2186
The Journal of Physical Chemistv, Vol. 83, No. 17, 1979
I 0 Beta
PuFG Pressure, torr
Figure 9. Correlation of a and
p radiolysis
results.
smaller initial PuF6 pressure. This agrees with the trend for k, found in the present work (Table 111). Figure 8 indicates the excellent correlation of the data with the rate law for a first-order reversible mechanism. Values for G(-PuF,) calculated from the values for k f in Table VI1 were 1.7 at 50 torr and 1.6 at 100 torr. These are within experimental error of the average value for G(-PuF6) for a radiolysis found in present study even though the fractions of a energy absorbed by the gases are different and the dose rates are very different. The fraction of energy absorbed by the PuF6 in the 0.13-L vessel was 0.33 at 100 torr and 0.15 at 50 torr compared to 0.60 and 0.33 for the 1.09-L vessel. These fractions along with values for C(P)for the two experiments in the 0.13-L vessel were calculated by the method described perviously. The a flux for 93% 239Pu(Table I) was used to calculate the dose rate. The initial dose rates of 6.32 X eV/ (torrnday) at 100 torr and 2.87 X lOI7 eV/(torr.day) at 50 torr were used to calculate the G values. These dose rates are 25 to 300 times less than those in the present study. The agreement in the G values from the two studies indicates that nonradiolytic thermal decomposition processes were also not significant in the experiments of Steindler and c o - ~ o r k e r s . ~ Comparison of a and p G Values. Values for G(-PuF6) for a and radiolysis are plotted vs. initial PuF6 pressure in Figure 9. Although the results are somewhat scattered, no significant effect of pressure is indicated. Values for G(-PuF6) for fl radiolysis are consistently lower than those for CY radiolysis. We conclude that this difference does not result from an error in calculating the amount of a and fl energy absorbed. The reason for the difference is not apparent since LET effects are not expected to be significant at these pressures. Perhaps in the a radiolysis, there is an enhanced decomposition at the wall since most
D. J. Nelson
of the a radiation was absorbed at the wall.
Acknowledgment. I thank three scientists at the Savannah River Laboratory for their contributions to this work. M. K. Jones prepared the PuF6 that contained plutonium with 4.8% 238Pu. L. M. Arnett and F. E. Driggers developed the computer programs for calculating the a and energy absorbed. M. J. Steindler of Argonne National Laboratory is thanked for his comments on the manuscript. The information contained in this article was developed during the course of work under Contract No. AT(07-2)-1 with the US.Department of Energy. References and Notes B. Weinstock, E. E. Weaver, and J. G. Malm, J. Inorg. Nucl. Chem., 11, 104 (1959). For a discussion of this process, J. M. Cleveland, "The Chemistry of Plutonium", Gordon and Breach Science Publishers, New York, 1970, p 505. A. E. Florin, I. R. Tannenbaum, and J. F. Lemons, J . Inorg. Nucl. Chem., 2, 368 (1956). C. J. Mandleberg, H. K. Rae, R. Hurst, G. Long, D.Davies, and K. E. Francis, J . Inorg. Nucl. Chem., 2, 358 (1956). 8. Weinstock and J. G. Malm, J. Inorg. Nucl. Chem., 2, 380 (1956). M. J. Steindler, D. V. Steldl, and J. Fischer, J . Inorg. Nucl. Chem., 26, 1869 (1964). R. P. Wagner, W. A. Shinn, J. Fischer, and M. J. Steindler, USAEC Report ANL-70 13 ( 1965). R. G. Nisle and I. E. Stepan, Nucl. Sci. Eng., 39, 257 (1970). F. L. Oetting, Phys. Rev., 168, 1398 (1968). M. J. Steindler and W. H. Gunther, Spectrochim. Acta, 20, 1319 (1964). R. K. Steunenberg and R. C. Vogel, J. Am. Chem. Soc., 78, 901 (1956). S. C. Llnd, "Radiation Chemistry of Gases", Reinhold, New York, 1961, p 31. C. F. Williamson, J-P. Boujot, and J. Plcard, "Tables of Range and Stopping Power of Chemical Elements for Charged Particles of Energy 0.05 to 500 MeV", Rapport CEA-R3042, Commlsseriat a I'Energie Atomique, France, 1966. M. J. Berger, J. Nucl. Med., 12, Suppl. 5, 7 (1971). W. L. Pillinger, J. J. Hentges, and J. A. Blair, Phys. Rev., 121, 232 (196 1). W. G. Cross. Phvs. Med. Biol.. 13. 611 (19681. J. K. Dawson, R:W. M. D'Eye, and A. E. Truswell, J . Chem. Soc., 3922 (1954). W. C. Mosley, private communication. L. E. Trevorrow, T. J. Gerding, and M. J. Steindler, J . Inorg. Nucl. Cbem. Lett., 5, 837 (1969). M. J. Steindler, USAEC Report ANL-6753 (1963), p 33. C. H. Shiflett, M. E. Steidlitz, F. D. Rosens, and W. Davis, Jr., J. Inorg. Nucl. Chem., 7, 210 (1958). S.Tsujimura, K. Hirano, A. Takahashi, and G. Frijisawa, J. Nucl. Sci. Techno/.,9, 534 (1972).
CIDNP/FTNMR as a Mechanistic Probe in the Pulse Radiolysis of Aqueous Acetone and Acetaldehyde' D. J. Nelson Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received February 20, 1979) Publlcation costs assisted by Argonne National Laboratory
The utility of CIDNP/FTNMR in the study of reaction mechanism in pulse radiolysis of aqueous acetone and acetaldehyde is discussed. Using this technique, one can identify the principal radiolytic products. The high field polarization observed in these products is correlated with that expected from reactions of the radical intermediates most likely present in the solution, enabling identification of important reaction pathways. In the pulse radiolysis of aqueous solutions having less than molar solute concentrations, the ionizing radiation interacts exclusively with water molecules, yielding the primary radicals e&, .OH, and .H [GD,o(pH3-13) = 3.0, 0022-3654/79/2083-2186$01.00/0
2.8, and 0.4, respectively] ,z These radicals subsequently react with the solute and/or with selective scavengers ' . which may be present in solution, such as N20 or H In many instances kinetic data regarding rates of solute 0 1979 American Chemical Society
Pulse Radiolysis of Acetone and Acetaldehyde
radical formation and reaction, spectroscopic evidence for radical intermediates, and product analyses are available. Such information has permitted the development of general mechanisms of the pulse radiolytic reactions of the aqueous solutes. However, details concerning these reaction mechanisms may be lacking. CIDNP, which monitors the polarized diamagnetic products of radical reactions, has been used extensively to investigate free-radical and photochemical reaction^.^ One aspect of our study of pulse radiolysis has been to develop CIDNP/flow FTNMR as a technique which can provide analogous mechanistic details about pulse radiolytic reactions. To this end kinetic data for reactions of the aqueous primary radicals with organic solutes are used qualitatively to predict ratios of the significant radicals formed by radiolysis as these change with the reaction conditions. In turn, proposed interactions and reactions of these radicals may be examined in light of the consistency with which such proposed reactivity predicts both the products of radiolysis and their polarization as observed by FTNMR spectroscopy. Study of the field dependence of the CIDNP is particularly useful in this regard because the mechanisms of product polarization at low fields are less restrictive than those at high fields. Therefore, the most complete polarized product identification may be achieved with low field spectra, whereas the changes in product polarization from lower to higher fields may provide details of radical-radical interactions. This method is used here in the study of the mechanisms of aqueous acetone and acetaldehyde pulse radiolyses. At present only a qualitative assessment of trends is described because of some limitations inherent in using a flow system of this design. For example, relatively high solute concentrations are required to avoid wide fluctuations in concentration caused by evaporation of volatile solutes during experiments. The integrated intensities which are reported for the respective polarized products are reproducible (&lo%). These values reflect the difference in integrated intensities between nonirradiated (dark) and irradiated (light) spectra and have not been corrected for relaxation. Acetone. Because of its usefulness as an electron scavenger in pulse radiolysis,2 the radiation chemistry of aqueous acetone is quite comprehensively described. In neutral or basic N2-saturated solution acetone reacts rapidly with eaq- (k N 6 X lo9 M-l s-l) a to give the 2propanol radical ion, (CH,),CO- (pK, E Reaction of acetone with .OH, the second aqueous primary radical which is formed in high yield, is slower ( h = 9 X lo7 M-l and yields the acetonyl radical, .CH2COCH3. The reactions of the hydrogen atom, H., with aqueous acetone are restricted both by the low yield of this aqueous primary radical (except at pH 1-2, see below) and by the slowness of its reactions; abstraction ( h = 1 X lo6 M-' s-' ) gives the acetonyl radical, and reduction (k N 3 X lo5M-' s-l) gives the 2-propanol radical.4c Thus, in neutral or basic N2saturated solution, irradiation of aqueous acetone should give predominately the 2-propanol radical and the acetonyl radical in roughly equal ratio. The former radical is detected by EPR spectroscopy6 and is the nominal precursor to the radiolytic product formed in highest yield, 2-propanol.' That the acetonyl radical is present is suggested by the formation of 2,5-hexanedione, a second product formed in lower yie1d.I (Low yields of hydroxyacetone have also been reported, but the mechanism of its formation has not been adequately d e ~ c r i b e d . ) ~ Saturation of irradiated aqueous solutions with N20, a technique used to study the effects of electron scavenging
The Journal of Physical Chemistry, Vol. 83, No. 17, 7979 2187
and increased .OH yield,2 should have only small effects in radiolysis of aqueous acetone solutions with concentrations exceeding M. (The rate constant for N20 reaction with e,; is comparable to that of acetone,4abut its solubility is only 0.03 M. Thus, at higher concentrations acetone becomes a better ea; scavenger than NzO.) However, in the radiolysis of acidic aqueous acetone solutions (pH 1-2) ea; reaction with H+ ( h = 2 X 10" M-' s-1)4cwill be competitive and will reduce the ratio of the 2-propanol radical to the acetonyl radical and increase the yield of -H. Acetaldehyde. Despite its structural similarity to acetone, the radiation chemistry of aqueous acetaldehyde shows some significant differences. The fact that in aqueous solution the concentrations of acetaldehyde and its hydrate, CH3CH(OH)2,are equal (Keq= 1.06)8cannot be disregarded. The other significant difference between these two carbonyl compounds lies in their reactivity with the aqueous primary radicals. In neutral or basic N2saturated solution reduction of acetaldehyde by e,; (k = 4 X lo9 M-l s-')~, proceeds at a rate comparable to that of acetone to give the ethanol radical ion, CH3CHO- (pK, = 11.6).5 Kinetic data are not available for the hydrate, but it should parallel other alcohols in showing a lack of Acetaldehyde reactivity toward ea; (k < lo5 M-' reacts more rapidly with .OH and .Hthan does acetone. ~ Hydrogen abstraction by .OH (k N 5 X lo* M-' s - ' ) ~yields the acetyl radical. (Hydrogen abstraction from the methyl group of acetaldehyde has not been documented but would not be expected to occur substantially faster than the rate constant of los reported for acetone.) The hydrate probably competes effectively for .OH (k N lo9 M-' s-l for chloral hydrate, for example.)4bReduction of acetaldehyde by -H(k N 6 X lo6 M-l s-l) gives the ethanol radical, and 3 X lo7 M-l s-' ) the acetyl hydrogen abstraction ( h radical.4c Hydrogen abstraction from the hydrate by .H should proceed at a rate comparable to that of other alcohols or ethers ( k = lo7 M-' ~-l).*~ The ethanol radical and the acetyl radical are detected by EPR spectroscopy: but kinetic data imply that the important radicals present in solution in neutral or basic N2-saturatedsolutions. should be the ethanol radical, and the hydrate radical CH,C(OH),, with the acetyl radical being of somewhat less importance. Once again saturation with N 2 0 of acetaldehyde solutions with concentrations exceeding M should only slightly alter the relative ratios of radicals present (favoring formation of the acetyl and hydrate radicals). As above, in acidic solution H+ will more effectively compete for e,; and will decrease the ratio of the ethanol radical and increase the yield of .H. Ethanol is the radiolytic product formed in highest yield in aqueous acetaldehyde radiolysis.'0 Lower yields of acetoin and acetic acid have been reported. A Cannizzaro reaction of the hydrate has been proposed to yield ethanol and acetic acid.lob Experimental Section Acetaldehyde and acetone (reagent grade) were used as obtained from the Aldrich Chemical Co. The pH of the deuterium oxide solutions was adjusted by the addition of HZSO4or NaOH. Solutions were continuously recirculated from a sample reservoir by using a flow system with transfer speeds of -50 mL min. The experimental system has been described.'
/
Results and Discussion Acetone. In the FTNMR spectra of 0.4 M aqueous acetone radiolysis (Figures 1-3) polarized acetone and
The Journal of Physical Chemistry, Vol. 83, NO. 17, 1979
2188
D. J. Nelson
TABLE I: Variations Observed with Reaction Conditions in Relative Intensities of High Field Polarization in Aqueous Acetone Radiolysis'" PH product (chemical shift, 6 )b
1.5
NZ
acetone (2.3)
6.3 -11000 - 12000 t1600/-1100 t1000/-600
- 13000 - 14000
N2O
+BOO/- 400 +700/-300
2-propanol
t2800/- 1800
f
Ha
OD
Hb
(3.9-4.1):;o
-11000 +1700/-700 t900/-700 +2400/- 2400 t2500/- 2400
+3300/-2700 +4200/-3300
t 2400/-1600 d d d d
N, N,O
('09)
11.8 - 10000
Ed >Ed
d
d, e d,e
d d
d
'" Based on differences in integrals of irradiated spectra vs. dark spectra. Intensity units arbitrary: + indicates enhanced absorption, -, emission. Intensities not corrected for relaxation. Data for multiplets presented in the order low field to high field. Emissively polarized HOD is also observed (4.96 ). Unless specifically referenced the spectral data are correlated with those reported in ref 30 and 31. Values given are intensities of strongest lines in the multiplet. Integrated intensity not available. Spectra not illustrated. e Polarization pattern not readily discerned due to lack of signal intensity and/or overlap in resonance lines,
CH3COCH3 pH 6.3
I
CH3COCH3
1
DH 1.5
I
1 6 I
6
I
5
I
4
I
3
I
I
I
2
I
0 ppm
Figure 1. CIDNP in aqueous acetone, pH 6.3. N,O: kG. N,: C, 50 G; D, 3.5 kG.
A, 50 G B, 3.5
2-propanol are seen a t all fields. In addition, 2,5-hexanedione and hydroxyacetone may be identified at low fields as minor radiolytic products. (These four products
I
I
I
I
I
1
5
4
3
2
I
0 ppm
Figure 2. CIDNP in aqueous acetone, pH 1.5. N,O: A, 50 G B, 3.5 kG. N,: C, 50 G; D, 3.5 kG.
are also those reported to be formed in highest yields in an earlier study of aqueous acetone radiolysis.)' In some experiments carried out on the radiolysis of neutral aqueous acetone solutions (spectra not illustrated) 1methylethenol, the enol of acetone, is also seen as a po-
Pulse Radiolysis of Acetone and Acetaldehyde
The Journal of Physical Chemistry, Vol. 83, No. 17, 1979 2189 CH3COCH3
TABLE 11: Qualitative Ratios of Radicals Expected t o be Present in Aqueous Acetone Radiolysis("
PH 1i.a
--(CH,),COD :.CH,COCH,:.D
PH
1 . 5 N2
9:10:2
N,O 6 . 3 N, N2O 11.8 N, N,O
8:3 1:2 1O:lO:l 1O:lO:l 1O:lO:l 1O:lO:l
a Calculated by using the relative rates of reaction with eaq-, .OH, and .H and the concentrations of reagents.
larized product a t all fields. The variations with reaction conditions in high field polarization intensity for the principal products are shown in Table I. Qualitative analysis of the kinetic data for reaction of the aqueous primary radicals with acetone (Table 11) suggests that under all reaction conditions interactions of the 2-propanol radical and the acetonyl radical should largely account both for the products formed and their polarization. Because the ratio of these radicals remains constant in neutral and basic solution, the polarization in the products of their reactions (Scheme I) should show little variation between neutral and basic solution. Saturation with N 2 0 of 0.4 M aqueous acetone solutions is predicted not to change the radical ratios and should therefore have no observable effect on the product polarization. In acidic solutions the relative ratio of .D is predicted to slightly increase and these changes may be reflected as small changes in the polarization of acetone and 2-propanol. The variations with reaction conditions in high field polarization in the principal products (Table I) show trends which are consistent with these predictions. For example, the emissive intensity of the acetone polarization, which is consistent with that predicted from interaction of the 2-propanol radical and the acetonyl radical (eq 3a), is very similar in neutral and basic solution. A significant increase in emissive intensity in acetone is observed in acidic solution, and this change is consistent with an increased contribution of a deuterium atom-acetonyl radical reaction (eq 4) in acetone formation and polarization. [That the net effect is pronounced in these two reactions is not surprising; the g factor difference in radical pairs involving the acetonyl radical (Table V) is substantial.] The acetone data in Table IV consistently show a small increase in emissive intensity with N 2 0 saturation of the solutions. This trend is probably indicative of a small percentage of eaq- scavenging by NzO, which slightly decreases the proportion of the 2-propanol radical and increases that of the acetonyl radical present under these conditions. (Such minor differences were neglected in the calculations for Table 11.) The possibility that this trend indicated the involvement of electron transfer from the 2-propanol radical to NzO ha5 also been considered. However, kinetic data for such a reaction12 suggest that this process would be too slow to effect significant changes in the proportions of radicals present. Similarly, variations in high field polarization intensity in 2-propanol (Table I) show trends which are consistent with the changes in radical ratios with reaction conditions (Table 11). The polarization intensities of the methinyl hydrogen are strikingly intense, considering that reactions are carried out in D 2 0 solution. The polarization intensities at this postion are very similar in neutral and basic solution and show a sharp decrease in acidic solution. The substantial hydrogen return to the methinyl position implies a carbon-bound source, and both this observation
I
I
I
I
I
I
6
I
5
4
3
2
I
0 ppm
Figure 3. CIDNP in aqueous acetone, pH 11.8. N,O: kG. NP: C, 50 G; D, 3.5 kG.
A, 50 G;B, 3.5
and the polarization pattern suggest significant involvement of reaction of the 2-propanol radical with a second like radical (Scheme I, eq 2a) in 2-propanol formation and polarization. Saturation with NzO of neutral or basic acetone solutions consistently results in decreased polarization intensity, which suggests again a small reduction in the ratio of the 2-propanol radical present as a consequence of some ea; scavenging by N20. The data for pH 1.6 in Table I suggest a more marked reduction in the ratio of the 2-propanol radical present is affected by acidifying the acetone solutions. We are presently unable to suggest a source for the enhanced absorption superimposed on the AE multiplet polarization of the methinyl hydrogen. The polarization in the methyl group of 2-propanol (A + AE) is consistent with that expected from interaction of the 2-propanol radical with .D (eq la). The trends in polarization intensity vs. pH are consistent with the changes in the relative ratios of these two radicals predicted.
2190
The Journal of Physical Chemistry, Vol. 83, No. 17, 7979
D. J. Nelson
Scheme I: Reaction Pathways and Predicted Nigh-Field Polarization in Aqueous Acetone Radiolysisa
tCH3COCHz’ *CHzCOCH31
---
(5)
(CH3COCH~)~6‘e
The radical pair model3 and the EPR data summarized in Table V are used in predicting the high-field polarization. Products underlined have been identified as correspondingly polarized products in Table I. No high-field polarization predicted. Identified as a polarized product at low fields.
The enol of acetone, 1-methylethenol, has been identified previously as an unstable product of aqueous acetone photolysis,15J6and the failure to observe enol polarization under most reaction conditions in this study probably reflects its facile tautomerization. Traces of acid- or base-catalyzed ketonization,l’ and the emissive intensity of the polarized enol (which is predicted in eq 3a), are probably seen as acetone polarization in the figures. Emissive polarization at low fields is observed for hydroxyacetone (4.46)18 and 2,5-hexanedione (2.9 No high-field polarization is observed in either product.20 The dione is nominally the acetonyl radical dimer (and, as such, no high-field polarization in the product is predicted). However, the reaction pathway yielding hydroxyacetone has not been adequately described. Kinetic arguments have been presented7 which suggest that it is not formed by reaction of .OH and the acetonyl radical, and that the equilibrium concentration of the enol (Keq= 2.4 X is too low for this product to result from addition of .OH to the enol (although the rate constant for such addition would be expected to be The correlation of Scheme I with the CIDNP data suggests, however, that the enol is a major product of aqueous acetone radiolysis. If so, the enol concentration in the reaction zone may greatly exceed its equilibrium concentration. Because reaction of .ODwith the other reagents present is relatively slow, hydroxyacetone formation by addition of .OH (-OD) to the relatively high concentrations of enol which apparently are present in the reaction zone (eq 1)becomes CH3
HO. t CH,=C
1
---$.
\
OD
HOCH,CCH, I OD
Re +HOCH,COCH,
(I)
more tenable. (However, the enol and other products derived from it are intensely polarized at high field. Thus, if hydroxyacetone is a product of enol reaction, the failure to observe high field polarization in hydroxyacetone is puzzling.) In summary, both product formation and high-field polarization have been satisfactorily accounted for by projections of the reactivity of the 2-propanol radical, the acetonyl radical, and, to a lesser extent, .D. The variations in high-field polarization intensity with reaction conditions correlate well with the qualitative predictions of changes in the ratios of these radicals present in solution. The observed products and their polarization suggest that
i o ;
I
;
k
5
4
;
;
I
Sppm
Figure 4. CIDNP in aqueous acetaldehyde, pH 6.3. A: “dark” spectrum. N,O: B, 50 G; C, 3.5 kG. N:, D, 50 G; E, 3.5 kG.
reactions 2a and 3a (Scheme I) are major reaction pathways and reactions la, 4, and 5 more minor. Little support has been provided for the reactions l b , 2b, and 3b. Acetaldehyde. The FTNMR spectra obtained in pulse radiolysis of 0.4 M solutions of acetaldehyde in D20 are shown in Figures 4-6. The uppermost spectrum in each of these figures is a “dark” spectrum, confirming the substantial hydration of acetaldehyde at all values of pH. (The ratio of methyl group integrals of hydrate to aldehyde is 2:l in acidic solutions, 4:l in neutral solutions, and 5:l
The Journal of Physical Chemistry, Vol. 83,No. 17, 1979 2191
Pulse Radiolysis of Acetone and Acetaldehyde
TABLE III: Variations Observed with Reaction Conditions in Relative Intensities of High Field Polqization in Aqueous Acetaldehyde Radiolysisa
DH 1.5
oroduct (chemical shift. 6 )b I
CH ,CHO
,
d d
(9.8)c Na N2Q
-2800 - 2800 d
:o
(2.3)
Nf (6.6) N,Q
2.4
d d d d d
o; (4.2) ":o (4.5)
OD
Hb
CH ,CH,OD
(3.7)
6.3 t 1308 t 1300 - 26001 t 800 - 20001 t 700 t 4001-400 t 4001- 400 t 6001- 600 t6001 500 t900/ 300d 17001 200d t 1700/- 1700 +1200/-1300 t 25001- 2600 t 18001- 1700
d d d d d d t 9001- 900
1
+ 6001- 100 + 4001- 200
"to
11.3 t 1500 t 1400 -2100 -2800
Ad/-600
t 1800/-500
t9001-700 t 38001- 200 + 1400/-40 d d -5001- 1000 - 5001- 1200 d d d d d d
d d
A/-800d CH,COCH( OD)CH,,
A/-1800" -400/A -400/A
d d
(4.3)
d d -t 2000/Ed
t 1800/Ed
Based on differences in integrals in irradiated vs. dark spectra, Intensity units arbitrary: t indicates enhanced absorption, -, emission. Intensities not corrected for relaxation, Data for multiplets presented in the order low field to high field. Ernissively polarized HOD is also observed (4.96 ). Except where specifically referenced spectral data are correlated with those reported in ref 30 and 31. Values given reflect intensities of strongest lines in the multiplet. Polarization pattern not readily discerned due to lack of signal intensity and/or overlap in resonance lines. a
"
E-
L
1
0
I
s
I
I
8
7
I
6
1
5
I
4
I
3
I
I
2
I
J Oppm
Plgure 5. CIDNP In aqueous acetaldehyde, pH 1.5. A: "dark" spectrum. N20: B, 50 G; C, 3.5 kG. N:, D, 50 G; E, 3.5 kG.
in basic solutions.) In the CIDNP spectra intense polarization in all fields is seen in acetaldehyde, vinyl alcohol, ethanol, and the hydrate (1,l-ethanediol). [The ready observation of CIDNP in vinyl alcohol, which has previously been identified as a polarized product of aqueous acetaldehyde p h o t o l y ~ i s , 2is~striking ~ ~ ~ when compared to
c
I
1
10
9
8
,
7
I
1
I
I
I
I
1
6
5
4
3
2
I
Oppm
Flgure 6. CIDNP in aqueous acetaldehyde, pH 11.3. A: "dark" spectrum. N,: B, 50 G C, 3.5 kG. N,: D, 50 G; E, 3.5 kG.
the difficulty encountered in observing the polarized enol of acetone (see above).] In neutral solution acetoin may be identified as a less intensely polarized product (all fields). Acetate polarization is most apparent at low fields, and in N2-saturated neutral solution very weak low-field polarization permits identification of biacetyl and methane. The variations with reaction conditions in the high field
2192
The Journal of Physical Chemistry, Vol. 83, No. 17, 1979
D. J. Nelson
Scheme 11: Reaction Pathways and Predicted High-Field Polarization in Aqueous Acetaldehyde Radiolysisa J
AiE]
CH3CHDOD 3:/EA
(la)
[ C H 3 k H O D D1 +
(1b)
LCH3iHOD CH3tHODI
LCH36HOD C H 3 k O I
-c
+
CH3CHzOD (AEI
CHz=CHOD
(EA1
[C H a C H(OD) 12
q-
+
+
CH3CH20D{cH2 CH3 A E + AE
CH2=C=0 (E] (DCH2COzD. E l
CH3CH(OD)COCH3 (A t AE) (E AE) ( E )
-c-
+
CH3CDO (E
[ C H 3 b O * D1
+
CHz=C=O (;.DCH,COzD,
(E) E)
CH,CD(OD), [ C H 3 t (OD) 2
AE)
+
(A
HD ( A
+
+
EA)
AE)
D1 CH2=C(ODIz (A) (*',DCH2C02D9 A]
CH3CH(OD),
+
( A t AE)
H D (E
+
+
EA1
(€1
CH2=C=0 (.'.DCH2CO,D,
E)
ICH,A t AE?
(A)
(A
+
AE)
(E
CH;!=C(OD);!
+
+
AE)
CH3CH(OD)2
(AEI
(9a)
[ C H 3 6 (OD12 CH$(OD121
(9b) a The radical pair model3 and the EPR data in Table V are used in predicting the high-field polarization. Products underlined have been identified as correspondingly polarized products in Table 111. N o high field polarization predicted. [CH ,C(OD) 2 12
polarization intensities of the principal polarized products are summarized in Table 111. Qualitative analysis of the kinetic data cited above permits projections of the relative ratios of the significant radicals expected to be present in solution under the various reaction conditions (Table IV). This analysis suggests that under all conditions the ethanol radical and the hydrate radical will figure significantly in both product formation and polarization. The contributions of acetyl radical reactions will apparently be more limited. The invariance of the radical ratios in neutral and basic solution suggests that the products and their polarization will be
very similar under these two conditions. Saturation with N20 of the acetaldehyde solutions would be expected, as a consequence of competitive eaq-scavenging, to slightly decrease the role of the ethanol radical, but changes in product polarization should not be large. In acidic solution the radical ratios suggest that reactions of the ethanol radical will contribute less to product formation and polarization, while the contributions to these processes of deuterium atom-related reactions will increase. The changes in relative ratios of radicals with reaction conditions are not large, portending some difficulty in correlating the CIDNP data in Table I11 with the ratios in
The Journal of Physical Chemistry, Vol. 83, No. 17, 7979
Pulse Radiolysis of Acetone and Acetaldehyde
TABLE IV: Qualitative Ratios of Radicals Expected to be Present in Aqueous Acetaldehyde Radiolysisa CH,CHOD:CH,CO: CH&OD),* : % D
PH
N, N*O N,
1.5 6.3 11.3
5:1:9:6 4:1:11:5
N2O
10:2:8:1 6:3:11:1
N, N,O
10:1:9:1 6:2:12:1
a Calculated by using the relative rates of reaction with e&, ,OH, and 5H and the concentrations of reagents. p hydrogen abstraction would be expected to occur only 1 0 - 1 5 % as frequently.22
*
TABLE V: g Factors and Hyperfine Coupling Constants radical g factor coupling constants, G e aci
-a
2.0023 2.00223
H* (D.)*
503.8 (77.45) t 5 . 1 (PI -15.37 (ct), t 22.19
2.0007 2.00323
*
2.00315 2.0034 2.00446
t 1 9 - 9 0 (p), 0.48 -+20 19.74
Reference 25. Reference 26. Reference 28. e Reference 29.
a
(0) (OH)
Reference 27.
Table IV and the projected reactions of these radicals (Scheme 11). Because an increasing number of radicals and their reactions come into play, analysis of the correlation between changes in radical ratios with reaction conditions is not so straightforward in aqueous acetaldehyde radiolysis as in the aqueous acetone case. The data from Table IV suggest that in neutral or basic solution reactions of the ethanol radical and the hydrate radical (reactions 7a-c, Scheme 11)will be major reaction pathways. In the spectra ethanol, vinyl alcohol, and acetate (in basic solution), products of reactions 7a and 7b, show polarization which is consistent with that predicted from reactions of these two radicals. Statistically less important reactions of these radicals (reactions 2a and 9a) also yield polarized products (ethanol and hydrate) which are observed spectroscopically. (The occurrence of reaction 2b cannot be verified with the lH-NMR data.) However, reactions of these two radicals fail to predict two observations regarding acetaldehyde polarization: the substantial return of hydrogen t o the aldehydic position (indicative of a carbon-bound source), and the intense net effect polarization of the methyl group (with accompanying EA polarization in neutral solution). Both of these observations are predicted by reactions of the acetyl radical with the ethanol radical (eq 3a) and with the hydrate radical (eq 6a). [The net effect from interactions of either radical pair should be pronounced, reflecting the large g factor difference between the radicals (Table V).] The source of the EA polarization in the methyl group of acetaldehyde (a multiplet effect seen only in neutral solution) is difficult to specify. Such multiplet polarization is not predicted in acetaldehyde formed directly via the reactions in Scheme 11. Rapid enol tautomerization would result in AE aldehyde polarization of both the methyl and methinyl hydrogens and does not explain the multiplet effect observed. Dehydration of polarized hydrate probably occurs a t a rate too slow to be important. One possible explanation is that ethanol radical, polarized by interaction with CH,CO, is scavenged by acetaldehyde,
+
2193
+
giving E EA polarization of the methyl group and A EA polarization of the aldehydic hydrogen. The CIDNP in basic solution is similar to that seen in neutral solution with three exceptions. Net emissive polarization is seen in the methyl group of acetaldehyde (expected from both acetyl radical reactions and ethanol radical scavenging), the enol polarization is much less intense, and the ethanol polarization is both less intense and is A + AE at the methyl group. Because tautomerization is both acid and base catalyzed,17 the loss in intensity of the enol polarization is not surprising. [Similar intensity losses are observed in acid solution (see below).] The changes in the ethanol polarization can be explained less directly. Methyl group polarization like that observed (A AE) in ethanol formed in basic solution is expected from reactions of the ethanol radical with .D (eq l a ) and with the acetyl radical (eq 3b), but changes in the significance of these reactions from their importance in neutral solutions are not predicted (Table IV). In acidic solution the largest change in product polarization is seen in acetaldehyde. The absence of polarization in the aldehydic hydrogen may be explained on the basis of increased contributions to aldehyde polarization of deuterium atom-acetyl radical reaction (eq 4a). The very weak enol polarization is expected. The diminished intensity in methylene polarization and the A + AE methyl group polarization in ethanol are expected if reaction l a contributes more substantially to product formation and polarization. Intense polarization is seen in the methyl group of the hydrate but not at the methinyl position. In acidic solution the methyl group is emissively polarized, CIDNP which is not consistent with that predicted from reactions of CH,C(OD), with .D (eq 5a), with COCH3 (eq 6c), with CH3CHOD (eq 7b), or with a second like radical (eq 9a). However, the polarization is consistent with formation of the aldehyde by the reaction pathways described above followed by hydration at a rate more rapid than that of spin-lattice relaxation. (This process also appears to explain the hydrate polarization in acetaldehyde photolysis in D20.)23A t other values of pH E + AE polarization of the methyl group is seen, consistent with that expected from hydration of polarized acetaldehyde, and to a lesser extent, reaction of CH,C(OD)2 with .D (eq 5a) or with -COCH3(eq 6c). The last processIwhich also yields ketene, would be expected to be energetically less favorable. Polarized acetoin, apparently a minor product, can be identified only in neutral solution. The polarization observed in the methyl group of the alcohol fragment (A + AE) is the only pattern that is readily discerned. Polarization of the methinyl hydrogen (E + AE?) is masked by the more intense vinyl alcohol multiplets and that of the methyl group in the acetyl fragment is either weak or is buried in the more intense acetaldehyde polarization. If the A + AE methyl group polarization alone is considered, acetoin formation by reaction of the ethanol radical and the acetyl radical (eq 3c) is expected to give polarization consistent with that observed at this position. Net emissive polarization of the methinyl hydrogen of acetoin which is consistent with that observed is also predicted from this reaction. The reason for the failure to observe polarization in the methyl group of the acetyl fragment (which is expected to be emission if reaction 3c applies) is not obvious. The possibility that it is masked by the aldehyde polarization has been considered. Very weak polarization is expected because the hyperfine coupling in the acetyl radical is small. Another possibility is that polarized acetoin rapidly hydrates, in which case
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The Journal of Physical Chemistry, Vol. 83, No. 17, 1979
the emission would be concealed in the intense polarization near 1.4 6. Low field emission is seen in acetate in neutral and basic solutions. Weak enhanced absorption at high fields is seen in acetate in basic solution, but acetic acid could not be identified at lower values of pH. On the basis of relative ratios of radicals predicted to be present, reaction of the ethanol radical and the hydrate radical (eq 7a) may be proposed as a pathway for acetate formation and polarization. The CIDNP support for the involvement of this reaction pathway is consistent with the proposal that a Cannizzaro reaction of the hydrate radical accounts for the interdependence of ethanol and acetate yields in aqueous acetaldehyde radiolysis.10b At low fields very weak polarization in biacetyl and methane is observed from irradiation of Nz-saturated neutral solution. Under these conditions very weak multiplet polarization near 4 6 is also discerned, suggesting the possibility that other very minor products such as 2,3-butanediol may be formed. In summary, both product formation and high field polarization appear to be satisfactorily accounted for by considering reactions of the ethanol radical, the acetyl radical, the hydrate radical, and .D. Consideration of the reactivity of more minor radicals which might be present, such as .CH2CH(OD)2and .CHzCHO, has been unnecessary. The CIDNP data suggest that reactions 7a and 7b (Scheme 11) are important reaction pathways, as expected, but that reactions la, 2a, 3a, 4a, 5a, and 6a are equally important.32 The significance of reactions 2b, 8b, and 9b is difficult to assess with the CIDNP data, but these reactions would be expected to be minor ones. The results are adequately explained without proposing the involvement of energetically less favorable reactions such as reactions lb, 3b, 4b, 5b, 6c, 8a, and 9a.
Conclusions This study of the pulse radiolysis of aqueous acetone and acetaldehyde suggests that the CIDNP/F’P”MR technique can be used advantageously to gain insights into the mechanisms of radiolytic reactions. The intense polarization that is observed enables rapid identification of the major radiolytic products. (For completeness, the identification of polarized products seen at both low and high fields is carried out.) Some restrictions in product identification because of overlapping chemical shifts are evident, but these may be minimized through ancillary studies using 13C-NMR spectroscopy. The CIDNP observed in the radiolytic products and its correlation with qualitative projections of the radicals expected to be present and their reactivity provides a broader picture of the radiolytic reaction pathways than does either EPR spectroscopy or product analysis. Using this technique, one finds that evidence is provided for the presence of all of the important radicals expected, and their roles in product formation and polarization are better defined. In this study, for example, attention is focussed on the acetonyl radical in one example and the hydrate radical in the second, major intermediates whose involvement in radiolytic reactions was heretofore poorly indicated. Several mechanistic details of aqueous acetone and acetaldehyde pulse radiolysis have been better described. The CIDNP/FTNMR study indicates more clearly than many other techniques the substantial return to reactants in radiolytic reactions. Attention has also been drawn to a number of specific reaction pathways which appear to be important in product formation. For example, in this study further support for the importance of’6-hydrogen
D. J. Nelson
elimination from the alcohol radical (to give the enol of the reactant) has been provided. (Indeed, enol formation apparently is a major reaction pathway, and the resultant enol a major reaction product.) Finally, the CIDNP study effectively compares and contrasts pulse radiolytic reactions with photochemical studies of the aqueous solutes. With respect to polarization pathways, the role of the aqueous primary radical .D in polarization has been more firmly established. (Rapid reaction of eaq-with the carbonyl compound restricts the contributions to polarization of e, --containing radical pairs.) Talen together, these factors imply that the CIDNP/ FTNMR technique can contribute meaningfully to the elucidation of the mechanism in pulse radiolytic reactions. Acknowledgment. We express our thanks to Dr. A. D. Trifunac, Chemistry Division, Argonne National Lahoratory, and to Professor R. G. Lawler, Brown University, for stimulating discussions of this work. We also acknowledge the work of R. H. Lowers, the operator of the Argonne Van de Graaf accelerator. References and Notes (1) Work performed under the auspices of the Office of Basic Energy Sciences of the U.S. Department of Energy. (2) J. W. T. Spinks and R. J. Woods, “An Introduction to Radiation Chemistry”, 2nd ed, Wiley-Interscience, New York, 1976. (3) (a) G. L. Closs in ”Advances in Magnetic Resonance”, Vol. 8 , J. S. Waugh, Ed., Academic Press, New York, 1974; (b) R. Kapteln in “Advances In Free Radical Chemistry”, Vol. 5, G. H. Williams, Ed., Academic Press, New York, 1975. (4) (a) “Selected Specific Rates of Reactlons of Transients from Water in Aqueous Solutlon. Hydrated Electron, Supplemental Data”, Natl. Stad. Ref. Data Ser., Natl. Bur. Stand., No. 43 (1975); (b) “Selected Specific Rates of Reactions of Transients From Water In Aqueous Solution. 111. Hydroxyl Radical and Perhydroxyl Radical and Their Radical Ions”, ibid., No. 59 (1977); (c) “Selected Specific Rates of Reactions of Transients From Water in Aqueous Solution. II. Hydrogen Atom”, ibid., No. 51 (1975). (5) K.-D. Asmus, A. Henglein, A. Wigger, and G. Beck, Ber. Bunsenges. Phys. Chem., 70, 756 (1966). (6) F. P. Sargent, Can. J. Chem., 46, 1029 (1968). (7) P. Riesz, J. Phys. Chem., 69, 1366 (1965). (8) R. J. Guthrie, Can. J. Chem., 53, 898 (1975). (9) D. R. G. Brimage arid J. D. P. Cassell, J. Chem. SOC.A, 2619 (1969). (IO) A. V. Efimov, Mater. Vses. Nauchn. Stud. Konf., Novosib. Gos. Univ., Khim., 72fh, 6 (1974): Chem. Abstr., 85, 12259 (1976); (b) E. P. Kalyazin, E. P. Petryaev, 0. F. Kaputskaya, and 0. I. Shadyro, Khim. Vys. Energ., 8, 548 (1974):Ugh Energy Chem., 8,471 (1974). (11 ) D. J. Nelson, A. D. Trifunac, M. C. Thurnauer, and J. R. Norris, Rev. Chem. Intermed., In press. (12) A rate constant of 4 X lo4 M-’ s-’ has been reported for this electron-transfer reaction: C. E. Burchill and G. P. Wollner, Can. J. Chem.,50, 1751 (1972). Because other electron-transfer reactions of the 2-propanol radical have been reported for which k IO8 M-‘ s‘’ (ref 13), the possibility that the method used in the determination of this rate constant Is inadequate was considered. However, the observations of Adams et al. (ref 14), using optical absorption spectroscopy, confirm that electron transfer from the 2-propanol radical to N,O is sufficlently slow as to be unimportant. (13) K.-D. Asmu& A. Wigger, and A. Henglein, Ber. Bunsenges. Phys. Chem., 70, 862 (1966). (14) G. E. Adams, B. D. Michael, and R. L. Willson, Adv. Chem. Ser.,
No. 81 (1968). (15) J. Baraon and K . 4 . Seifert, Ber. Bunsenges. Wys. Chem., 78, 187 (19741. (16) The enol has also been identified as a product of acetone photolysls In nonaqueous solvents: 6. P. Laroff and H. Flscher, Hdv. Chirn. Acta, 56, 2011 (1973). (17) J. March, “Advanced Organic Chemistry: Reactions, Mechanisms, and Structure”, 2nd ed, McGraw-HIM, New York, 1977. (18) G. A. Olah and A. T. Ku, J. Org. Chem., 35, 3916 (1970). (19) M. Lehnig and H. Fischer, Z. Naturforsch. A , 25, 1963 (1970). (20) In basic solution very weak high field enhanced absorption in the dione is seen. Such polarization would be expected If the dione were an “escape” product of reaction of a polarized acetonyl radical with a second acetonyl radical or if the dione were formed by addition of a polarized acetonyl radical to the enol. (21) A. Gero, J. Org. Chem., 19, 469, 1960 (1954). (22) K.D. Asmus, H. Moeckel, and A. Henglein, J. Phys. Chem., 77, 1218 (1973). (23) H. E. Chen, M. Cocivera, and S. P. Vaish, Can. J. Chern., 53, 2548 (1975). (24) J . Bargon and K.-G. Seifert, Chem. Eer., 108, 2073 (1975). .
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The Journal of Physical Chemistry, Vol. 83, No. 17, 7979 2195
Reaction of Oxygen Atoms with Hydrogen Sulfide (25) (a) E. C. Avery, J. R. Remko, and B. Smaller, J. Chem. Phys., 49, 951 (1968); (b) F. P. Sargent and E. M. Gardy, Chem. Phys. Lett., 39, 188 (1976). (26) R. W. Fessenden and R. H. Schuler, J. Chem. Phys., 39,2147 (1963). (27) J. E. Bennett, B. Mile, and B. Ward, Chem. Commun., 13 (1969). (28) R. Livingston and H. Zeldes, J. Chem. Phys., 44, 245 (1966). (29) R. Livingston and H. Zeldes, J. Chem. Phys., 45, 1946 (1966). (30) “High Resolution NMR Spectra Catalog”, Varian Associates, Palo Alto, Calif., 1962. (31) “The Aldrich Library of NMR Spectra”, C. J. Pouchert and J. R. Campbell, Ed., Aldrich Chemical Co., Milwaukee, Wisc., 1974. (32) I n view of the low ratio of acetyl radical which is expected to be
present, this intermediate seems to play an inordinately important role in product formation and polarization. Correspondingly, the contributions of the hydrate radical to these processes are less than would be expected. These observations have led us to conskler the possibility that the hydrate radical dehydrates to the acetyl radical: CH,C(OD),
* CH,CO
-F D20
While this process woukl contribute to the acetyl radical concentration at the expense of that of the hydrate radical and explain the above observations, it must be noted that, for it to be significant, a rate constant greater than 10’ is required.
Kinetics and Mechanism of the Reaction of Oxygen Atoms with Hydrogen Sulfide’ Donald L. Singleton,* Robert S. Irwin, Wing S,, Nip,t and R. J. Cvetanovl6 Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada K I A OR9 (Received April 3, 1979) Publication costs assisted by the National Research Council of Canada
Rate constants were determined by a phase shift technique for the reaction of oxygen atoms with hydrogen sulfide. Over the temperature interval 297-502 K, the results are consistent with the Arrhenius equation k l = (1.56 f 0.83) X 1O’O exp[(-2171 f 202)/T] L mol-’ s-l, where the uncertainties are 95% confidence limits for three degrees of freedom. Potential sources of systematic errors are discussed and their magnitudes estimated. In separate experiments, oxygen atoms were generated by mercury photosensitized decomposition of NzO at 700 torr, and the products of their reaction with HzS were analyzed by gas chromatography. Besides N2, the only detected products were H20,H2, and a very small amount of 02.The results suggest that about 52% of the reaction proceeds by way of hydrogen abstraction from H2S,and less than 11%by way of addition of oxygen atoms to H2S followed by decomposition of the adduct to form H and HSO.
Introduction The kinetics and mechanisms of the reactions of ground state oxygen atoms with hydrogen sulfide and organic sulfides have been the subject of several recent studies which produced somewhat surprising results. Mass spectrometric detection of the products resulting from crossed beams of oxygen atoms and organic sulfur compounds suggested an addition mechanism’ in which the initial adduct fragmented or was stabilized, analogous to the well-known addition of O(3P) to olefim2 The very large rate constants and the negative Arrhenius activation energies for the organic sulfides could be accomodated by this rne~hanism.~ It has been suggested that the reaction with hydrogen sulfide also could proceed by an addition mechanism (Ib) 0 + H2S OH + SH (la) 0 + H2S [H2SO]* HSO + H (1b) instead of entirely by direct abstraction (la), although it was emphasized that not enough data existed to reach definite concl~sions.~ Evidence that an addition mechanism could occur, at least under very special conditions, came from a matrix isolation study in which the species HSOH was identified as a product of the photolysis of mixtures of O3 and H2S in an argon matrix? Presumably the direct abstraction route, reaction la, has too high an activation energy to occur at the temperature of the matrix, 8 K, and it was suggested that HSOH was formed by decomposition and secondary reactions of an HzSO adduct in a matrix cage, Le., 0 + H 2 0 [H2SO] HSO + H HSOH. Subsequent studies of chemiluminescence from 4
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NRCC No. 17548. NRCC Research Associate. 0022-3654/79/2083-2195$0 1.OO/O .. -
this system were consistent with this m e c h a n i ~ m . ~ ! ~ The reported room temperature rate con~tants~*~-’l for reaction 1vary by more than a factor of 4, although the two most recent ~ a l u e s are ~ 3 ~within 20%. However, the same two studies yielded quite different temperature dependences, such that the reported rate constants at 500 K differed by loo%, and both results differed significantly from the only other reported Arrhenius parametersS8 The present work was done to provide additional information, using quite different experimental methods, which could help resolve some of the questions concerning the kinetics and mechanism of reaction 1. Rate constanints were determined with a phase shift technique, and several separate experiments were carried out to determine the products.
Experimental Section Rate Constant Determinations. Oxygen atoms were generated by modulated mercury photosensitized decomposition of nitrous oxide.l2-I4 The relevant reactions are
+ + + + + + - +
Hg6(’So) + hv(254 nm) Hg6(3Pl) N20
Hg6(lSo)
O(3P) H2S o ( 3 ~ ) NO
Hg6(3P1)
M
N2
products
NO^
M
O(3P) (1) 2)
The phase shift between the modulated 254-nm light incident on the reaction cell and the chemiluminsecence resulting from reaction 2 was measured with photomultipliers and a lock-in amplifier. Pressures and flow rates of the gases were measured as before14 with the exception of HzS, for which the pressure drop across the capillary
0 1979 American Chemical Society
