establishing the basis of the spur model of positronium formation in The model essentially proposes that positronium (Ps) is formed by the reaction of a positron with one of the electrons released by ionization of the medium at the end of the positron track, in competition with other spur reactions, such as the electron-ion recombinations. On these grounds, correlations have been sought and were effectively found between the formation probability of Ps and information drawn from pulse radiolysis (PR) experiments on various properties of excess electrons, such as their mobility3 or their work function4 in nonpolar solvents, and the solvation time in polar solvent^.^-^ In water in particular, very strong support to the model was given by the correlation found between the ability of a solute to inhibit Ps formation, expressed by an inhibition constant k, (defined in eq 3), and its reaction rate constants for ed- scavenging2and, to a higher degree, between k, and the ability of the solutes to scavenge the precursors (sometimes referred to as the dry electron), ew-, of Q-,expressed by the 1 ICj7c o n ~ t a n t . ~C,, is defined as the concentration of a reactant that will decrease the initial yield of e=[ to I / e (37%) of its initial value. From the wealth of information derived in polar solvents, a provisional scheme for the elementary reactions in the positron spur was proposed.2 This pathway involves two pathways for Ps formation at very short times, either by the reaction of the quasi-free e+ and e- or by the reaction of the localized particles. It may be noted that, in the case of water, this proposal was made before the localized, infrared absorbing electrons were observed experimentally.s Chemical evidence for different channels for the formation of e-; has been found from pulse radiolysis experiments in alcohol^.^ Although the gross features of the processes occurring in the positron spur have thus far been shown to correlate well with what is known of the nascent electron spur, some differences are anticipated due to the specific nature of the positron, its privileged interaction with the electron, and the special spatial distribution that would be expected between the electron and the positron. At this level, it seems that more information could be derived to assess these differences by closely examining some particular processes observed in the fields of either positronium annihilation (PA) of PR studies. Several such processes can be found in the literature, of which two have been selected for the present studies: (i) The sensitivity of Ps formation probability to ion-pairing effects in 'Permanent address: Laboratoire de Chimie NuclCaire, Centre de Recherches Nucltaires, B.P. 20, 67037 Strasbourg, Ccdex, France.
0022-365419112095-0897$02.50/0
nitrate solutions in various polar, protic, and nonprotic solvents.I0 Although the NH4+ and Li+ cations have no effect on Ps formation, NH4N03appears to be a more efficient Ps inhibitor than LiN03, k , decreasing the increased concentration in the latter case. Correspondingly, strong ion-association effects on the reactivity of solvated electrons have been found previously in nitrate solutions in ethanol," but no report has appeared on this subject as regards epn-. (ii) The enhancement of Ps formation in the presence of efficient hole scavengers, essentially halide ions and amines,'*-I4 ascribed to the resulting increased availability of electrons which, otherwise, would have recombined with the holes? In PR, the possibility of increasing the e[, yield is poorly documented, with only a few indications such as for basic anions in methanolI5 and hydrazineI6 or for halides in methanolI7 and (less certain) water.I8J9 In this paper, results are reported on quasi-free and solvated electron scavenging by the nitrate ion in methanol, with Li+ or NH4+ as counterions and the ionic strength effect on this reaction. A comparison is made with Ps formation inhibition. By use of studies showing Ps enhancement by amines in methanol as a guide,I4 the e+-, yield in methanol was measured in the presence of triethylamine (TEA) and of N,N-dimethylaniline (DMA). ( I ) Mogensen. 0. E. J. Chem. Phys. 1974, 60, 998. (2) AbW, J. Ch.; Duplitre, G.; Maddock, A. G.; Talamoni, J.; Haessler, A. J. Inorg. Nucl. Chem. 1981, 43, 2603. (3) Jacobsen, F. M.; Mogensen, 0. E.; Trumpy, G. Chem. Phys. 1982,69, 71. (4) Jansen, P.; Mogensen, 0. E. Chem. Phys. 1977, 25, 75. ( 5 ) AbW, J. Ch.; Duplitre, G.; Maddock, A. G.; Haessler, A. Radia?. Phys. Chem. 1980, 15,617. (6) AbW, J. Ch.; Duplltre, G.; Maddock, A. G.; Haessler, A. J. Radioana/. Chem. 1980,55, 25. (7) Duplitre, G.; Jonah, C. D. Radia?. Phys. Chem. 1985, 24, 557. (8) Migus, A.; Gauduel, Y.;Martin, J. L.; Antonetti, A. Phys. Reo. Le??. 1987,58, 1559. (9) Lewis, M. A.; Jonah, C. D. J . Phys. Chem. 1986, 90,5367. (IO) AbM, J. Ch.; Duplltre, G.; Maddock, A. G.; Haessler, A. Radiochem. Radioanal. Let?.1979, 38, 303. (11) Hickel, 8. J . Phys. Chem. 1978,82, 1005. (12) AbW, J. Ch.; Duplitre. G.; Haessler, A. Radio?.Phys. Chem. 1986, 28. 19.
(13) Duplitre, G.; AbW, J. Ch.; Talamoni, J.; Maddock, A. G . Chem. Phys. Len. 1983, 100,553. (14) Talamoni, J.; AbM, J. Ch.; Duplitre, G. Radia?. Phys. Chem. 1984,
--.(IS) Johnson, D. W.;Salmon, G. A. Radial. Phys. Chem. 1977, IO, 291. 24. 449. .---
(16) Seddon, W.A.; Fletcher, J. W.;Sopchyskyn, F. C. Can. J . Chem.
1976, 54, 2807.
(17) Arai, S.;Kira, A.; Imamura, M. J . Phys. Chem. 1970, 74, 2102. (18) Pucheault, J.; Ferradini, C.; Julien, R.; Deysine, A.; Gilles, L.; Moreau, M. J. Phys. Chem. 1979,83, 330. (19) Khorana, S.;Hamill, W.H. J . Phys. Chem. 1971, 75, 3081.
0 1991 American Chemical Society
Duplltre and Jonah
898 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991
-
0.0 0.0
0.5
1 .o
C (M)
I
0.0
Figure 2. Variation of the apparent reaction rate constant for solvated electron, k(ed-+NO IO M), occurs in NaC104 and LiCI04 solutions in methanol, up to 0.4 M. Figure 1 shows the variation of k(e,C) at low concentrations. The NH4+ ion reacts rather effectively with e,,- at low concentrations, where k(e,,-) > IO9 M-l s-I can be anticipated. The decrease with concentration is characteristic of ionic strength effects'JI and is very well fitted by using the well-known expression from the Debye-Huckel theory7v2'
with B = 1.8246 X 106/(tT)3/2and g = 502.9/(tT)II2, where ko(e,;+S) is the reaction rate constant at zero ionic strength; c, the dielectric constant of the solvent (33.6); I, the ionic strength; T, the absolute temperature: ZAand ZB,the charge numbers of the reactants; a, the distance of closest approach of the ions, in nm. In the present case, fixing a = 0.6 nm, which is also the value derived when this parameter is also fit, eq 1 gives the following expression, where the positive and the negative signs apply in the case of anions and of cations, respectively:
log k(eWl-+S) = log ko(e,y+S)
f
3.1451'/2
1
(2)
+ 3.04311/2
The derived fitting parameter is ko(e,;+NH4+)
= 2.77
X
(20) Jonah, C. D. Reo. Sci. Instrum. 1975, 46,62. (21) Davies, C. W.Ion Associotion; Butterworths: London, 1962.
IO9
0.0
0.5 C (M)
1 .o
Figure 3. Variation of the apparent reaction rate constant for solvated electrons, k(ed-+NO> 10' M-I, which means that, at the concentrations used, N H 4 N 0 3is always 100% associated. It may be seen that the reaction rate constants at zero ionic strength are well-defined for NH4+, with a value agreeing well with that found when fitting the data for NH4CI04alone and for NH4N03;ko (e,[+NO
