J. Phys. Chem. 1985,89, 3343-3347
3343
On the basis of the values of p for the microemulsion t,, t2, and t3, we find that 6 for w/o microemulsion is of the same order of magnitude as for o/w microemulsion. We can therefore conclude that the behavior and the dimensions of the outer shell are quite independent of the type of microemulsion (o/w or w/o). Experimentally, we see that the relaxation frequencies of both types of microemulsion, o/w and w/o, are in the region of 1 GHz. With the help of our model and measured values, the extension of the shell is found to be of order 10 A in both types of microemulsion. The independence of the outer shell irrespective of the core has also been observed by other techniques.12 The foregoing analysis is for want of a molecular dielectric theory of interfacial ionic and dipolar regions necessarily based on representing their behavior by macroscopic parameters. Pending such a theory, the derived values of these parameters cannot be taken too literally. Nevertheless, they lead to semiquantitative conclusions which seem quite satisfactory and consistent with attributing the observed relaxation to an interfacial layer of molecular thickness with considerable rotational mobility of constituent molecular dipoles. The experimental results and interpretation we have developed are similar in some respects to those of Foster and co-workers for oil in water microemulsions at frequencies from 1 to 15 GHz with nonionicz4 and ionic surfactant^,^^ but rather different in others. Their system most closely resembling ours was composed of mineral oil with sodium cetyl sulfate and 1-pentanol in 2 to 3 weight ratio as emulsifier, 9 to 1 emulsifier to oil weight ratio,
and 4 0 4 0 % water by weight. For this and other systems, they found considerably depressed circular arc loci for e centered about frequencies varying from 2 to 15 GHz with increasing water content, but with no indication from their plots of a resolvable relaxation region such as we have found for both o/w and w/o emulsions. The interpretation of Foster et al. is in terms of interfacial water bound to charged groups and counterions of the surfactants and to the OH groups of the alcohol, with relaxation regions centered in the range 2-4 GHz and amounts involved inferred from magnitude of the relaxation observed at sufficiently higher frequencies to be attributed to bulk water. We have found no good explanation for the difference between our finding of two quite distinct relaxations, as contrasted with the broad structureless relaxation found by Foster et al. in their systems. Further studies varying relative composition as well as amount of emulsifier could help clarify the explanations. Our results for water in oil emulsions are less definitive for the relaxation of water in the droplets for two reasons: the effects are expected from mixture formulas to be relatively considerably smaller and the precision of the present data is not as good because the cell used was better suited for measurement of much higher permittivities. Further measurements with better sensitivity would be desirable for the systems already studied and for emulsions with added salt to obtain information about counterion polarization processes associated with the interfacial layer in both o/w and w/o systems.
(24) Foster, K. R.; Epstein, B. R.; Jenin, P. C.; Mackay, R. A. J . Colloid Inrerfaee Sei. 1982, 88, 233. (25) Eptein, B. R.; Foster, K. R.; Mackay, R. A. J . Colloid Interface Sei. 1983, 95, 218. (26) For a review of ion atmosphere polarization, see: Mandel, M.; Odijk, T. Annu. Rev. Phys. Chem. 1984, 35, 7 5 .
Acknowledgment. G. Delbos thanks NATO organization for the grant of a fellowship which permitted him to work on this project at Brown University. Other support was provided by Grant CHE-7822209-A from the National Science Foundation and by Brown’s Materials Science Laboratory, also funded by NSF. Registry No. SDS, 151-21-3; 1-butanol, 71-36-3; toluene, 108-88-3.
Time Resolution Enhancement Technique Applied to a Study of the Absolute Rate of Reaction of Ketyi Radicals with a Spin Trap Using Flash Photolysis Electron Paramagnetic Resonancet Tak-Ming Chiu, Aleksander Siemiarczuk, S. King Wong,* and James R. Bolton* Photochemistry Unit, Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 5B7 (Received: November 27, 1984)
In electron paramagnetic resonance (EPR) measurements of the transient kinetics of photochemically produced radicals the measured output of the detection system is a convolution of the exciting pulse profile and the instrumental response with the undistorted kinetic response of the chemical system under study. However, the latter can only be obtained if the exciting pulse is a 6 function and the detection system has an infinite bandwidth. We have chosen as a test system the spin-trapping reaction of acetone ketyl radicals by the cyclic spin trap 5,5-dimethyl-l-pyrroline1-oxide(DMPO). In this study the measured EPR signal has k e n deconvolved by using an assumed “analytical”spectrometer response profile which was found qualitatively to agree well with the experimental profile obtained separately. The absolute bimolecular trapping rate constant obtained at 100-kHz field modulation (time resolution -10 ps) was found to be (1.2 f 0.1) X lo* M-I s-’. This compares favorably with the value of (1.1 & 0.1) X lo8 M-’ s-’ obtained at 2-MHz field modulation (time resolution -1.5 ps) where signal distortion is minimal.
Introduction In recent years the development of transient electron paramagnetic resonance (EPR) spectroscopy has been stimulated by progress in the area of chemically induced dynamic electron polarization (CIDEP).’ Thus kinetic studies have been carried out with conventional EPR spectrometers a t 100-kHz field modulation (response time 10 ~ s (the ) ~time resolution of our
-
Contribution No. 000,Photochemistry Unit, Department of Chemistry, The University of Western Ontario.
-
EPR spectrometer as received is 200 ps, but it can be modified to give an overall response time of -10 p ~ ) . Further ~ improvements in response time can be made by modifications in the (1) Muus, L. T., Atkins, P. W., McLauchlan, K. A., Pedersen, J. B., Eds. “Chemically Induced Magnetic Polarization”;D. Reidel: Dordrecht, Holland, 1977.
(2) McIntosh, A. R.; Manikowski, H.; Bolton, J. R. J . Phys. Chem. 1979, 83, 3309. (3) (a) McIntosh, A.; Bolton, J. R. J . Magn. Reson. 1978, 32, 167. (b) The response time has been further improved to about 10 ws: McIntosh, A., Bolton, J. R., unpublished results.
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3344 The Journal of Physical Chemistry, Vol. 89, No. 15, 1985
Chiu et al.
-
-
detection system, such as with 2-MHz field modulation4 (response time 1.5 ps) or direct detectionS (response time -400 ns). Of these three detection systems, the 100-kHz field modulation detection system affords the best signal-to-noise ratio; however, its modulation frequency puts a limit of 10 ps on the response time for the measurement of kinetic processes. With the 2-MHz field modulation or the direct-detection mode, better response times are achieved, but at the expense of signal-to-noise ratio. The kinetic response of any detection system is a convolution of the pulse profile and instrument response with the true kinetic response of the chemical system under study. Hence, if the kinetic response is comparable to the instrument response time, the observed response will be distorted and generally rate constants will be underestimated. It is thus desirable to seek a method by which the true kinetic response can be recovered. Deconvolution techniques6 have been used extensively for some time in improving time resolution in fluorescence lifetime measurements. Several methods are available in the literature and have been reviewed recently.16 In this paper, we explore the application of such deconvolution techniques to flash photolysis electron paramagnetic m n a n c e (FPEPR) spectroscopy; however, the numerical procedures we have developed should be applicable to any transient kinetic response in digital form (e.g., from an optical flash photolysis apparatus). As an example we chose a simple spin-trapping reaction R. ST ---* S A where a transient radical R. is allowed to react with a spin trap (ST) to form a relatively persistent spin adduct which can be detected by EPR. The subject of spin trapping has been r e v i e ~ e d . ~ Under favorable circumstances, the structure of the radical Rcan be identified from the hyperfine splitting constants of the spin adduct. The spin-trapping technique is potentially very useful in elucidating reaction mechanisms when transient-free radicals are involved. In particular, it is important to have available kinetic rate constants for a wide variety of spin traps with a large number of radicals. For example, Ingolds and co-workers have undertaken an extensive study of trapping primary and secondary alkyl radicals with a variety of spin traps. Earlier Janzen7 and co-workers and Perkinsg and co-workers obtained trapping rate data based on competition reactions with measured rate constants that are sometimes uncertain. This uncertainty is thus transferred to the trapping rate constants obtained by such relative methods. Nevertheless, such studies are useful in providing a relative estimate of the trapping efficiencies of various spin traps provided the spin adducts are relatively long-lived. In this study, we chose the simple photochemical reaction of acetone with 2-propanol (reactions 1 and 2) which produces a
1 mT
n
-
+
R
9
II
II CH3CCH3
CCH3CCH31"
(1)
(2) ?H
CH3LCH3
+
DMPO spin trap
SA
(3)
spin adduct
(4) (a) Wong, S.K.; Chiu, T. M.; Bolton, J. R. J. Phys. Chem. 1981,85, 12. (b) Hore, P. J.; McLauchlan, K. A. Reo. Chem. Intermed. 1979, 3, 89. (5) Wong. S. K. J. Mugn. Reson. 1982, 47, 500. (6) Ware, W. R. In 'Creation and Detection of the Excited State"; Ware, W. R., Ed.;Marcel Dekker: New York, 1974; Vol. 1, Part A, Chapter 5. (7) (a) Janzen, E. G.; Evans, C. A.; Davis, E. R. In "Organic Free Radicals", Pryor. W. A. Ed.; American Chemical Society: Washington, DC, 1978; ACS Symp. Ser. No. 69, Chapter 26. (b) Janzen, E. G. In "Free Radicals in Biology"; Pryor, W. A., Ed.;Academic Press: New York, 1980; V O ~4, . pp 115-154. (8) (a) Schmid, P.; Ingold, K. U.J . Am. Chem. Soc. 1977,99,6434. (b) Schmid, P.; Ingold, K. U. Ibid. 1978, 100, 2493. (c) Maeda, Y.; Ingold, K. U. J. Am. Chem. Soc. 1979, 101, 4975 and references therein. (9) (a) Perkins, M. J.; Roberts, B. P. J. Chem. Soc., Perkin Trans. 2 1974, 297. (b) Perkins, M. J.; Roberts, B. P. Ibid. 1975, 77.
F w e 1. EPR spectrum of the ketyl spin adduct of DMPO in a mixture of acetone and 2-propanol in water (1:1:2); uN = 15.4 G, uH = 24.3 G; microwave power = 10 mW, modulation amplitude = 0.16 G, gain = 1.25 X 10'.
single radical product, the ketyl radical, which is then trapped by the spin-trapping reaction (reaction 3) where the spin trap in this case is 5,5-dimethyl-l-pyrroline1-oxide (DMPO). On
. I
0DMPO
photolysis, ground-state acetone molecules are converted to excited singlets which intersystem cross to the corresponding triplets (reaction 1). The excited triplets then abstract a hydrogen atom from 2-propanol (reaction 2) to give acetone ketyl radicals. Reaction 3 completes the addition step of the ketyl radicals to DMPO where the spin adduct (SA) is
The spin adduct is a relatively stable free radical and does not decay significantly during the kinetic period of interest. k3 is the rate constant of interest in this study. This reaction is essentially pseudo first order since DMPO is present in excess. The bimolecular rate constant can thus be determined via FPEPR on an absolute basis by varying the concentration of DMPO. Experimental Section A Varian E-12 X-band spectrometer was used with 100-kHz field modulation, modified to give an instrument rise time of 10 ps for transient studiesa3 For experiments performed a t 2-MHz field modulation, the instrument response time is 1.5 ps.& The flash system consists of a pulsing unit (EG & G, PS-302) and flashlamp (EG & G, FX-265) and generates a light pulse with full width at half-maximum (fwhm) of 1.5 ps. All sample handlings were performed under dim red light. Nitrogen gas was bubbled through all solutions before and during the experiment. An appropriate concentration of the spin trap, DMPO (5,5-dimethyl- 1-pyrroline 1-oxide, Aldrich), was prepared in a solvent consisting of acetone (Fisher spectrograde), 2-propanol (Fisher spectrograde), and doubly distilled water in a 1:1:2 ratio by volume acidified with 12 M HCl(1 mL per 100 mL of aqueous mixture). The sample solution was flowed slowly, by means of a peristaltic pump [Cole Parmer Masterflex (C7520-00)], through a Varian EPR flat cell (S-802) in a Varian EPR TM110 (Model E-238) cavity. On photolysis, an EPR spectrum of six lines (see Figure 1) assigned to the ketyl adduct of DMPO (aN = 15.4 G, uH = 24.3 G) was obtained. An earlier reportlo of this adduct in benzene gave similar hyperfine splittings (aN = 14.58 G , uH = 23.91 G). This is consistent with previous findings" that nitrogen hyperfine splitting constants of nitroxyl radicals decrease in nonpolar solvents. For the kinetic studies, the transient EPR signals were captured by a Nicolet Model 2090 I11 transient recorder with 8 bits of
-
-
-
(10) Janzen, E. G.; Liu, J. I.-P. J . Mugn. Reson. 1973, 9, 510. (1 1) Kawamura, T.; Matsunami, S.;Yonezawa, T.; Fukui, K. Bull. Chem. SOC.Jpn. 1965, 38, 1935.
The Journal of Physical Chemistry, Vol. 89, No. 15, 1985 3345
Rate of Reaction of Ketyl Radicals
7.6 ps
A
6
Figure 2. Response profile R(r) for (a) 100-kHz detection system with R ( f )= (r2/B2)e-’/B’;B = 13.8 pus;fwhm = 15.9 p (trace A). (b) 2-MHz detection system, B = 1.9 p,fwhm = 2.3 p s (trace B).
dynamic resolution and a minimum dwell time of 0.5 ps per point. Each digitized kinetic trace was then transferred to a Nicolet Model 1180 computer for signal averaging. The averaged signal traces were stored on magnetic disk (Nashua 0015-8) for subsequent signal processing on the computer.
Results and Discussion I . Determination of the EPR Spectrometer Response Function and Flashlamp Profile. The measured EPR kinetic response is the convolution of three functions: the instrument response of the spectrometer, the flashlamp profile, and the ketyl radical production kinetics. This latter step (reaction 2), which follows the flash, is very fast (see below) compared to the addition of ketyl radical to DMPO (reaction 3). We show later that in most cases the kinetic rise of the ketyl radicals has a negligible influence on the measured kinetic response. One can denote a composite function R ( t ) which includes the effects of the instrument response and the flashlamp profile. The detected EPR kinetic response S ( t ) is thus a convolution of the kinetic response F(t) of the chemical system and R ( t ) S ( t ) = J ‘0F ( t ) R ( t - t’) dt’
(4)
For the present study
F(t) = A( 1 k3‘ = k,[DMPO] = 1 / ~ ~A ; = constant
(5b)
Initially we attempted to determine R ( t ) by assuming an expansion into a series of Gaussian functions without any prior assumption of the shape. However, this was unsuccessful due to an inadequate signal-to-noise ratio in the kinetic data. After some trials we chose the function
R ( t ) = (t2/B2)e-ff/Bf
(6)
to describe the response function where B is an adjustable parameter. The optimum value of B was determined by using the following spin-trapping reaction hu
+ DMPO
2.OH
-
+
kd
that is used to fit to the measured EPR time profile of reaction 8. The parameter B is then adjusted so as to give the best fit to this measured profile. The fwhm of R ( t ) are found to be 15.9 and 2.3 for 100- and 2-MHz systems, respectively (Figure 2a,b). This is expected, since the 100-kHz system has a much longer response time. At a high concentration of DMPO ([DMPO] = 0.1 M), the kinetic profile obtained from reaction 8 represents the integrated composite profile R(t) (Figure 3a). This is the result of combining eq 4, 6, and 9 with kS’ = k5[DMPO] = (4.3 X 109)0.1 s-l = 4.3 X lo8 s-l and T = 0.693(k5’)-l = 1.6 ns. F(t) is thus effectively a unit step function of magnitude A and hence S ( t ) becomes
S ( t ) = A J r0 R ( t - t’) dt’= A S 0‘ R ( t ) dt
(5a)
where
H202
Figure 3. (a, top) Formation kinetics S(r) of the OH adduct of DMPO at [DMPO] = 0.1 M in 1% H202 obtained at 100-kHz field modulation. (b, bottom) Differential of the formation curve in a. The solid line is given by (t2/B2)e-f2/B’ with E = 13.8 ws.
(7)
so that 1 Wt) 1 R ( t ) = - -- ,Sf(t) A dt
Thus R(t) can be obtained by differentiating S ( t ) . This is shown in Figure 3b. It can he seen from this figure that the analytical expression 6 fits the differentiated curve S f ( t )rather well. Once the profile R ( t ) has been determined it is then convoluted with A(l - e-k3’r)to give the best fit to the kinetic results by a nonlinear least-squares pr0~edure.l~The convolution of F(t) with R ( t ) is performed by numerical integration with some initially chosen value of 7 3to give Y(t). In each iteration 73 in incremented in such a way as to minimize the expression X’ = E [ Y ( t j )- s(tj)12 i
is then convoluted with the composite profile eq 6 to yield a curve
2. Ketyl Radical Spin Trapping Kinetics. With the assumed reaction scheme (reactions 1-3) acetone ketyl radicals are generated on photolysis via hydrogen abstraction by the excited triplet acetone from 2-propanol. Under the conditions of our experiment the concentration of 2-propanol is 3.2 M so that the production of acetone ketyl radicals is pseudo first order. The hydrogen abstraction rate constant k2 (1 X lo6 M-’ s-l ) has been measured in an optical flash-photolysis
(12) Neta, P.;Steenken, S.; Janzen, E. G.; Shetty, R. V. J. Phys. Chem. 1980, 84,532.
(1 3) Bcvington, P. R. “Data Reduction and Error Analysis for the Physical Sciences’’; McGraw-Hill: New York, 1969.
.OH
spin adduct
(8)
where k5 = 4.3 X lo9 M-’ s-l as determined by pulse radiolysis.’2 Reaction 8 is pseudo first order at high concentrations of DMPO. The kinetic expression A(l - e-ks’t); k5’ = k5[DMPO]
(9)
3346 The Journal of Physical Chemistry, Vol. 89, No. 15, 1985
Chiu et al.
Q
1.0
0.5
20
1.5
3 [DMPOIXIO M
Figure 5. 100-kHz field modulation experiments: plot of S-I) (X) or ( T ~ , ~ ) - I (0) vs. [DMPO].
-
(X
(l/s,)Xro52
a3 VS
I
l6
1.2
1.4
t
t
Figure 4. Formation curves of the acetone ketyl radical-DMPO spin adduct at [DMPO] = 1.0 X lo-) M obtained at (a, top) 100-kHz field
modulation (curves A and B are respectively the fitted curves with and without reconvolution) and (b, bottom) 2-MHz field modulation. Curve C is the fitted curve after reconvolution. TABLE I: Rise Times of the Formation Curves (at 100-Wz Field Modulation) of the Acetone Ketyl-DMPO Spin Adduct at Different Concentrations of DMPO
[DMPOI, M
2.0 X 1.5 x 10-3 1.3 X 1.0 X 0.75 X lo-' 0.5 X lo-'
73, MS
73,cr
10.8 9.9 10.8 11.5 13.0 17.2
15.1 14.0 14.2 15.4 15.5 19.5
(1/73)9
x 10-5
M~
S-I
0.93 1.oo 0.93 0.87 0.77 0.58
0.66 0.71 0.70 0.65 0.65 0.51
study.14 Hence, the ketyl radical production kinetics has a half-time rise of -0.2 ps. After approximately five half-times (- 1.0 ps), the production of ketyl radicals is essentially complete and can thus be considered almost "instantaneous" if a 6 pulse is applied. The experimental response profile R(t) is thus largely determined by that of the EPR spectrometer and the flashlamp profile. The bimolecular rate constant of spin trapping k3 can be determined by monitoring the formation of the spin adduct S A as outlined in the following rate equations:
- d[(CH3)2C0H1 --- = d[SA1 dt
dt
k3[ (CH3)&OH] [DMPO]
[SA] = A( 1 -
k3' = k3[DMPO],
T~
= (k3')-'
At high concentrations of DMPO, reaction 3 is pseudo first order. Figure 4a shows a typical rise curve of the spin adduct M at 100-kHz field modulation. Each at [DMPO] = 1.0 X such kinetic trace was analyzed as outlined earlier. Table I shows the kinetic rise times ( T ~ obtained ) at 100-kHz field modulation and at various concentrations of the spin trap. A nonlinear least-squares fit of the experimental rise C U N ~ S gives the rise times 4) Porter, G.;Dogra, S.K.;Loutfy, R. 0.;Sugamori, S. E.; Yip, R. W. 'rem.Soc. Faraday Trans. 1 1973, 1462. 5) Greenstock, C. L.;Wiebe, R. H. Can. J . Chem. 1982, 60, 1560. 6) See,for example: McKinnon, A. E.;Szabo, A. G.;Miller, D. R. J. Chem. 1977, 81, 1564. OConnor, D.V.;Ware, W. R.; Andre, J. C . bys. Chem. 1979, 83, 1333. I.
as
0.0
(1/73c),
x io-$ S-1
1.0
1.5
[DMPOIXlO M
Figure 6. 2-MHz field modulation experiments: plot of (74-I (XlO-' s-l) vs. [DMPO] with slope = (1.1 f 0.1) X lo8 M-I s-l. TABLE II: Rise Times of the Formation Curves (at 2-MHzField Modulation) of the Acetone Ketyl-DMPO Spin Adduct at Different Concentrations of DMPO
[DMPO], M
73,
MS
Ms
1.5 X 1.2 X 1.0 X lo-' 7.5 X lo4 5.0 X lo4 4.0 X lo4
6.3 8.7 8.5 10.4 13.3 17.4
6.6 9.1 8.8 10.7 13.5 17.7
7 3 , ~ ~
(1/73),
(1/73c)r x10-5'~-1
1.59 1.15 1.18 0.96 0.75 0.57
1.52 1.10 1.14 0.93 0.74 0.56
x 10-5 s - ~
( T j P ) which are longer than the rise times ( T ~ obtained ) from the deconvolution procedure. This is a direct consequence of the effect of the instrument response and light pulse on the formation kinetics of the spin adduct. Even at relatively low concentrations of the spin trap, when the formation of the spin adduct is slow, the rise curves are distorted and the rise times level off. This is shown in Figure 5 where the circles (0)represent the plot of (r3,J1vs. [DMPO]. In contrast, the plot of ( T J ' vs. [DMPO] represented by the crosses (X) is less distorted and does not level off until a higher concentration of DMPO. A tangent a t the origin to the curve gives a slope equal to the bimolecular trapping rate constant of the acetone ketyl radicals by DMPO. This is found to be (1.2 f 0.1) X lo8 M-I s-'. To provide a more stringent test, the experiments were repeated at 2-MHz field modulation where the response time is 1.5 ps. Figure 4b shows a typical rise curve obtained at 2-MHz field modulation with [DMPO] = 1.0 X lW3 M. Table I1 summarizes the results. It is seen that the T ~ ' Sand 73,:s are quite similar, in contrast to the large difference between them for the 100-kHz system. The close agreement between T3 and T3,c)S is perhaps not unexpected, bearing in mind that the instrument rise time (- 1.5 w s ) a t 2-MHz field modulation is -7 times faster than that of the 100-kHz system. In other words, the spin-trapping reaction
-
J. Phys. %hem. 1985, 89. 3347-3352 at the highest concentration of DMPO used (1.5 X M in our experiment) is not sufficiently fast enough for the deconvolution procedure to effect a significant difference between the T ~ ' Sand q ; s when the response time of the detection system is not as yet approached. Figure 6 is a plot of ( ~ ~ ) -vs. l [DMPO]. The slope of the straight line through the origin gives the bimolecular trapping rate constant k3 as (1.1 f 0.1) X lo8 M-' s-'. This is in excellent agreement with the results obtained at 100-kHz field modulation. DMPO has been shown to be more efficient than the,acyclic spin trap phenyl-N-tert-butylnitrone(PBN). Thus InggldEand co-workers have determined the absolute trapping rate constants of primary and secondary alkyl radicals by DMPO in benzene at 40 OC to be 26 X lo5and 4.2 X los M-' s-', respectively. For PBN, the corresponding absolute trapping rate constants are 1.3 X lo5 and 0.68 X los M-' s-l, respectively. Greenstock and Wiebels have determined by pulse radiolysis the trapping rate constant of acetone ketyl radical by PBN to be 1.0 X lo7 M-' s-'. Our measured value of 1.1 X lo8 M-' s-' for trabping of acetone ketyl radicals by DMPO is 10 times larger than the corresponding rate constant for trapping of acetone ketyl radicals by PBN. This exhibits the same trend as that found for the previously mentioned reactions studied by Ingold.E
-
3347
Conclusions In this time-resolved EPR study of a spin-trapping reaction, the deconvolution procedure seems able to recover reasonable rise times from highly distorted kinetic profiles. The results obtained at 100-kHz field modulation compare favorably with those a t 2-MHz field modulation. With a careful choice of the form of R(t) one can measure reliably kinetic rate processes with rise times which are comparable to the instrument response time of the EPR spectrometer. Although we have developed and applied this method for FPEPR kinetic studies, the general method should be applicable to the kinetic response of any detection system.17 Acknowledgment. This work was supported financially by an Operating Grant from the Natural Sciences and Engineering Research Council of Canada. Registry NO. CH$(=O)CH,, 67-64-1; CH,CH(OH)CH3,67-63-0; DMPO,3317-61-1; SA, 74670-74-9; CH,C(OH)CH3, 5131-95-3. (17) A referee has pointed out that a similar treatment to correct for the effects of the pulse profile and instrumental characteristics has been applied to pulse radiolysis kinetic profiles.'* (18) Janata, E.; Schuler, R. H. J. Phys. Chem. 1982,86,2078. Scaiano, J. C. J . Phofochem. 1981, 16, 71.
Gas-Phase 'H NMR Studies of Keto-Enol Tautomerism of Acetylacetone, Methyl Acetoacetate, and Ethyl Acetoacetate Michael M. Fokendt, Boris E. Weiss-Lopez, J. Paul Cbauvel, Jr., and Nancy S. True* Department of Chemistry, University of California, Davis, California 9561 6 (Received: November 27, 1984)
The effect of intermolecular interactions on the keto-enol tautomeric equilibria of acetylacetone, ethyl acetoacetate, and methyl acetoacetate has been studied by using 'HNMR spectroscopy. For all three molecules gas-phase spectra obtained in the 445-373 K temperature range are consistent with equilibrium constants which favor the enol tautomer to a larger extent than is found in condensed phases at corresponding temperatures. Temperature-dependent intensity ratios yield the following gas-phase relative enthalpies (W(eno1-keto), kcal/mol): -4.66 (18), -3.17 (24), and -3.04 (59) for acetylacetone, ethyl acetoacetate,and methyl acetoacetate,respectively. The correspondingdifferencesin neat liquid samples are 2-3 kcal/mol smaller. For all three molecules in all phases the keto tautomer is favored entropically (ASo(enol-keto) is -8 cal/(mol K)). These results are discussed in terms of structural differences and dielectric effects.
Introduction For gaseous molecules, N M R spectroscopy can provide high quality thermodynamic parameters characterizing equilibria in suitably selected cases.' Comparison with similarly obtained condensed-phase data yields a straightforward measure of the direction and extent of solvent effects on both relative enthalpies and relative entropies of the associated molecular forms. The present study reports gas-phase thermodynamic data for the keto-enol equilibria of acetylacetone (2,4-pentanedione) (R, =
R2 = CH,), methyl acetoacetate (R, = CH3, R2 = OCH,), and ethyl acetoacetate (R, = CH3, R2 = OC,H,). The high energy bamer to tautomer interconversion permits observation of distinct N M R spectra of both forms at temperatures well above ambient thus allowing quantitative gas-phase thermodynamic data to be obtained for the first time for these relatively nonlabile molecules. In each case a comparison with condensed-phase data reveals significant phase-dependent relative energy and relative entropy (1) Chauvel, J. P., Jr.; True, N. S. J . Phys. Chem. 1983,87, 1622-1625.
0022-3654/85/2089-3347$01.50/0
differences between the keto and enol tautomers. The keto-enol equilibrium of acetylacetone has been well characterized in solutions by using 'H N M R spectroscopy. The relative thermodynamic parameters are found to be both solvent and concentration dependent. Recent temperature-dependent studies of acetylacetone in seven solvents in the range 273-353 K demonstrated that in all cases the enol form is favored energetically (AHo298(E-K) ranged from -1.8 (Me2S0 solution) to -3 kcal/mol (cyclohexane solution)) and disfavored entropically (hS0298(E-K) ranged from -4 (cyclohexane solution) to -6 cal/(mol K) (neat liquid)).2 These results are consistent with several earlier N M R studies of equilibria in a~etylacetone.~-~ In contrast to the liquid-phase system, only semiquantitative data concerning the gas-phase keto-enol equilibrium of acetylacetone exists. Spectroscopic&' and electron diffracti~n'~J~ studies have, (2) Spencer, J. N.; Holmboe, E. E.; Kirshenbaum, M. R.; Firth, D. W.; Pinto, P. B. Can. J . Chem. 1982, 60, 1178-1182. (3) Reeves, L. W. Can. J . Chem. 1957, 35, 1351-1365. (4) Burdett, J. L.; Rogers, M. T. J . Am. Chem. SOC.1964, 86, 2105. (5) Rogers, M. T.; Burdett, J. L. Can. J . Chem. 1965, 43, 1516-1526. (6) Powling, J.; Bernstein, H. J. J. Am. Chem. Soc. 1951, 73,4353-4356. (7) Jarrett, H. S.; Sadler, M. S.;Shoolery, J. N. J. Chem. Phys. 1953.21, 2092-2093. (8) Harris, R. K.; Rao, R. C. Org. Magn. Reson. 1983, 21, 580-586. (9) Funck, E.; Mecke, R. In "Hydrogen Bonding"; Hadzi, D., Ed.; Pergamon Press: London, 1959; p 433.
0 1985 American Chemical Society
