J . Phys. Chem. 1988, 92, 7260-7262
7260
0
Acknowledgment. The author thanks discussions with J. M. Bowman and with H. 0.Pritchard prior to publication of his note (ref 3). This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, US. Department of Energy, under Contract W-31-109-Eng-38. Registry No. H, 12385-13-6;CO,630-08-0. Albert F. Wagner
Theoretical Chemistry Group Argonne National Laboratory Argonne, Illinois 60439
--
-8"
- $@
Figure 1. The (H,O-OH-) model for electrons in water favored by Robinson and co-workers (ref 7).
Received: August 3, I988
Aquated Electrons, H,O- Anions, and OH-/H,O Units Sir: There is a major dichotomy in the current descriptions of those species usually described as "solvated electrons". In one, often described as the "cavity" model, the electron is thought to be solvated in a cavity, in much the same way as a monoatomic anion.'S2 In the other, strongly favored by Tuttle and his cow o r k e r ~by , ~ Hamill ~~ and co-workers,5,6 and by Robinson and co-workers,' "solvated electrons" are not invoked. Instead, solvent anions, such as (NH3)'- or (H20)'- are envisaged. The particular model now presented by Hameka et al.' differs from the other "solvent-anion" models in several interesting aspects. Various arguments are presented which purport to show that the new model is preferable to any cavity model. Indeed, common to all these pre~entationsj~ is a clear feeling that the solvent cavity concept is open to criticism. My aim is to indicate why I consider the new model to be less satisfactory than the cavity model for "solvated electrons". Description of the Robinson Model This is depicted as an (H30- - -OH-),, unit, shown pictorially in Figure 1 . The OH- part is described as being a normal solvated hydroxide ion. The H30' unit is less well defined. Two theoretical models for isolated H20'- radicals are described in detail. One (A) is a u* model, the unpaired electron being in the u* orbital of one stretched 0-H bond. The other (B) is a "Rydberg" model,
,---.
A(ii)
ti-O--'--H
I ti H30*
(a*)
H,O*
IRydbsrg)
the excess electron being accommodated in a 3s orbital and the water molecule being almost unchanged from normal H 2 0 . It is not clear which model is envisaged for the H,O' unit in the ( H 3 0 ---OH-) species. However, only the "Rydberg" model for H 3 0 ' can explain the optical absorption normally assigned to solvated electron^.^ Some time ago,* we showed by fairly simple (1) Symons, M. C. R. Q. Rev. 1958, 12, 17. (2) Symons, M. C. R. Chem. SOC.Rev. 1976, 5 , 337. (3) Tuttle, T. R.; Graceffa, P. J. Phys. Chem. 1971, 75, 843. (4) Golden, D. M.; Tuttle, T. R. J. Phys. Chem. 1984, 88, 3781. (5) Razem, D.; Hamill, W. H. J . Phys. Chem. 1977, 81, 1625. (6) Razem, D.; Hamill, W. H. J. Phys. Chem. 1978, 82, 488.
(7) (a) Hameka, H. F.; Robinson, G.W.; Marsden, C. J. J. Phys. Chem. 1987, 91, 3150. (b) Robinson, G. W.; Hameka, H. F. SPIE 1987, 742, 82.
Figure 2. Overlap between the 1s orbital of a hydrogen atom and a lone-pair orbital of water generating a and u* orbitals which give rise a*) in the 50000-cm-' region. to a characteristic transition (a
-
- -
calculations that the u* model would give an allowed transition u* in nature (Figure in the 50000-crn-' region, this being u 2). However, as stressed by Hameka et al.,' the 3s 3p transition for a Rydberg model should be of much lower energy and could, in principle, be the origin of the intense visible band for these systems (13 700 cm-'). I conclude that the Rydberg model (B(ii)) but, in my view, there is favored for H30' in this pre~entation,~ are good experimental results that, of the two, favor the u* model.
versus Rydberg Structure for H30' Consider first the u* structure for H30' in water. In fact, H' "atoms" in aqueous systems have ESR spectra closely resembling those for free H' atoms (500-510 G hyperfine coupling to a single ' H n u c l e u ~ ) . ~ *Also, ' ~ for H' in aqueous solutions, an absorption band in the 50000-cm-' region has been detected." This accords with the 'H- - -OH2 model (A@)), with the SOMO largely located on H'. The visible band from electrons in water and the narrow singlet ESR spectrum are clearly due to a species with utterly different physical properties. In order to accommodate these observations and retain the Robinson model' (Figure 1 ) it is necessary to suggest that the OH- ion induces a switch from the normal u* 'H- - -OH2 structure to the "Rydberg" structure. I can see no reason for this. We have detected two quite distinct species in irradiated aqueous NaOH glasses, one being H,' and the other e;. The H,' center has an ESR spectrum typical of H' atoms and is either 'H- - -OHz or 'H- --OH-. (I expect the latter to be favored in terms of Figure 2.) The H, centers react on warming above 77 K to give more e; with loss of the H' signal.I2 The optical absorption is typical of electrons in water. The electrons couple weakly with 6 H from 6 H 2 0 and only very weakly with the cations. Clearly the electrons are strongly confined, we think in anion vacancies in the glass.I3 These results underline the distinct nature of the two species. Further evidence against a Rydberg-type structure for HzOor H 3 0 comes from our studies of the 'H- - -hal- centers (hal- = F, C1-, Br-, I-).'4 In all cases, a very large proton splitting was u*
(8) Claxton, T. A,; Symons, M. C. R. J. Chem. SOC.,Chem. Commun. 1970, 379.
(9) Shields, L. J . Chem. Phys. 1966, 44, 1685. (IO) Rexroad, H. N.; Gordy, W. Phys. Rev. 1962, 125, 242. ( 1 I ) Nielson, S. 0.; Pagsberg, P.; Rabani, J.; Christensen, H.; Nilsson, G. J . Phys. Chem. 1969, 73, 1029. (12) Zimmerman, D.; Symons, M. C. R. Int. J . Radiat. Phys. Chem. 1976, 8, 395. (13) Blandamer, M. J.; Shields, L.; Symons, M. C. R. J . Chem. SOC.1964, 4352. (14) Raynor, J . B.; Rowland, I . J.; Symons, M . C. R. J . Chem. Soc., Dalton Trans. 1987, 421.
0022-3654/88/2092-7260$01.50/00 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 26, 1988 7261
Comments observed together with strongly anisotropic coupling to the halogen nuclei. For ' H - - - F (as with H' in water) delocalization is minimal, but for the others it is appreciable, the results fitting perfectly with the u* structure. These centers have no visible coloration. I conclude that there is much evidence in favor of a u* (H-atom) structure, but the new model requires a "Rydberg" structure for H30'.
The Cavity Model The apparent prejudice against the cavity model would be understandable if this postulate had no precedent. However, it is closely linked to the F-center, which must be one of the best understood units in defect chemistry. ESR and ENDOR results leave no doubt that F-electrons are anion vacancies, with only the electron in the cavity. Since these well-established centers are remarkably stable, I fail to understand the barrier to accepting the concept of electrons in solvent cavities. There may seem to be an energy barrier to forming cavities. However, if the gegenions are also considered, this is no longer a problem. Thus, dissolution of NaCl in water to give Naaq+and Cl,; presents no structural or energetic problems. The same The Na+ ions should apply to Na metal giving Na,,' + e,;. solvate via "lone pairs" on oxygen thereby liberating O H groups of high reactivity. These will welcome anything to solvate, including e-. "Disruption" of the solvent structure is induced by Na+, and overall structure is restored on solvation of e-. Evidence in favor of a cavity model comes from a study of irradiated ethan01.l~'~At 4 K ejected electrons are trapped, but the absorption band is in the near-IR (ca. 7000 cm-'). The ESR singlet is very narrow, showing that interaction with solvent protons is small. On annealing to 77 K there is an irreversible change in the absorption band which is lost in the IR and a new band grows in the visible region, close to the band for e- in liquid ethanol. At the same time, the ESR spectral width greatly increases. By use of C2H50D,this width enhancement is shown to stem primarily from hyperfine coupling to OH protons. In marked contrast, no such color centers are formed in irradiated crystalline ethanol. The most obvious explanation is that electrons formed by radiolysis are trapped in solvent cavities characteristic of the glassy state. On annealing the local 0-H groups swing in toward the charge center. This stabilizes the unit and shifts the absorption band to high energies, and the ESR width is increased because the OH protons move into the wave function of the electron, It is not easy to understand these results in terms of EtO-- - -H20Et or related units. An interesting contrast comes from irradiated MeCN. Here molecular radical anions are unambiguously formed. The electron is clearly localized, no cavities are involved, and the radical-anion species are quite stable at 77 K.18J9 Thus, when solvent anions are stable, they form, as expected.
acceptor is a unit of four water molecules. I do not think this is a necessary conclusion. The phenomenon is purely a function of how rapidly the electron can become solvated. It is well established that water solvates electrons, and anions generally, far more rapidly than alcohols. For mixed solvent systems both H 2 0 and MeOH must be involved, the shape/slope of the curve simply reflecting how rapidly the solvent can adjust. I find it difficult to see that the number 4 has a major significance. Thus all four H 2 0 units in Figure 1 will be H-bonded to other water or alcohol molecules. Primary and secondary solvent molecules must adjust, and there will be imperfect adjustment at the time of electron departure. I imagine a steady fall in the rate at which e- can be sufficiently stabilized for ionization to occur as [ROH] is increased, simply because ROH molecules move (rotate) more slowly. There is no evidence that ROH molecules cannot constitute part of the solvent cage in mixded solvents. If this is the case, a major reason for postulating the OH-/H30 unit is removed.
Theory For bulk aqueous systems it is very difficult to use high-level M O theory effectively. However, insofar as this is possible, there seems to be no case against the "cavity" theory, which can clearly hold its own quite well.21 An often satisfactory compromise is the use of computer modeling techniques. These methods have given results for aqueous systems that match experiment remarkably well for both solutions of nonelectrolytes and electrolytes. It is therefore important that, for electrons in water, solvent cavities are clearly f a v ~ r e d .Such ~ ~ ~modeling ~~ can be used to reproduce the optical spectrum for electrons reasonably well.24 Reactions of Solvated Electrons Some chemical evidence is given in support of the H30---OHmodel7 These reactions are now discussed in terms of both models, the primed reactions using the H20*---OH- structure. The most important reaction of electrons in water is surely electron transfer resulting in addition (1,l') or dissociative electron capture. (S is any electron-affinic substrate.) Both (1) and (1') must have ea,- + S * S'(1)
+
x
nv
x"
-
x*+
-
-
H2
+ 20H,[
+ Ha, H2 + OH,; (OH-sH),, + Ha, H2 + OH,; OH,,ea,- + OH,, (OH-*H),, + OH,, OH,; + H20 ea,- + H 2 0 Ha, + OH,,Ha, + OH,; (OH-*H),, ea,Ha, + OH,; e,,-
-
hv'
+
+ (OH-sH),,
(OH-*H),,
Formation of Solvated Electrons by Photoionization Robinson and co-workers base much of their ideas on a fascinating series of experiments using competition between light emission and electron ejection from certain photoexcited molecules (X) . 7 3
7 X +
+
H30---OH- S H ~ O +--OHS*(1') barriers, but is is difficult to see why H30-- -OH- should be a highly efficient electron donor unless a proton transfer proceeds concomitantly with e- transfer. Certainly the model is not helpful in understanding electron transfer. e,; + ea,-- H2 + 20H,,(2)
-
(3')
-+
(4')
-
--c
e:,
In pure alcohols, the first process dominates whereas in water the second dominates. From a study of mixed H20-ROH solvents, they find a very nonlinear change, the shift from the latter to the former as the [ROH] is increased being quite rapid. A detailed analysis of the results has led to the conclusion that the electron (15) Smith, D. R.; Pieroni, J. J. Can. J . Chem. 1967, 45, 2723. (16) Hase, H.; Warashina, T.; Noda, M.; Namiki, A,; Higashimura, T. J . Chem. Phys. 1972, 57, 1039. (17) Blandamer, M. J.; Shields, L.; Symons, M. C. R. J. Chem. SOC.1965, 1127. (18) Egland, R. J.; Symons, M. C. R. J. Chem. SOC.A 1970, 1326. (19) Le Roy, R. J.; Sprague, E. D.; Williams, F. J. Phys. Chem. 1972, 76, 546. Wang, J. T.; Williams, F. J . Am. Chem. SOC.1972, 94, 2930. (20) Lee, J.; Robinson, G. W. J . Chem. Phys. 1984, 81, 1203.
Ha,
+ OH,,-
ea;
(OH-.H),,
(2')
+ H@aq+
+ H30,,+
(3) (4)
(5) (5') (6)
(OH--H),,
(6')
-
(7)
-
Ha,
Ha,
+ 2H20
(7')
(21) Clark, T.; Illins, G. J . Am. Chem. SOC.1987, 209, 1013. Clark, T. Faraday Discuss. Chem. SOC.1984,78, 203. Reed,A. E.; Clark, T. Faraday Discuss. Chem. SOC.,in press. (22) Sprik, M.; Klein, M. L.; Chandler, D. J. Chem. Phys. 1985,83, 3042; Phys. Rev. B Condens. Matter 1985, 31, 4234. (23) Sprik, M.; Impey, R.W.; Klein, M. L. J . Chem. Phys. 1985,83,5802. (24) Wallqvist, A.; Martyna, G.; Berne, B. J. J . Phys. Chem. 1988, 92, 1721.
J . Phys. Chem. 1988, 92, 7262-7263
7262
Reactions 2-7 are invoked as strongly supporting the H30--OH- model and disfavoring the e?< model.7b A general problem for the ( H 3 0 -- -OH-) reactions IS that they rely extensively on their “hydrogen ,atom” character. This works well using a u* structure for H30. However, in that case there will be no visible absorption band. Using the Rydberg structure requires just the same type of orbital switch (delocalized Rydberg a*) as is the case for eaq-, and most of the possible advantages are lost. Reaction 2 is slow and could proceed by various routes. One possibility is that dielectron units, analogous to the well-known F’-centers are intermediates. These diamagnetic species could react with H20to give H- OH-, the former rapidly giving H2. So both reactions 2 and 2’ are acceptable. For (3), e,, is expected to add to H’ H-, and therefore H2. Thus, (3) seems to be as good as (3’). Reaction 4 is a clear example of reaction 1: (4’)is no help at all. Reactions 5 and 6 go together since ( 6 ) is the reverse of ( 5 ) . Reaction 6 is fast in alkaline solution and even occurs in the solid state in alkaline glasses, as discussed above.I2 Any mechanism must fit the principle of microscopic reversibility. For eap we need to invoke e- transfer into the H-O u* orbital + solvent adjustment. For the reverse, H’ will “stick” to OH- by H-bonding, so solvent adjustment will be rate determining. Clearly the H 3 0- -OH- picture appears to be ideal-but is it really too good? Reaction 5’ is the dissociation of this complex. I cannot see any significant barrier to this (nor is one discussed in ref 7), so (5’) should be very rapid. However, this cannot be the case or the model fails. One way out of this difficulty is to argue that H30’ is not like H’ (Le., not a*) but has a Rydberg structure. In other words, both these structures seem to be required. Finally, (7) is very fast, as it should be on either model. In addition to these reactions we need to consider the reaction which interconnects the two models.
-
-
+
eaq- * H,O- - -OHThis can be subdivided into ionization of water (9) to give an ion pair, desolvation of ea;, and addition of e- to H30+. 2 H 2 0 * H30+-- -OH-
(9)
Reaction 9 is, of course, highly unfavorable. As written, this is even more so than usual, since, for the model, the ions need to be retained as pairs, making the reverse even more favorable. Desolvation of eaq- is also strongly unfavorable in terms of the localized cavity model, which seems to be strongly favored over delocalized m o d e l ~ . ~ lThen - ~ ~ e- addition to H30+to give H30’ is not strongly favorable even in the gas phase. Given that H 3 0 behaves more like a neutral molecule than a cation, there must be a major loss of solvation energy. I conclude that the forward reaction 8 is highly unfavorable overall. It would only become favored if ea< were a high-energy state so that its destruction via (8) resulted In a large energy gain. The simple concept of e- in a solvent cavity requires a solvation stabilization that is not too different from that for an anion such as C1-. Furthermore, quantum mechanical calculations2’ and molecular dynamics calculation^^^-^^ both suggest that this is a stable entity. Conversely, the reverse of (8) which contains the reverse of (9) is surely strongly favored provided ea; is, indeed, a reasonable entity.
Conclusions Other aspects raised in ref 7 include mobility comparisons and, particularly, comparisons between “solvated electrons” and protons in water. In my view, though suggestive, these are not compelling, and I feel that the arguments given above rather strongly favor the eaq- cavity model. Acknowledgment. I thank Professor G. W. Robinson for very helpful and friendly correspondence. Department of Chemistry The University Leicester, LE1 7 R H , U.K
Martyn C. R. Symons
Received: May 9, 1988; In Final Form: July 20, 1988 0022-3654/88/2092-7262$01.50/0
Reply to the Comment “Aquated Electrons, H,OAnions, and OH-/H,O Units” Sir: We welcome the open discussion about our model for the hydrated electron engendered by the Symons comment.’ First of all, let us point out again2s3that this fascinating species may exist, and may very well participate in its chemical reactions, in more than a single form. There are two broad classes of such forms. One in comprised of “physical” models, of which the cavity model is an example; the second is comprised of “chemical” models, to which OH--H30 structures belong. Symons’ arguments concerning the u* structure and the Rydberg structure promotion are well-taken points for an isolated OH--H30 entity. According to our present ideas, the hydrated electron is a Rydberg electron in the condensed phase. However, until the correct electronic description of fully hydrated H 3 0 radical is known, or until the hydration of the entire excess electron entity is taken into account, the picture will not be entirely clear. These questions can in principle be sorted out with high-level MO calculations of H30.(H20), and of small negatively charged water clusters with full structural optimization. The important issues that have been raised by Symons could not be addressed in our earlier preliminary Gaussian-82-level computations3 because of inappropriate basis sets and inadequate structural optimization. New computations are under way in our laboratory4 in order to clarify such points, both with respect to the optical and ESR spectra of the hydrated electron and with respect to questions of stability of hydrated H30 and hydrated OH--H30 structures. So far, the new results are encouraging and illuminating. However, because of a shortage of CRAY time, it will be another few months before definitive answers can be obtained. It should be pointed out here, however, that the Rydberg radicals H 3 0 and NH, in the gas phase are quasi- table.^ Symons is correct in judging that the results of our experimental picosecond work on hydrated electrons&” (and proton^'^-'^) are partly (not purely, as Symons says) =a function of how rapidly the electron (or proton) can become solvated.” Again,” our picture is this. For endoergically produced species, the precursor molecule in its excited state and, typically in equilibrium with the surrounding solvent, “waits patiently” until an appropriate fluctuation in the local solvent structure occurs. At this point, and on a much faster (femtosecond) time scale, the electron (or proton) in a non-rate-limiting step transfers (tunnels?) to this solvent structure, stabilizing it. This new solvent structure, while favorable for binding the charge, is energetically unfavorable in the pure solvent. Thus, the fluctuations require an activation barrier14 and occur only occasionally. The time scale for the overall reaction thus depends on the relatively slow rotational (Debye) time of the ~ o l v e n t , ’ which ~ ~ ’ ~ then becomes a part of the frequency factor in the rate expression.]’ For exoergically produced ions (electrons or protons), Le., those from t ~ o - p h o t o n ’ ~or , ’ ~radiolysis’* ex(1) Symons, M. C. R. J. Phys. Chem., preceding paper in this issue. (2) Robinson, G. W.; Hameka, H. F. Proc. SOC.Photo-Opt. Instrum. Eng. 1987, 742, 82. (3) Hameka, H . F.; Robinson, G. W.; Marsden, C. J. J . Phys. Chem. 1987, 91, 3150. (4) Muguet, F.; Bassez, M.-P.; Robinson, G. W., work in progress. ( 5 ) Griffith, W. J.; Harris, F. M.; Beynon, J. H. In?. J . Muss. Spectrom. Ion Processes 1987, 77, 233. (6) Lee, J.; Robinson, G. W. J . Chem. Phys. 1984, 81, 1203. (7) Robinson, G. W.; Lee, J.; Moore, R. A. In Ultrafast Phenomena Iv; Auston, D. H., Eisenthal, K. B., Eds.; Springer-Verlag: Berlin, 1984. (8) Lee, J.; Robinson, G. W. J . Phys. Chem. 1985, 89, 1872. (9) Moore, R. A.; Lee, J.; Robinson, G. W. J . Phys. Chem. 1985,89, 3648. (10) Lee, J.; Robinson, G. W. J . A m . Chem. SOC.1985, 107, 6153. (1 1) Robinson, G. W.; Thistlethwaite, P. J.; Lee, J. J . Phys. Chem. 1986, 90,4224. (12) Lee, J.; Griffin, R. D.; Robinson, G. W. J. Chem. Phys. 1985, 82, 4920. (13) Lee, J.; Robinson, G. W.; Webb, S . P.; Philips, L. A,; Clark, J. H. J. A m . Chem. SOC.1986, 108, 6538. (14) Lee,J.; Robinson, G. W.; Bassez, M.-P. J . Am. Chem. SOC.1986,108, 7471. (1 5 ) Hasted, J. B. Aqueous Dielecfrics; Chapman and Hall: London, 1973. (16) Wiesenfeld, J. M.; Ippen, E. P. Chem. Phys. Lett. 1980, 73, 47.
0 1988 American Chemical Society
