J. Phys. Chem. 1982, 86. 3094-3105
3094
bypass HOI. At lower acidities oxygen is important and involved in I2oxidation, but at higher acidities a sequence is needed where oxygen is not inv01ved.l~ The model used here has not included O2 as a reactant.
in the H20z,IO3-, H+, Mn2+,CA system, and for oxidation of Iz in the Hz02,IO 2.3 X lo' kB(2,) = 2.8 x lo9 k I - 4.4 x 10, S-I koQ(AcH)= 2.3 x 10' L
mol-'^-^
See the text for the source of these estimates.
TABLE VII: Estimates of the Fraction of the Vibrationally Rich, Triplet Acetaldehyde Molecules Formed at 3000 A Which Do Not Decompose To Form Products
pco,, torr
@kQ 1 - ($1
+ @n+ $m)b
0 50 100
0.00 0.00 0.03 0.11 0.08 0.21 0.24 0.35 200 300 0.33 0.41 400 0.40 0.52 This represents the quantum yield of triplet biacetyl formation (as read from Figure 1 of Gandini and Hackettz9);presumably this is the fraction of CH,CHO(T,) molecules which lives long enough t o undergo triplet energy transfer t o biacetyl. Calculated for the conditions used by Gandini and Hackett through our primary photodissociation quantum yield equations 20-22 of the text.
with this nonradiative decay constant is one leading to vibrationally excited triplet molecules through intersystem crossing and that process I occurs from this state. We estimate from our eq 19 based upon @CO(OZ)- @ C ~ ~ O that Z) at low pressures $I N 0.99 f 0.06, and from 4I = 1 - $11 - 4111we estimate independently that $I N 0.93 f 0.01. Hence, our data are consistent with the rate constant for intersystem crossing, kk k,& N 4.0 X lOe s-l. These and all other estimated rate constants for the excited states of acetaldehyde excited-at 3000 A are summarized in Table VI. Our limited evidence for the view that process I occurs from the triplet molecule should be reviewed here. Gandini and Hackett31determined the quantum yields of sensitized triplet biacetyl formation ($&) from acetaldehyde excited at 3000 A in mixtures of CH3CH0 and (CH3C0)z,with varied amounts of added COz. Their measured $i, monitors in theory that fraction of the triplet molecules which are sufficiently vibrationally relaxed to be captured by the energy transfer process. These data are shown in Table VII, where they may be compared with our estimates of the fraction of the molecules which are not decomposed in processes 1-111 for the same conditions of added COz. A reasonably good correlation between the two columns is seen. The data suggest that the fraction of vibrationally rich acetaldehyde molecules which decompose at the lower pressures, but become relaxed at higher COzpressures, are in the triplet state. Whether those which decomposed in process I were originally in the triplet state or whether the
3104
The Journal of Physlcal Chemistry, Vol. 86, No. 16, 1982
triplets were formed by a collision-induced intersystem crossing as they were relaxed cannot be discerned from these data alone. However, it is difficult to conceive of any state other than the triplet to which the observed rapid t r a n ~ f o r m a t i o nfrom ~ ~ the singlet could occur yet retain the necessary potential for nearly complete regeneration of the triplets which Ganbini and Hackett observed by extrapolation of the & values to high pressures of added C02.32 Further support for the triplet involvement in process I can be derived from the suppression of CO yields with added isobutene. Thus, the extent of lowering of @aseen here by isobutene addition is very similar to the extent of triplet quenching observed by Archer et al.39in their cis2-butene-acetaldehyde mixture photolyses at 3130 A. In our experiment at the highest isobutene pressure (50.1 torr) ac0is suppressed by about 21% from the value in the comparable C,HE-free runs. From the data of Archer et al. at 3130 A and 35 "C, we can estimate that the addition of 50.1 torr of cis-2-buteneto 40 torr of acetaldehyde would have resulted in the capture of about 22% of the precursor responsible for the observed cis-trans isomerization (presumably the triplet acetaldehyde). Seemingly the CO suppression that we see reflects largely the triplet acetaldehyde vibrational and/or electronic quenching, since HCO radicals do not appear to add readily to i s o b ~ t e n e . ~ ~ Our current observation that added isobutene and acetaldehyde quench ac0with similar low efficiencies, and the estimate of Gill et al. of k (AcH) E 2.3 X 10' L mol-' S-', suggests that kQ(CdwN lo7% mol-' s-'. To the extent that this represents a triplet energy transfer reaction, the probable endothermicity of the reaction CH3CHO(Tl) + C4HE CH3CH0 C4H8(T1)could account for its slowness.33 In contrast to the interpretation given here and that offered in most previous work which supported the role of vibrationally excited triplets in process I, the results of Gill et aL5 provide compelling evidence that this view may be incorrect. The measured rates of appearance of HCO(O,O,O)following the flash excitation (3400-2400 A) of acetaldehyde (0.2 torr) were much too slow to be consistent with their origin from a vibrationally rich triplet state whose lifetime for excitation at 3000 A must be less than 2.5 X lo4 s for their conditions. However, the suggestion of Gill et al. that process I11 must dominate over all decay pathways and that CH3C0 radical thermal interactions with acetaldehyde generate HCO slowly in this system is probably not correct. There is no known free-radical reaction which is analogous to that required to fit the hypothesis of Gill et al.: CH,CO CH3CH0 CH3COCH, HCO. Although our data do support their contention that process 111does occur, its quantum efficiency is very low. Our experiments with small additions of O2show clearly that HCO and CH, radicals are the major primary products of acetaldehyde photolysis at 3000 A. As we have seen in this study, conventional free-radical chemistry can explain the product quantum yields well. We must conclude that the hypothesis of Gill et al. of CH3C0 involvement in the slow appearance of HCO(O,O,O) in their experiments a t 0.2 torr of acetaldehyde is probably incorrect. In contrast to the results of Gill et al. carried out at 0.2 torr of acetaldehyde, their results from the flash photolysis of acetaldehyde at 10 torr show the very rapid rate of HCO(O,O,O) formation which is consistent with the occurrence of process I from a short-lived, vibrationally rich triplet state. However, the results at 0.2 torr remain unexplained. I t is possible, but not likely in the opinion
-
+
+
+
-
Horowitz et al.
TABLE VIII: Estimation of the Radiative Decay Constant for the CH,CHO(T,O) State
1.14 1.14 1.35 1.35 1.88 1.88 2.42
2.3 1.5 2.3 2.5 1.5 2.1 2.6 1.7 2.4 2.5 1.7 2.5 3.8 1.6 2.2 3.4 1.6 2.4 4.0 1.9 2.6 Data reported by Parmenter and no ye^.^' Calculated by using eq 23 of the text; values for ki, and kf for the particular wavelength were derived from a combination of data from Hansen and Lee,33Gandini and Hackett and our present estimates of &; k , + k d ( ~ , oand ) ~ Q ( A & were from Gill et al.5 2.5 2.5 2.6 2.6 3.8 3.8 3.9
of Gill et al., that at the lower pressures some vibrationally rich levels of HCO were the dominant early product of acetaldehyde decay and escaped detection; in such a case the slow HCO(O,O,O) appearance rate might reflect merely the time required for relaxation to the detected levels of HCO. Gill et al. have suggested, but did not favor, an alternative explanation for the origin of the HCO(O,O,O) radicals in their experiments which we feel also bears further testing. They pointed out that T,O could conceivably be the precursor to these radicals in the low-pressure experiments if the measured lifetime for the triplet phosphorescence decay of CH3CHO(T,0),2.3 X s observed by them, reflected simultaneous and significant decomposition as well as emission. Indeed, we find that published phosphorescence and fluorescence quantum yield data and other evidence do support this view, in our opinion. We may review this evidence here. First note that, if kd(T1~) >> k,, then 4.4 X lo4 s-' E Ad(T,o)e-Ed(T1ol'RT. (The symbolism used here is that defined in Table VI.) Assuming Ad,,o, is a normal preexponential factor (about 1015 s-') common for simple bond rupture reactions like 14 kcal mol-'. Such a barrier process I, then &(Tlo) would be in accord with the very high rate of decay of the - ETl0 = 15 unrelaxed, 3000-A-excited acetaldehyde (EIlw kcal mol-') observed here and in other work. A much larger barrier to triplet dissociation in process I is expected theoretically,6although the previous history of such barrier calculations in excited states shows that an overestimation of the magnitude of the barrier is not uncommon. More direct evidence can be had from the &/4, results of Parmenter and N ~ y e swho , ~ ~excited acetaldehyde in experiments at 3130 and 3340 A using high pressures (100-400 torr, 36 OC). We may couple their data with other recent luminescence data from acetaldehyde to derive an estimate for the radiative decay constant 12, for the acetaldehyde(TlO)state: T,O hu, So. Thus, for the high pressures of acetaldehyde where a large share of the original TIW excited state will be relaxed to Tlo,then the &/4, ratio should be given approximately by relation 23; the
-
+
constants refer to those outlined in Table VI. Relation 23
3105
J. Phys. Chem. 1982, 86, 3105-3114
reduces to eq 24. At 3000 A Hanson and Lee find kf N kp
(4p/$f)(kf/kisc)(kp + kd(Tlq + ~Q(A~H)(AcH)) (24)
5.0 X lo5 s-l; we estimated in this study that kist = 4.0 X lo8 s-l; Gill et al. report k, kd(T1~)N 4.4 X lo4 s-l and k Q ( A a ) N 2.3 X 10' L mol-' s-'. Thus,for the 3130-A data, k, = (4p/4f)(55 + 2.88 X 104(AcH))s-l. In a similar fashion we may derive from 3340-A data, k, = (+,/&) (75.86 + 3.97 X 104(AcH));here (AcH) is in mol L-l. We have used the 4f/$pratios reported by Parmenter and Noyes together with these relations to estimate k values from data at both 3130 and 3340 A (see Table VIIf). The data are reasonably consistent, and insofar as the assumptions made are correct, we find k,.= (2.0 f 0.4) X lo2s-l. The Parmenter and Noyes relation derived from 4, in pure acetaldehyde photolyses at 3340 A, 1/4, N 630 + 5.3PAcH(torr),gives further evidence of the magnitude of k,. Applying the conventional triplet formation and quenching mechanism of Table VI, we anticipate that the intercept in the above function should equal (k,. + kd(T10)/4,&, and with the slope N ~ Q ( A ~ H ) / & ~ , . Taking the recent estimate of k, + kd(T 0 ) from Gill et al. and 4isc these data &e k, 70 s-l and ~Q(A,) N 1.0 for 3340 N 7 X lo6 L mol-'s-l. Both estimates are about a factor of 3 lower than the seemingly more accurate estimates of k, N 2.0 X lo2 s-l derived here, and kQ(AcH)N 2.3 X lo' L mol-l s-l from Gill et al. However, these results also support the view that k, is much smaller than the value of k + kd(Tlo) (4.4 f 0.5) X lo4 s-', measured by Gill et al. b e suggest that the phosphorescence decay constant is controlled largely by the rate constant for triplet decomposition. If this interpretation is correct, then k, + kd(T1O) should show a marked temperature dependence
+
which reflects the high E, which we speculate must be associated with kd(T1O). We conclude that, if only the vibrationally relaxed triplet forms the HCO(O,O,O) radicals for the conditions of Gill et al., then the observed appearance time is consistent with their origin in Tio. Obviously further quantitative work on HCO appearance times at different temperatures is called for to test the current hypothesis. Process 11, unimportant at 3000 A, becomes a major primary process at the shorter wavelengths. It probably originates from higher vibrational levels of the excited singlet state as suggested by many Our data show that process III originates from a state different from process I. This observation and the apparent increased importance of H2 as a product at shorter wavelengths suggest that process I11 also originates from the higher vibrational levels of the excited singlet state. All kinetic information on the primary processes derived here and in previous work and our speculations on the mechanism of CH3CH0 photochemistry at 3000 8, are summarized in Table VI. Further studies of the wavelength dependence of the primary processes in acetaldehyde are underway and will appear in a subsequent publication. Acknowledgment. This work was supported by the US Environmental Protection Agency under the research grant R-806479-03 and the US-Israeli Binational Science Foundation grant No. 2509/81. We are grateful to the Mound Laboratories of the Monsanto Chemical Co. for the professional leave granted to Dr. Kershner which enabled him to participate in this research effort. We thank Dr. Charles S. Kan for his help with computer simulations in the tests of mechanisms and for the preparation of azomethane used in the actinometry.
Wavelength Dependence of the Primary Processes in Acetaldehyde Photolysis Abraham HorowRz' and Jack 0. Calvert' Department of Chemistry, The Ohio State Universny, Columbus, Ohio 43210 (Received: December 23, 1981)
The quantum yields of the gaseous products CO, CHI, and H2were determined in photolyses of acetaldehyde and its mixtures with O2 and C02with excitation at 2900,3000,3130,3200, and 3312 A and 25 f 2 "C. The results are interpreted in terms of the occurrence of three primary photodecomposition modes: CH3CHO*+ CH3 HCO (I); CH,CHO* CHI + CO (11);CH3CHO* H + CH3C0 (111). The quantum yields of these 411,& I ~ ,respectively) at low pressures are as follows: 2900 A, 0.887,0.063,0.076;300G A, 0.930, processes 0,011, 0.059; 3130 A, 0.924,0.018,0.055;3200 A, 0.470,0.079,0.00;3312 A, 0.051, 0.035,O.OO. The values are sensitive to pressure of added gases with the precursor of I less affected than that for I1 and 111. Present evidence suggests that I originates from a vibrationally excited-triplet state; I1 and I11 become increasingly important at the short wavelengths and may arise from the high vibrational levels of the first excited singlet. The present data, coupled with other published information, are used to estimate rate constants for the primary photodecomposition processes for acetaldehyde in the sunlight-irradiated, lower troposphere.
-
+
Introduction In our recent study of the photochemistry of acetaldehyde at 3000 A2 new evidence was presented for the nature and extent of the occurrence of three primary processes in acetaldehyde photolysis: CH3CHO* CHQ+ HCO (1) +
(1) Permanent address: Soreq Nuclear Research Center, Y a m , Israel.
0022-3654/82/2086-3105$01.25/0
-
CH,CHO* CH&HO*
-- + + CH4
H
CO
CH3CO
(11) (111)
We interpreted our results to suggest that the important process I occurs from a vibrationally rich, first excited triplet state of acetaldehyde. Processes I1 and I11 were much less important at 3000 A, and their origin was a state with different quenching properties, presumably the higher vibrational levels of the excited singlet state. A quanti0 1982 American Chemical Society
