J. Phys. Chem. 1988, 92,4835-4837
1O3HZS2O3+ S203’-
-
IOz-+ S4062-+ H2O (M7)
The consideration of the second preequilibrium is made necessary by the finding that, in the route leading to tetrathionate, the concentrations of both thiosulfate and the hydrogen ion are on the second power. The catalytic effect of thiosulfate on the oxidation of hydrogen sulfite is explained by (M8) and (M9)
IO3H2S2O3+ HS03-
+ H+
-
S042- + S2032-+ 1 0 2 -
+ 4H+ (M9)
These reactions, together with the preequilibria (M4) and (M6), are responsible for the third and the fourth terms of the rate law (l”’), resp. In this set of elementary steps, (M8) is responsible for the inhibition effect of hydrogen sulfite in the oxidation of thiosulfate, that is, for the k,[HS03-] term in the denominator of rate laws (2”) and (3’). In principle, a similar inhibition effect was due to (M9). According to the experiments, however, its weight is negligible. However, due to the obvious differences in the catalytic and inhibition effects, the catalytic effect due to (M9)
4835
appears to be significant and therefore even the fourth term in (1”’) should have been considered. The finding that with increasing hydrogen ion concentration, and in an excess of thiosulfate, the oxidation of thiosulfate to hydrogen sulfite is suppressed in comparison with the competing oxidation to tetrathionate can be explained by (M6) and (M7). These steps can be responsible for the k/[Sz032-][H’] term in the denominator of (3’). Of course, a great number of further reactions must occur involving iodite and hypoiodite. There are good reasons to assume that these reactions are rather complex and involve the formation of adducts between the reactants. These fast reactions, however, probably are of secondary importance in the dynamics of the system. It goes without saying that reactions Ml-M9 do not represent the mechanism of the reaction. However, it seems that it is the simplest and chemically plausible set of elementary and quasielementary reactions which may be responsible for the observed phenomena.
Acknowledgment. We are grateful to Drs. Vilmos GBspBr and Gyorgy Pdta for valuable discussions, to Miss Zsuzsa V. Nagy for her help in some experiments, and to the Hungarian Academy of Sciences for supporting this work by research grants A-MM-268 and OTKA 156.
Dissociation Energy of an Isolated Triplet Acetone Molecule Hanna Zuckermann, Bernhard Schmitz, and Yehuda Haas* Department of Physical Chemistry and The Fritz Haber Center for Molecular Dynamics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel (Received: May 19, 1988)
Fluorescence lifetime measurements of acetone in a seeded supersonic molecular beam reveal a sharp increase in the decay rate at an excitation energy of about 32700 cm-I. There is a concomitant sharp decrease in the emission intensity. It is suggested that these changes are due to the onset of the homolytic dissociation of acetone to methyl and acetyl radicals. The barrier for this reaction on the triplet TI surface is determined to be at 32700 f 50 cm-’ above the ground state.
Introduction The gas-phase photodissociation of acetone involves the primary reaction
-
CH3COCH3
CH3C0 + CH3
= 81 kcal/mol (1)
It is believed to take place on the triplet n.n* surface. Previous estimates of the energy barrier corresponding to this reaction ranged between 6 and 17 kcal/moI.l4 Table I summarizes these results and the experimental techniques used in obtaining them. All previous work was done at ambient temperatures, making it necessary to correct for the initial energy distribution of the molecules. Furthermore, since relatively high-pressure conditions were used, extrapolation to zero pressure was required in some cases. W e report a measurement of the dissociation energy on the triplet energy surface, taken in a supersonic seeded molecular beam. Under these conditions, the molecules are both cold and isolated from collisions, allowing an almost direct measurement. Our result is 32 700 50 cm-I, corresponding to 93.4 kcal/mol
*
(1) Cundall, R. B.; Davies, A. S. Proc. R. SOC.London A, 1966,290,563. ( 2 ) ONeal, H. E.; Larson, C. W. J . Phys. Chem. 1969, 73, 1011. ( 3 ) Gandini, A,; Hackett, P. A. J . Am. Chem. SOC.1977, 99, 6195. (4) Copeland, R. A.; Crosley, D. R. Chem. Phys. Lett. 1985, 115, 362.
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TABLE I: Previous Estimates of the Barrier to Reaction 1 energy barrier, method kcal/mol ref 1. kinetic analysis of photolysis experiments 6.4 1 2. kinetic analysis using RRK theory 10.3 2 3. photosensitization of biacetyl 15.0 3 4. low-pressure bulk phosphorescence 11.5’ 4
“Assuming that the triplet zero point energy of the triplet is 80 kcal/mol above So. above the ground state. Assuming that the TI surface is at 80 kcal/mol, this value corresponds to a barrier of 13.4 kcal/mol. Experimental Section The pulsed supersonic beam source was described earlier.5 Acetone was seeded in helium and expanded through an orifice into a vacuum chamber maintained at lo-’ Torr. Orifice diameters used were 0.03 and 0.3 mm. The expanding jet was intersected by a tunable dye laser beam about 20-50 nozzle diameters downstream. The resulting fluorescence was observed at right angles, through a 370-nm cutoff filter. The laser was either an excimer laser pumped dye laser (Lambda Physik, 0.2 cm-’ ( 5 ) Anner, 0.; Haas, Y. Chem. Phys. Lett. 1985, 119, 199.
0 1988 American Chemical Society
4836 The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 ACETONE - d,
Letters
ACETONE- h e
;i
’
33112
33003
32922
/
J 0
2
4
6
8
;Ll 0
2
4
6
248
8
Figure 1. Decay profiles of acetone-h6 (left) and acetone-d, (right) near the energy at which the decay time shortens. Total emission is shown, upon excitation at the energies (in cm-I) specified in the figure; 0.07% of acetone in 9 atm of helium was expanded through a 0.03 mm orifice into a vacuum of mbar.
bandwidth, 20 ns pulse duration) or a Nd:YAG pumped dye laser (Quanta Ray, 1 cm-’ bandwidth, 10 ns pulse duration). Experiments were usually run at 10 Hz, keeping the pressure on the vacuum side at less than Torr. Signal averaging of up to lo4 shots was required for obtaining a satisfactory signal to noise ratio. Results Low-pressure room temperature studies6 revealed that the emission of acetone consists of several decay components. A very fast decay component, commonly referred to as the spike (less that 10 ns, the time resolution of the experiment), is followed by a much longer, nonexponential decay that lasts about 3 pus near the origin. As the excitation energy is increased, the long-lived component decay time increases first, and beyond about 32 700 cm-I decreases, until it disappears completely at about 33 300 cm-’. The same general trend is also observed in the jet, except that the disappearance of the long component occurs over a much narrower energy range. Figure 1 shows representative decay curves for acetone-h6 and acetone-d6. The sharp decrease in lifetime occurs in acetone-h6 near 32 700 cm-I and in acetone-d6 at about 33 000 cm-I. The data shown in Figure 1 were taken with an orifice of 0.03 mm, using a mixture of 0.07% acetone in 9 atm of helium. Under these conditions, cooling was not as extensive as upon using the 0.3-mm orifice, as judged by the rotational envelopes of the excitation bands. The thermal distribution of the molecules in these experiments is much narrower than at room temperatures but still might obscure the exact energy at which the long-lived component disappears. It was thus attempted to achieve better cooling by using larger orifices. It turned out that this procedure actually led to the appearance of a persistent residual emission having a decay time of 600-800 ns upon excitation at energies above the previously observed cutoff. This emission followed the usual spike and exhibited a remarkable constant decay time, not only in the transition region of 33 000 cm-I but also up to 37 500 cm-I. As discussed in more detail in a separate report,’ this fluorescence is due to acetone dimers (or higher adducts) that photodissociate into electronically excited monomeric acetone molecules. It proved very difficult to find expansion conditions that simultaneously produce a large number of cold monomers and no clusters. Dilution of the mixture necessitated unpractically long averaging periods. Using the larger nozzle, a remarkable change in the appearance of the fluorescence excitation spectrum was observed around 304.5 nm (32 840 cm-I). As seen in Figure 2, at lower frequencies the spectrum consists of a series of distinct, sharp features. At higher frequencies, the fluorescence intensity drops considerably, and the spectrum is diffused. This residual emission disappears altogether
3b7
316
E X C I T A T I O N WAVELENGTH, nm
TIME ( m ~ c r o s e c o n d r l
Figure 2. A portion of the fluorescence excitation of acetone around 305 nm; 0.1% acetone in 5 atm of helium was expanded through a 0.3-mm nozzle. The spectrum was taken a t the time interval 0.1-2 f i s after the laser pulse.
when the smaller nozzle is used. Under these conditions, as remarked above, only the very short-lived emission (the spike) persists.
Discussion It is proposed that the rather sudden disappearance of the long-lived emission is due to dissociation on the TIsurface. This contention is based on the following arguments: (a) Acetone is known to decompose according to eq 1 at about the energy at which the change takes place. (b) A sudden decrease in fluorescence lifetime and intensity is often associated with dissociation. (c) Other radiationless processes are less likely to take place. Intersystem crossing to So probably takes place, but its rate is expected to rise much more gradually with energy. Dissociation to H + CH2COCH3requires more energy.* Proton transfer to form an enol is possible but has not been reported to be an important process in the gas phase. (d) RRKM calculations were performed to test the possibility of a reaction from So. It is found that the rate of change in the region of interest is much slower than observed. In contrast, calculations on the triplet surface showed a very steep increase of the rate constant as soon as the barrier was exceeded. We attempted to correlate the observed changes in the rate constant with an RRKM calculation using the barrier as a parameter. The steep rise could be reproduced with a barrier of 15 f 1.5 kcal/mol. The rather large uncertainty is due to the fact that we could not yet attain good data at rates exceeding lo7 SKI,where the calculation becomes more sensitive. In any case, the calculation shows that the process is compatible with a “small molecule” behavior, as expected for the triplet state at the energy range, with a density of states of the order of 10’ states/cm-I. (e) The SI state is nondissociative in this energy region. This is demonstrated by the persistence of the nanosecond fluorescence components. By comparison with previous studies on aldehydes? the short component is due to dephasing of the initial state, prepared by optical excitation, into the eigenstate of the system which is a longer combination of SI and TI components. As long as the triplet state is bound, the eigenstate exhibits a long-lived fluorescence. At the energy corresponding to the barrier, the triplet state becomes dissociative and the long (microsecond) component disappears. The threshold energy for dissociation may be compared to that determined recently for the formaldehyde C-H bond at about 91 kcal/mol and the C-D bond at 93 kcal/mol,I0 on the T, surface. For acetaldehyde the threshold is about 90 kcal/mol for C-C bond fission and 2-3 kcal/mol higher for C-H bond The (8) McMillen, D. F.; Golden, D. M. Annu. Reu. Phys. Chem. 1982, 33,
493. The value quoted by these authors is 98.3 kcal/mol. ( 6 ) Greenblatt, G. D.; Ruhman, S.; Haas, Y . Chem. Phys. Lett. 1984,112, 200. (7) Zuckermann, H.; Schmitz, B.; Haas, Y., paper submitted to the XI1 IUPAC Symposium on Photochemistry, Bologna, 1988.
(9) Muhlbach, J.; Huber, J. R. J . Chem. Phys. 1986, 85, 4411. (10) Chuang, M.-C.; Foltz, M. F.; Moore, C. B., to be submitted for
publication.
(11) Horowitz, A.; Kershner, C. J.; Calvert, J . G. J . Phys. Chem. 1982, 86, 3094.
J . Phys. Chem. 1988, 92, 4837-4839 result obtained for acetone is thus seen to be compatible with that of simple aldehydes. The actual barrier above the triplet surface origin can only be estimated, as the location of that origin has not yet been determined. In acetaldehyde, the S1-T1 separation is 2500 cm-1,13,14and assuming a similar value for acetone, the (12) Horowitz, A,; Calvert, J. G. J . Phys. Chem. 1982, 86, 3105. (13) Moule, D. C.; Ng, K. H. K. Can. J . Chem. 1985, 63, 1378. (14) Baba, M.; Hanazaki, I.; Nagashima, U. J. Chem. Phys. 1985, 82, 3938.
4837
T, origin would be at about 28 000 cm-I (80 kcal/mol). This leads to a barrier of 13.5 kcal/mol, similar to that deduced for acetaldehyde." Acknowledgment. This research was partially supported by grant 84-00037 from the United States-Israel Binational Science Foundation (BSF), Jerusalem, Israel. B.S. is grateful to the Minerva Foundation for a research fellowship. We thank Professor C. B. Moore for communicating his recent results on formaldehyde dissociation (ref lo), prior to their publication.
Consequences of Thermodynamic Restraints on Solvent and Ion Transfer during Redox Switching of Electroactive Polymers Stanley Bruckenstein* Chemistry Department, University at Buffalo, State University of New York, Buffalo, New York 14214
and A. Robert Hillman School of Chemistry, University of Bristol, Bristol, BS8 ITS, England (Received: March 3, 1988; In Final Form: July 5, 1988)
A general framework is given which describes ion and solvent populations within hydrophilic polymers containing fixed redox
sites in terms of thermodynamically defined quantities. The results are presented in a form suitable for analysis of weight changes occurring in surface-immobilized polymer films as a result of redox switching. Two important general features associated with the switching process emerge. Firstly, solvent transfer will occur. Secondly, the stoichiometric coefficients describing the changes in solvent and ionic populations are not required to be integral.
Introduction A complete description of charge transport in electroactive polymers requires consideration of concomitant changes in ion and solvent populations. Despite the existence of theoretical treatments of related problems for polyelectrolyte systems,' only limited attention has been given to the thermodynamic aspects of such processes.* We show how analogous concepts to those applied to polyelectrolytes' may be used to describe the behavior of certain classes of surface-immobilized redox polymers. Several possible classes of behavior arise because of the microscopic nature of the polymer/bathing solution interfacial region. One limiting case, not treated here, is that in which the polymer phase excludes solvent and during the redox switching process permits only unsolvated ions to enter or leave. Electroneutrality determines the sum of anion and cation ingress and egress, the polymer structure determines the relative amounts of anions and cations moved, and solvent participation is limited to specific solvation of the two forms of the electroactive polymer. A second limiting case is that in which the polymer is permselective, so one does not have to consider the distribution of co-ions. Separation of charge gives rise to a potential difference at the polymer/solution interface, the so-called Donnan potential. A special example of this case, which we show below to be included in our more general formulation, has been investigated by Naegeli et al.* We develop a thermodynamic description for a more general situation. We treat the case in which solvent in the polymer phase is distinguishable from solvent in the external phase (bathing solution). The novel feature is that we allow the activities of cations, anions, and solvent in the polymer phase (polymer associated solvent) to be determined by thermodynamic restraints in addition to electroneutrality, which only involves charged species. Thermodynamic parameters are the key to deducing the sources
+
(1) Marinsky, J. A . J . Phys. Chem. 1985, 89, 5294. (2) Naegeli, R.; Redepenning, J.; Anson, F. C. J . Phys. Chem. 1986, 90, 6221.
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and sinks for the mobile species participating in the overall redox reaction. Experimental verification demands a technique capable of quantitatively determining small changes in polymer film populations of species in the presence of vast excesses of the same species in the bathing solution. One simple experimental technique meeting this requirement is the in situ electrochemical quartz crystal microbalance (EQCM) t e ~ h n i q u e . ~Thus - ~ our first goal is to obtain the governing thermodynamic relationships, and our second goal is to relate the mass changes that accompany the polymer redox process to the relevant thermodynamically defined quantities. In keeping with our second goal, we particularize our treatment to describing mass functions, rather than, for example, activity functions, as would be utilized for electrochemical data analysis. Theoretical Section The Model. We consider a hydrophilic redox polymer with fixed positive charge sites and containing a well-defined amount of solvent (e.g., water) and ions from the bathing solution. We treat the general case which involves solvated protons, solvent and counterions. The case of fixed negative charges may be developed by analogy. Half-reaction 1 represents the redox reaction for the monomeric analogue in solution. Xm+ + ne + qH+ = XHq(m+q-n)+ (1)
The polymeric redox reaction, explicitly including hydronium ion, counterions and water in the polymer (subscript p), and the solution phase (s), is Xm+p+ ne + (bp + qs}H30++ ( ( m b), + ( 4 - n),JAwpH20= XHq(m+Tn)+p + ( ( 6 - y ) , + ys)H30++ ( ( m + 6 + 4 - n - Y ) p + YJA- + ( ( w + 4 - x)p + x,JH,O (2)
+
+
(3) Bruckenstein, S.; Shay, M. Electrochim. Acta 1985, 30, 1295. (4) Varineau, P. T.; Buttry, D. A. J . Phys. Chem. 1987, 91, 1292. (5) Orata, D.; Buttry, D.A. J . Am. Chem. SOC.1987, 109, 3574.
0 1988 American Chemical Society
