464
KORJIAN FADNOS A N D W.ALBERTSOYES, JR.
Photolysis of Biacetyl-Oxygen Mixtures at 4358
A.1
by Norman Padnos and W. Albert Noyes, Jr. Department of Chemistry. G'niaersity of Rochester, Rochester g 7 , S e w York
(Received August 2, 1963)
The photolysis of gaseous mixtures of' biacetyl (2,3-butanedione) and 0.02-2.0 mm. of oxygen by light of 4338 A. has been studied. Carbon monoxide, carbon dioxide, water, methanol, and formaldehyde were fourid as products. The variation of carbon dioxide yields and oxygen consumption with oxygen pressure is similar to that which would be predicted if the only effect of oxygen pressure were on the ratio of triplet biacetyl deactivated in first-order processes to that reacting bimolecularly with oxygen.
Introduction
Oxygen was prepared in the line by heating potassium permanganate and purified by passage through The photochemistry of biacetyl at 4358 A. has been a trap immersed in liquid nitrogen. Oxygen-18 was studied by Sheats and Noyes2q3and more recently by obtained from the Weizmann Institute. Koyes, AZulac, and A I a t h e ~ o n . ~At room temperature Light from a Hanovia Type S-100 medium pressure the dissociation arises mainly from collision of two mercury arc was collimated and passed through a triplet molecules. Corning CS 5-74 glass filter to remove wave lengths The emission from biacetyl irradiated a t 4358 A. A. Screens of copper wire were inshorter than 4358 has been studied by many ~ o r k e r s . It ~ consists terposed t o reduce intensity when this was desired. of a short-lived (7 < 8 X 10-6 sec.)6 and a longThe reaction cell was cylindrical, made of quartz, lived component (. = 1.8 X 10-3 S ~ C . ) . ~ - The ~ long20 cm. long and 2.8 cm. in diameter. Its volume was lived component is strongly quenched by 0xygen,6~~-l' 86 ~ m . ~ A . stirrer and a U-tube trap and the connectwhile the short-lived component is unaffected by oxying tubing brought the total volume of the cell system gen. Taylor and BlacetlZophotolyzed mixtures of biacetyl and oxygen a t 3130 A. and found carbon monoxide, (1) This work was supported in part b y the Directorate of Chemical Sciences, Air Force Office of Scientific Research, under Contract carbon dioxide, formaldehyde, and water as products A1'49(638) 679. with lesser amounts of methanol and methyl hydro(2) G . F. Sheats and W. A. Noyes, Jr., J . A m . Chem. Soc., 7 7 , 1421 peroxide. Their mean @co2and @;H,O at 60" were (1955). 0.72 and 0.33, respectively. The quantum yields dis(3) G. 1;. Sheats and W. A . Soyes, J r . , ibid., 77, 4532 (1955). (4) U'. A. Noyes, Jr., W. A . Mulac, and 31.S. Matheson, J . Chem. played considerable scatter in their investigation, Phys., 3 6 , 880 (1962). as in the present one. PorterI3 photolyzed biacetyl(5) Cj". 1%'. A. Noyes, J r . . G . B. Porter, and J. I;. Jolley, Chrm. Rev., oxygen mixtures a t 4358 A. and 30" and found carbon 5 6 , 49 (1956). dioxide, carbon monoxide, methanol, formaldehyde, (6) €I. .J. Groh, J r . , J . Chtm. P h w . , 2 1 , 674 (1953). acetone, and acetaldehyde. His yields were - @ 0 2 = (7) G.M . Almy arid S. Anderson, ibid., 8, 805 (1940). ( 8 ) It. D. IZawclilTe, Ret. Sci. Instr., 13, 413 (1942). 1.2-2.3, @coI= 0.9t5-1.49, @co = 0.15-0.34. (9) W. E. Kaskan and A . B. 17. Duncan, J . Chem. Phys., 18, 427 The present work was undertaken in the hope of (1950). correlating the photochemical behavior a t low oxygen (10) G . M . Almy. 11. Q . Fuller, and G. D. Kinzer, P h y s . Rev., 5 5 , pressures with the behavior of the long-lived emission. 238 (1939). (11) G . 51. Almy, H. Q. Fuller, and G . D. Kinzer, J . Chem. P h y s . ,
Experimental
8 , 37 (1940).
The vacuum line was similar to that used in most investigations in this laboratory. l 4 Eastman White 1,abel biacetyl was purified in the line by bulb-to-bulb distillation.
(12) R. I-'. Taylor and 1.; E. Blacet, Ind. Eng. Chem., 4 8 , 1505 (1956). (13) G . B. Porter, J Chem. Phys., 3 2 , 1587 (1960). (14) J . Ileicklen, Dissertation, University of Rochester, Rochester, N. Y., 1958.
The Journal of Phvsical Chemistry
PHOTOLYSIS OF BIACETYL-OXYGEN MIXTURES
465
to 316 C M . ~ . Transmitted light was focused on an RCA Type 929 phototube, whose output was measured. Oxygen-free biacetyl was used as an actinometer, using Sheats and Koyes2values for the quantum yields. Noncondensable products and excess oxygen were analyzed by combustion over copper-copper oxide wire. Carbon dioxide was removed a t -130' and its identity (and in the oxygen-18 experiments its composition) checked in the mass spectrometer. The remaining material was analyzed in a variety of ways. Formaldehyde was identified by illatsukawa'~'~ color test, methanol by vapor fractometry and mass spectrum, and H2018 by mass spectrometry, but these products were not quantitatively determined.
Table I : Product Yields in the Photooxidation of Biacetyl" p
biacetyl,
mm.
The products identified from the photolysis of biacetyl--oxygen mixtures were carbon monoxide, carbon dioxide, methanol, formaldehyde, and water. Methane was probably present in small quantities, and ethane was not present. Quantitative data were obtained for carbon monoxide, carbon dioxide, and oxygen uptake. The scatter in the data was quite pronounced. The yields do not vary with biacetyl pressure within the accuracy of the data (Table I). There appears to be a trend toward decrease of quantum yields with increasing time of reaction a t a given intensity and mean oxygen pressure, but this is by no means certain. The number of molecules of oxygen removed per photon absorbed and the quantum yield of carbon dioxide are independent of absorbed light intensity over the range studied, 2 x 10" to 2 x l O I 3 quanta absorbed/ml. /see., while the carbon monoxide yield appears to increase with decreasing intensity. At oxygen pressures greater than about 0.15 mm. the variation of yields with oxygen pressure is small, but the yields fall off considerably a t lower oxygen pressure. The relation between mean oxygen pressure during a run and the product yields was found by the method of least squares. The data for the 900-see. runs and for the single 600-sec. run were used in these calculations. Several equations were fitted to the data but only eq. 1 is used since other equations were found not to be better.
I/@ = a/(Oz)
+b
(1)
where @ is a quantum yield, a and b are constants which vary from yield to yield, and (02)is oxygen concentration. Equation 1 may also be derived from a simple mechanism in which an excited species either reacts with oxygen to give products or decays by one or more first-order processes. The ratios of slope to intercept
see.
I,
-
25 20 24 20 21 27 19 25.5 22 24.5 23
2160 374
24 12 13 22 18 18 13 13
1800 900 900 900 900 1800 2700 7920
24 24 25 37
11700 12015 11700 11700
26
46800
Results
Po1, mm.
Duration,
900 900 900 900 900 900 900b 900
600
I,
I,
I,
-
-
-
aco
'PCOl
a02
2 X 1013 quanta/cm.3 sec. 0.227 0.029 0.17 0.104 0.025 0.12 0.135 0.035 0.21 0.082 0.033 0.17 0.134 0.031 0.24 0.121 0.035 0.20 0.206 0.035 0.22 0 086 0.028 0.17 0.161 0.034 0.25 0.108 0.028 0.12 0.161 0.05 0.39
0 0 0 0 0 0 0 0 0 0 0
8 X loL2 quanta/cm.3 sec. 0,164 0.042 0.34 C C 0.4 1.0 C C 1.0 C C 1.8 C C 2.1 C C 1.0 C C 0.9 C C
0 15 0 24 0 33 0 33 0 18 0 24 0 28 0 19
1
12 10 14 12 13 16 15 10 14 10 18
x
lo'* quanta/cm.3 sec. 0.003 0.16 0.068 0.064 0.062 0.21 0.058 0.089 0.17 0.084 0.18 0.29
0 0 0 0
16 19 15 22
2 X 10" quanta/cm.3 sec.
0.081
0.055
0 14
0.22
a Cell system volume = 316 cm.3; temperature = 28 f 3'; illuminated volume = 86 cm.3, These d a t a were obtained after many experimental difficulties had been overcome. Many early runs were made but are not recorded here. * Three 300sec. exposures separated by 1200-sec. dark periods. c N o t measured.
(a,%) vary from about 3 t o 25 X lo'* molecules/ml. for the yields determined, t ~ i z . , CO formation, COz for mat ion , O2 disappearance. Table I1 shows the composition of the carbon dioxide in the runs with added 0 2 ' 8 . The carbon dioxide is mostly COW1*. The fraction of C016016and of COl80l8 decreases with increasing Ia.t, with the decrease in much more marked. This implies that after the first few minutes of reaction, the rate of formation of C0l8O1*is very small.
Discussion Since we have not presented any new data concerning the secondary processes in this system, an extended (15) D. Matsukawa, J. Biochem. (Tokyo), 30, 386 (1939).
Volume 68, Number 3
March, 1964
466
I\TORMAN P-4DNOS AND
w.ALBERT NOYES, JR.
+ 3B0= D + B 3Bm + P 3Bm + = B +
(14)
3Bo
(16)
3B0
Table 11 : Isotopic Composition of COZ from Runs with Oxggen-18"
0 2
Poz, mm.
Pb,aoetyi,
mm.
I, 1.0 1.0 1.0 1.0 0.4 1.0 0.1 0.9 0.9 0.5
13 12.5 13 22 13 13 13 12.5 13 12
1.0 0.4
2.5
13 13 17
1.0 1.0
12.5 12.5
a
%
73
%
C02'6
C0160'8
Coal6
8 X 1OI2quanta/cm.3 see. 360 8.Sb 51b 450 12 52 900 7 66 900 6 78 900 7 70 1,800 . 7.5b 72.5' 3,600 6 78 3 600 6b 80.S5 7,200 4 88 7,200 4 89 ~
I,
I,
=
Duration, see.
=
4 X 1 O I 2 quant'a/cm.s sec. 7 63 2,700 11,160 15 72 142,560 3 89
= 0.46 X
24,300 96,060
Temperature is 27 & 2".
quanta/cm.3 sec. 6 73 5 84
40.5' 36 27 16 22 206 16 13.5b 8 7 30 13
8 21 11
Average of two rune.
discussion of these would be pointless. TWOobservations are of interest: a chain reaction undoubtedly
occurs ('f' steps and but the chains must be short a t the temperatures used as demonstrated by the low quantum yield. The absolute values of the quantum yields are not very accurate and are in poor agreement with However, the ratios of yields are-consistent with his work. The Primary Process. The steps necessary for biacetyl a t 4358 8.are
B
+ hv
=
'B
'B = D 'B = B 'B = 3B,
'B = B
+ hvf
3Rm = D
=B 'Bo
=
3Bo
=
D
=
The Journal of Physical Chemistry
B
+ 02
=e
P
+ 0,
=
B
3Bo
+ hv,
+ O2
(15)
(17)
D represents dissociation; B, ground state biacetyl;
'B,biacetyl in its first excited singlet state; 3Bj biacetyl in its first excited triplet state; the subscript m refers to vibrational excitation, and subscript zero to lack of vibrational excitation; hvf represents emission from the singlet, and hv,, emission from the triplet; P represents products. Steps 2-12 are kinetically first order in the gressure range used. At room temperature a t 4358 A. the dissociatjon steps 3, 7, and 10 are not important.2 It must be remembered that a t 4358 8. 'B molecules are formed with very little vibration energy. Grohe has shown that oxygen does not affect the singlet emissioii, hvf. Therefore reactions of oxygen with IB need not be considered. Since oxygen reduces the concentration of 3B, reaction 13 can be neglected. In a simple reaction mechanism in which an excited species either undergoes a first-order decay (part of Tvhich at least leads to emission of radiation) or second-order collisional deactivation, the emission efficiency is given by equations of the form 1 / ~ 3=
a
+ b(02)
(18)
where Q 3 is the emission efficiency from the presently considered triplet state. Hence Qp3/Qo3 = 1
+ b(02)la
(19)
and b l a is called the "quenching constant." (Qo3is the emission efficieilcy in the presence of and Q p 3 in the absence of oxygen.) If the same state is destroyed by oxygen as emits the radiation, mean lifetimes will be given by equations of the form l / r o = a and 1,'. = a b ( 0 2 ) . ( 7 is the mean life of the triplet state a t oxygen concentration ( 0 2 ) and ro is the mean life at zero oxygen concentration. I n both cases the triplet state with vibration energy equilibrated with the surroundings, 3B0, is referred to since vibration energy would be very rapidly equilibrated at the pressures used.) Hence
+
ro/r = 1
= B
0 2
0 2
(13)
+ b(Oe)/a
(20)
The quenching constant calculated by Kaskan and Duncan9 from mean lifetimes and the ones calculated from either the data of Almy, Fuller, and Kinzerll OF
BIACETYL-OXYGEN n!hXTURES
PHOTOLYSIS O F
467
of Coward arid Xoyes16 agree well within experimental error. The average value is (1.7 f 0.4) X
ml./molecule. One is justified therefore in assuming that the quenched and reacting states are the same. Hence steps 14 and 15 are neglected. Reaction 33 will be unimportant a t the low conclentrations of 3130which would prevail in the presence of oxygen. Hence
( 3 B ~= )
+ + + k6)(k7 + + + + k16)(02))(k10 + kli + + +
k6kglra/(h k8
IC5
k4
(k14
k9
k12
(k16
k17)(02))
(21)'
If steps 14 and 15 are neglected for the reasons given, one may obtain the detailed equation for Q 3 , the phosphorescence efficiency, as Q3
=
k12(3Bo)/l,
=
+
k6)(k7
k6k9k12/(k3
f
IC8
+ + + + hl + + + ki?)
k9)(klO
kl2
(kl6
(02))
(22)
If @ is ono of the quantum yields based on eq. 16, one finds @J =
kd3BO)( 0 2 ) / 1 r . 4'1
+ + k5
h6k5kdOZ)/ [ ( I C 3 k6)(k7 f k8 k9)(klO =
+
k12
T,
+ + ( h 6
+ + kll +
kl7)(02))1
(23)
+ k n + + ( h e f kl,)(Oz)) = kK/(k3
+ + + k5
k4
'T
(24)
(25)
k6)
If we return to the question of whether 3B, or 31Bo is the state which reacts with oxygen, one may conceivably proceed by either of two sets of assumptions. AssumptionI. (ku
+ kid(02)
k7
+
-/- kg
(26)
Thus the primary reaction yield would be @p
= (k3
=
+
kKkl4 k4
constant
f kK
+
k6)(k14
+
=
0.58 X 10'G[02]-1
@GO,-'
=
and the quantum yield of formation of the triplet state will be
913
+ 3.3 @'coZ--l= 1.05 X 1016[02]-' + 4.1 @cop' = 0.55 X 1016[02]-1+ 11 = 0.33 X 10'6[02]-1 + 2.9 0.83 X 10'6[02]-1 + 4.6 @.co-l = 0.70 X 1016[02]-1+ 25 @02-l
@02-'
the mean lifetime of the triplet state, will be 1/(h
+
+
125
A4
Equation 29 is of the proper form to fit the data since 4 3 should be constant to a good first approximation. Thus it appears that a t least for the three yields studied the product yields are proportional to the primary yields, i.e., to the rate of reaction of the vibrationless triplet state with oxygen. One check on this assumption is that the ratio of slope to intercept for the experimentally determined equations of form 1 should be the reciprocal of the quenching constant, (k10 -I- k ~ , k12)/(h6 k17). The equations of this form with [02] in molecules/ ml. and @ the experimentally determined quantum yield are
k16)
(27)
Equation 27 d'oes not fit the facts. Hence one of the constants 167, k8, or k~ must not be negligible. Howeve,r, a t room temperature the data show k7 to be very small and k8 probably to be negligible. Hence it is reasortable to assume that kg is not negligible. I n reality jt is impossible to tell whether step 9 is first or second order since the concentration of the colliding molecules will cancel.
(30) (31:)
(32) (33) (34) (35)
Equations 30-32 refer to the results of the 900-see. runs and eq. 33-35 to the results of 600-see. runs. The slope-to-intercept ratios of these equations are, respectively, 18, 25, 5.0, 11, 21, and 2.8 X 1014molecules/ ml. The reciprocal of 1.5 X is 6.7 X loll4. The agreement is fair. Incidentally, it is noted that, while the slope-to-intercept ratios for the equations involving oxygen uptake and carbon dioxide formation are in fair agreement with one another, the ratios for eq. 32 and 35 for carbon monoxide formation are quite different from these. Some secondary steps must lead to products but the extent of their occurrence must not vary greatly with conditions. The agreement of the slope-to-intercept ratios for this work with the ratios from emission data is independent of the absolute yields. If = 0, which must be at least approximately true, and if 4 3 = 0.15 (a minimum value since Almy, et al., find this to be the phosphorescence yieldlOlll), one can calculate the number of molecules formed or of (16) N A Coward and W -4.S o j e s , J r (1954)
J Chem Phys , 22, 1207
Volume 68, Sumber 3
March, 1964
NORMAN PADNOS A N D W. ALBERTKOYES,JR.
468
oxygen consumed per triplet state molecule formed. The figures vary from about 0.5 to about 2 . The values are all small which means that reaction chains must be short at room temperature. A higher value of d3 or if k17 is qot zero would mean that shorter or longer chains, respectively, would occur. It has been pointed out to us by Dr. K. 0. Kutschke that
where q = 1.5 X 10-15 ml. molecule-’, the quenching constant. Hence kl6/ICl2 could be evaluated. The data are not accurate enough, however, to make such an evaluation mean much. An estimate of d3 has been made by Backstrom and Sandros. l7 They find that the phosphorescence yield in benzene solution for biacetyl directly photo-excited is equal to that for biacetyl phosphorescence sensitized by benzophenone (when the biacetyl concentration is high enough for maximum energy transfer from benzophenone). They find that the sensitized emission is pure phosphorescence. Therefore, they conclude that the efficiency of formation of triplet biacetyl from the singlet is near unity, L e . , k 5 >> IC, Ice. However, they note that “since the phosphorescence lifetime of
+
The Journal of Phpical Chemistry
biacetyl in the vapor state at 25” has been foundg to be 1.80 X l o p 3 sec., whereas the natural lifetime may sec., the phosbe assumed18 to be about 2.25 X phorescence yield observed by Almy and Gillette corresponds to a conversion efficiency of only about 2O%.” (The value quoted for the natural lifetime is the lifetime measured by lIcClure’* in E.P.A. a t 77°K.) Thus Biickstrom and Sandros conclude that + P = 0.20. Ishikawa and no ye^'^^^^ have found that the ratio of biacetyl-phosphorescing to biacetyl-excited, for the emission sensitized in the gas phase by benzene, is 0.15, which is the same as that found by Almy for the unsensitized emission. Thus the argument of Backstrom and Sandros holds equally well for the vapor. The present results, though somewhat low, are consistent with @P = 0.20. Unfortunately, the interpretation t o be placed on the present data is by no means clear. In any case, this work does not yield any information on the efficiency of formation of triplet biacetyl as distinct from @p. (17) H. J. L. Backstrom and K. Sandros, Acta Chem. Scand., 14, 48 (1960). (18) D. S. McClure, J . Chem. Phys., 17, 905 (1949). (19) H. Ishikawa and W. A. Noyes, J r . , J . A m . Chem. Soc., 84, 1502 (1962); J . Chem. Phys., 37, 583 (1962). (20) H. Ishikawa, Dissertation, University of Rochester, Rochester
N. Y., 1962.
