J . Am. Chem. SOC.1986, 108, 5135-5142
5135
Pulsed-Laser Flash and Continuous Photolysis of Aqueous Solutions of Methyl Viologen, Oxalate, and Their Ion-Pair Complexes Dasari R. Prasad,+ Morton Z. Hoffman,*+Quinto G . Mulazzani,* and Michael A. J. Rodgerss Contribution from the Department of Chemistry, Boston University, Boston, Massachusetts 0221 5, Istituto di Fotochimica e Radiazioni d'Alta Energia, Consiglio Nazionale delle Ricerche, 40126 Bologna, Italy, and the Center of Fast Kinetics Research, University of Texas, Austin, Texas 7871 2. Received January 21, 1986
Abstract: MV2+forms a 1:l ion-pair complex (Keg= 21 M-I) with C2042-in aqueous solution at natural pH (7.1) that exhibits an enhanced tail absorption in the 310-400-nm region; at X 300 nm. Figure 1 also shows t h e dependence of t h e absorption a t 310-420 n m o n [C2042-] (0.015-0.10 M) a t fixed [MV2+] (0.010 M ) ; t h e spectra a r e unchanged in t h e p H range 7.1-10.0. U s e of t h e Benesi-Hildebrand analysis3* for 1: 1 donor-acceptor complexes (Figure 2) for the absorbance a t 350 n m yields Kq = 21 M-' and e350 = 7.2 M-I cm-' a t t h e ambient p H of these solutions (7.1); t h e linearity of the plot demonstrates that Kq is constant within t h e range of the ionic strengths (0.08-0.33 M) of t h e solutions. Further evidence t h a t t h e absorbing complex has a 1 :1 stoichiometry comes from Job's plots of the absorbance of the system a t 340 n m a s a function of t h e mole fraction of MV2+ a t constant ([MV2+] [C20d2-]) = 0.050 M (Figure 3). Knowledge of t h e values of Keq a n d q enables t h e equilibrium concentrations of all three components, and their contributions to the total absorbance a t each wavelength, t o be calculated. For example, t h e absorption of t h e solutions a t 254 n m fits Beer's law when measured e-values of C204*-(25 M-* cm-I) a n d MV2+ (1.84 X lo4 M-I cm-' ) a r e coupled with a calculated value for t h e complex of 2 X lo4 M-' cm-I.
+
(38) Benesi, H. A.; Hildebrand, J. H. J . Am. Chem. SOC.1949, 71, 2703.
J . Am. Chem. SOC.,Vol. 108, No. 17, 1986 5137
Pulsed- Laser Flash and Continuous Photolysis
Table 1. Q(MV'+) as a Function of Excitation Wavelength and the Initial Substrate Concentrations i, [MVz+l, [C@?-l, anm mM M fi fi h (MVT~b + ?
o,lil 0.15
> I
v
380
20
0.10
>0.99
0.99
>0.01
0.00
0.23 (1)
340
5.0 10 15 20 25
0.10 0.10 0.10 0.10 0.10
>0.99 >0.99 >0.99 >0.99 >0.99
lo8 s-l. release of the radical pair into the bulk solution (reaction 3) occurs The linear generation of MV" during the initial phases of the competitively with back-electron transfer within the solvent cage continuous photolysis, from which the values of @(MV'+) are (reaction 4), solvent-separated redox products will not form. The calculated, argues that back-electron-transfer reaction 7 can be irreversible transformation of C204'- (reaction 5 ) into COz and ignored when [MV'+] is low. C02'-, species that cannot engage in back-electron transfer with In the 340-380-nm region, the only absorbing species is the MV", permits the radical cation to accumulate. In reaction 6, MVZ+/C2042-complex; the fraction of light absorbed by unC02'- reduces MV'+, undoubtedly both complexed and uncomcomplexed MV2+ and C2O4*- is XO.01. The values of @(MV'+) plexed, to generate a second equivalent of M V + ; k6 is known from thus obtained (Table I) yield an average value of 0.24, with an pulse radiolysis to be in the range 0.4-1.6 X IO'O M-' s-l, deaverage deviation of f l l % , independent of the concentration of pending upon the ionic strength and composition of the s o l ~ t i o n . ~ ~ the ~ ~ complex (3.2-13 mM) and uncomplexed MV2+ (1.8-12 mM) at essentially constant [C2042-](0.10 M). For excitation at 100 pM, kow/[MV2+] is significantly less than that value, becoming almost a factor of 2 less a t 200 p M MV2+. It appears that, although the secondary formation of the MV" species can be generally identified with reaction 6, the system becomes unexpectedly complex as [MV2+] is increased. This conclusion becomes secure when the results of the 355-nm laser experiments are examined. At the higher concentrations
-
M.D.; Sutin, N. Annu. Reu. Phys. Chem. 1984, 35, 437. Jones, G.,11; Becker, W. G.J . Am. Chem. SOC.1981, 103, 4630. Jones, G., 11; Becker, W. G. Chem. Phys. Lett. 1982.85, 271. Jones, G., 11; Becker, W. G. J . Am. Chem. SOC.1983, 105, 1276. Jones, G., 11; Malba, V. Chem. Phys. Lett. 1985, 119, 105. hasad, D. R.; Hoffman, M. 2. J. Chem. SOC.,Faraday Trans., in
(48) Newton, (49)
(50)
(47) Balzani, V.;Scandola, F. In Energy Resources through Photochemistry and Catalysis; Gritzel, M.,Ed.; Academic press: New York,1983; p
-
(51) (52) (53)
press.
J . Am. Chem. Soc., Vol. 108, No. 17, 1986 5141
Pulsed- Laser Flash and Continuous Photolysis Scheme I
of MV2+employed, necessitated by the absorbance requirements, the formation af the second equivalent of MV” via reaction 6 should occur in the ns time frame. Even with the use of the mode-locked laser, no secondary formation of the 600-nm absorption was observed at short times after the pulse. As shown in Table 111, a secondary formation is observed, but in the @ time frame. This secondary formation, which cannot be attributed to reaction 6, is observed a t 395 and 605 nm, and is independent of beam intensity and [MV2+];it is, however, inversely dependent on [C2042-](Figure 7). The same first-order rate constant is also = 500 nm, where a spectral decay is observed obtained when A,, (Figure 6). Thus, the initial and final spectra obtained upon the 266- and 355-nm laser excitation are unquestionably those of M V ” species; however, the differences in these spectra, which are very apparent in Figure 6, argue that the species are not identical. In fact, the final spectrum is virtually identical with the well-known spectrum% of MV’+ obtained in continuous and flash photolysis, continuous and pulse radiolysis, electrolysis, and thermal reduction and can, indeed, be ascribed to MV’+ free in solution. The initial spectrum, on the other hand, shows an enhanced absorption in the 450500-nm region, reminiscent of that displayed by reduced-viologen dimers and a g g r e g a t ~ . ~ ~ In order to understand the nature and kinetics of the secondary spectral change, which, as [MV2+]is increased, clearly is not due to reaction 6, it is necessary to examine in detail the environment in which MV’+ finds itself upon excitation of the MVZ+/C2042complex. Although the absorption of the complex in the near-UV arises from a species with a 1:l stoichiometry, consideration must be given to the existence of higher aggregate species that do not contribute to the spectrum. MV2+ has been observed, by lightscattering measurements, to form aggregates in neutral aqueous solution at a concentration of about 10 mM;S5 although the presence of C1- at 0.1-1 M does not have an appreciable effect on this “critical aggregation concentration” (CAC), 0.1 M phosphate buffer at pH 7 lowers the CAC. The effect of high concentrations of C2042-on CAC is unknown, but one can surmise that the strong ion-pairing interaction between MVZ+and C20d2lowers CAC significantly (perhaps to 1 mM or below), so that, for the 355-nm laser excitation experiments, MV2+ exists mainIy as higher ion-paired aggregates. In fact, one can visualize that the aggregated MV2+ species exist in a hydrophobic core surrounded by a hydrophilic sheath of C2O:- ions; the structure can be regarded as a “pseudomicelle”. The absorption of light by a MV2+/C2042-unit within the “pseudomicelle” would generate MV’+ and C204- therein via the equivalent of reaction 2; unless the latter radical undergoes decarboxylation within the “pseudomicelle” in competition with geminate pair back-electron transfer, no net production of M Y + could occur. The C02’- radical thus generated within the “pseudomicelle”could easily reduce a second MV2+in a time much shorter than that required for the reaction to occur diffusionally in homogeneous solution. The result would be the existence of two MV’+ units in close proximity, yielding an initial spectrum that would have the characteristics of the reduced “dimer”; it has been suggested recently” that M V “ does not dimerize but, rather, aggregates into micelle-like particles. However, now the ionpairing interaction between M V ” and C20,2-would be less strong than between the original substrates; the MV’+ species within the “pseudomicelle” can be visualized to exist in an unstable environment. The slow first-order conversion would be due to the rearrangement of the aggregate structure as MV’+ is released into bulk solution in its final relaxed, equilibrated condition. The rate constant for this transformation would be expected to be independent of [MVz+]and the intensity of the laser beam. However, as [C2042-]is lowered in the bulk solution, the rate of equilibration of the cation radicals would be enhanced; it should be noted that
the dynamic breakup of aqueous micelles involving the release of one of the surfactant units occurs on the I.LS time frame.57 Scheme I shows the represntation of this model; only two MV2+ ions are shown in the core of the “pseudomicelle”, and only one C20d2-in the sheath, for simplicity. In the 266-nm experiments at lower [MV2+],aggregation would exist to a lesser extent; only a very small contribution of the “pseudomicelle” to the initial spectrum and its slow transformation would be made. MV‘+ would be produced in two stages in approximately equal amounts from the excitation of the complex and the secondary reaction of C02’- in homogeneous solution; by and large, the rate of the latter would be proportional to [Mv2+]. As [MV2+] is increased, the onset of aggregation would become apparent. For example, the effective concentration of free MV2+ would not increase proportionally with the total MV2+concentration, so that the rate of secondary formation of MV” would not continue to rise linearly but would fall off with increasing [MV2+]. Reactions involving the aggregates would not become apparent until they became a large fraction of the total process and rate determining, as is the case in the 355-nm experiments. Fine-Tuning the Mechanism. Finally, consideration must be given to the effect of aggregation on 9(MV’+) in continuous photolysis. It would appear that under the conditions of very low [MV2+]that were employed in some of the 254-nm experiments, reactions 2-6 and 12 in homogeneous solution would be operative with qcr, reflecting the competition between reactions 3 or 12 and 4. At higher [MVz+], this same [MVZ+,C2042-] absorbing unit would be contained within the “pseudomicelle” aggregate; the processes occurring subsequent to the absorption of a photon are given in Scheme I. The independence of qn on the concentrations of the substrates at 254 and 340 nm argues that the probability of the breakup of the initially formed geminate pair is largely independent of the environment in which it is found; certainly, there is no analogue to reaction 3 in the “pseudomicelle” model. One is drawn to the inescapable conclusion that the propensity of the caged C204*-species toward decarboxylation in competition with geminate cage electron transfer is the main factor controlling the value of the cage release yield; a similar conclusion was reached by other workers5* regarding the dissociative nature of electron capture when tetranitromethane is used as an electron acceptor. This point of view is further supported by the range of values of @(MV’+) we have recently observed59in the photolysis of ion-pair complexes of MV2+and hydroxycarboxylates, where the charges on all species are the same, but the stabilities of the resulting RC02’ radicals are different.
(54) Summers, L. A. The Bipyridinium Herbicides; Academic Press: London, 1980; Chapter 4. (55) Ebbesen, T. W.; Ohgushi, M. Photochem. Photobiol. 1983,38, 251. (56) Rieger, A. L.; Rieger, P. H. J. Phys. Chem. 1984, 88, 5845.
(57) Fendler, J. H. J . Phys. Chem. 1985, 89, 2730. (58) Masnovi, J. M.; Huffman, J. C.; Kochi, J. K.; Hilinski, E. F.; Rentzepis, P. M. Chem. Phys. Lett. 1984, 106, 20. (59) Prasad, D. R.; Hoffman, M. Z., work in progress.
J . Am. Chem. SOC.1986, 108, 5142-5145
5142
Acknowledgment. This research was supported in part by grants from the Office of Basic Energy Sciences, Division of Chemical and the National Sciences, U S . Department of Energy (M.Z.H.) Institutes of Health (M.A.J.R.) and in part by Consiglio Nazionale delle Ricerche of Italy. The collaboration between Q.G.M. and M.Z.H. is part of the US-Italy Cooperative Research Program.
CFKR is supported jointly by the Biomedical Research Technology Program of the Division of Research Resources of N I H (RR 008866) and by the University of Texas. The authors thank Dr. S. J. Atherton (CFKR) for technical assistance and many highly stimulating discussions; they also thank Dr. V. Malba and Professor G. Jones (B.U.) for their continued interest in this work.
The CloHSPotential Energy Surface: The Azulene-to-Naphthalene Rearrangement Michael J. S. Dewar* and Kenneth M. Merz, Jr. Contribution from the Department of Chemistry, The University of Texas at Austin, Austin, Texas 7871 2. Received February 12, I986
Abstract: The azulene-to-naphthalene rearrangement (AN rearrangement) has been studied, using MNDO and MIND0/3. All of the proposed unimolecular pathways have been examined in detail and found wanting. The results presented here agree with the first step of a mechanism proposed by Becker et al. A new mechanism is proposed for the rest of the reaction.
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We recently reported' a preliminary study of the azulene-tonaphthalene (1 2) rearrangement (AN reaction) which eliminated from consideration several of the mechanisms that had been previously proposed and we also suggested a new alternative. Here we present the results of a systematic study of the C 1 d 8potential energy (PE) surface, carried out in the hope of finally solving the problem. We will not discuss its historical developement because an excellent review has recently appeareda2 Experimental Procedure MNDO' was used for closed-shell species and the spin-unrestricted version of MNDO' (UMNDO) for biradical or open-shell species. AI1 geometries were fully optimized, using the DFP method.J In certain cases where MNDO is known to give large errors (e&, hydrogen transfers), MINDO/36 (or UMINDO/J) calculations were also carried out for comparison. All transition states (TS) were located by using the reaction coordinate method,' refined by minimizing the norm of the gradient: and characterized by establishing that the Hessian (force constant) matrix had one, and only one, negative eigenvalue.* Options for all these procedures are included in the MOPAC package of computer program^.^ Results and Discussion Schemes I and I1 show the unimolecular mechanisms proposed, respectively, by Scottlo and Becker," while Scheme I11 shows the mechanism proposed here on the basis of our calculations. First we will explain why our results eliminate the Scott and Becker mechanisms as possible major contributors to the A N rearrangement. Unfortunately, no mechanism yet proposed, not even the one suggested here, can explain all of the results obtained from labeling studies.I2 We have therefore also studied various possible M.J. S.;Merz, K. M.,Jr. J. Am. Chem. Soc. 1985,107, 6111. (2) Scott, L. T. Acc. Chem. Res. 1982, 15, 52. (3) Dewar, M.J. S.;Thiel, W. J . Am. Chem. SOC.1977, 99, 4899,4907. (4) Pople, J. A.; Nesbet, R. K. J . Chem. Phys. 1954, 22, 571. (5) Davidon, W. C. Compur. J. 1958, 1,406. Fletcher, R.;Powell, M.J. D. Compur. J. 1963, 6, 163. (6) Bingham, R. C.; Dewar, M..J. S.; Lo, D. H.J. Am. Chem. SOC.1975, 97, 1285, 1294, 1302, 1307. (7) Dewar, M. J. S.;Kirschner, S. J. Am. Chem. SOC.1972, 94, 2625. (8) McIver, J. W.; Komornicki, A. Chem. Phys. Leu. 1971, 10, 303. McIver, J. W.; Komornicki, A. J . Am. Chem. SOC.1972, 94, 2625. ( 9 ) QCPE publication 455, Department of Chemistry, Indiana Unversity, Bloomington. IN 47405. (10) Scott, L. T.; Kirms, M. A. J . Am. Chem. SOC.1981, 103, 5875. (11) Becker, J.; Wentrup, C.; Katz, E.; Zeller, K. P. J . Am. Chem. SOC. 1980, 102, 51 10. (12) (a) Alder, R. W.; Whittaker, G. J. J . Chem. Soc., Perkin Trans. 2 1975, 714. (b) Alder, R. W.; Whilshire, C. J . Chem. Soc., Perkin Trans. 2 1975, 1464. (c) Alder, R.W.; Whiteside, R.W.; Whittaker, G. J.; Whilshire, C. J. Am. Chem. SOC.1979, 101, 629. (d) Zeller, K. P.; Wentrup, C. Z. Nafurforsch. E 1981, 36,852. (e) Scott, L. T.; Kirms, M.A,; Earl, B. C. J. Chem. SOC.,Chem. Commun. 1983, 1373. (f) See ref 2, 10, 11, 16. (1) Dewar,
Scheme I. The Scott Mechanism
3
4
5
Scheme 11. The Becker et al. Mechanism
la
m 2
QLm-rg C:
6
E
"
7
e
Scheme 111. The MNDO Mechanism
I
6
I1
pathways for the scrambling of the labels in azulene and naphthalene in the hope of resolving the remaining discrepancies. This work is presented in the following paper.
0002-7863/86/1508-5142$01.50/00 1986 American Chemical Society
