ISOTOPIC EXCHANGE OF EXCITED OXYGEN ATOMSWITH CARBON MONOXIDE
1767
Isotopic Exchange of Excited Oxygen Atoms with Carbon Monoxide
by E. A. Th. Verdurmen FOM-Laboratorium voor Massascheiding, Kruislaan 407,Amsterdam, Netherlands (Received Odober I& 1966)
Isotopic exchange of the excited singlet oxygen atoms, O(singlet), produced by photolysis of N20 at 1850 A, with CO labeled with '*O has been studied at room temperature and within the NzO partial pressure range 6-48 torr. The fraction of the O(sing1et) atoms that exchanged with CO has been calculated from the measured '*O content of the product 02. The exchange depends on the total pressure of a particular mixture, is suppressed by addition of Kr and SFe, being inefficient collisional deactivators of the electronic energy of the O(sing1et) atoms, but is stimulated by addition of Xe, an efficient deactivator of these excited atoms. The N2 quantum yield in NzO is only slightly reduced by addition of CO. Relatively small amounts of COZ are formed. The results are interpreted in terms of a mechanism in which the isotopic exchange is ascribed to the reactions
CO
+ O(sing1et)
--j
COz*(singlet) -+ O(sing1et)
+ CO
The excited association complex COz*(singlet) should have a lifetime long enough to allow its collisional deactivation in the applied pressure range. The effect of increasing total pressure and adding foreign gases is vibrational deactivation of the COz*(singlet)resulting in predissociation to ground-state oxygen atoms. These atoms are mainly removed by formation of COz. Consequently part of the singlet oxygen atoms reacting with CO are withdrawn from exchange and prevented from entering the product 02. It is assumed that effective isotopic exchange occurs between the excited singlet oxygen atoms and the product 02. An upper limit of the ratio of the rate constant k; of O(sing1et) N20 --c Nz 0 2 and the rate constant ks of O(sing1et) CO + CO2*(singlet) has been obtained (kalks < 1.2).
+
Introduction Study of isotopic exchange reactions can provide valuable information on the elementary steps of a chemical process, information that only with difficulty can be obtained otherwise. Particularly in the case of reactions of an atom with a simple molecule isotopic exchange may be concerned with,the formation of an excited association complex, its lifetime, and the requirement or the effect of a third body. The present work was undertaken in order to determine whether such an excited association complex occurs in the direct reaction of 0 atoms with CO. This reaction has been suppomd to take place in the isotopic exchange of oxygen between CO and O2.' An excited association intermediate in fact has been found in the direct isotopic exchange of ground-state oxygen atoms with 0 2 , NO, and NOZ,~JFrom the point of view of
+
+
chemical kinetics it would be of interest to use excited oxygen atoms, as reactions of these atoms have been investigated relatively little. The metastable states O(lD) (2 ev) and O ( 9 ) (4.2 ev) have radiative lifetimes of, respectively, =lo0 and = 1 sec. In the study of reactions of these species the electronic deactivation of the atoms has to be taken into account. Raper and DeMore' investigated the reaction of CO with O(lD) by photolysis of 0 3 at 2537 A in liquid CO at 77°K. The mechanism is an association process in which the excited association complex may undergo (1) E.A. Th. Verdurmen and C. A. Bank, J. I o r g . Nucl. Chem., 25, 1521 (1963). (2) J. T. Herron and F. S. Klein, J . Chem. Phys., 40, 2731 (1964); 41, 1285 (1964). (3) W.Brennen and H. Niki, ibid., 42, 3725 (1965). (4) 0.F. Raper and W. B. DeMore, ibid., 40, 1053 (1964).
Volume YO, Il'umber 6 June 1966
E. A. TH. VERDURMEN
1768
unimolecular decomposition to the original reactants, or to products of lower electronic energy in the course of stepwise vibrational deactivation. Isotopic exchange of O(lD), generated from O3by 2537-A ultraviolet light, with COz was studied by Katakis and Taube6 and ascribed to a direct exchange reaction leaving the excited oxygen atom in the same excited electronic state. Recently data became available6-* on the efficiency of collisional deactivation by various kinds of molecules of the singlet oxygen atoms produced by photolysis of NzO at 1850 A. That information is essential in kinetic studies of reactions of these excited atoms. At 1850 A there is enough energy to form from NzO both O(lD) and O(lS) atoms. Usually it is assumed that the oxygen atoms formed are in the O(%) state,@~lo but yet there are arguments in favor of O(1D).697 Yamazaki and Cvetanovi66 studied isotopic exchange of these oxygen atoms with C02 and interpreted their observations in terms of a mechanism in whichJ as a result of oxygen exchange, the excited oxygen atom is deactivated to the ground state, either by a "knockoff" process or a complex format>ionprocess. The present paper reports a study of the mechanism of isotopic exchange of the excited singlet oxygen atoms, produced by photolysis of N20at 1850 A, with carbon monoxide labeled with lSO. The results indicate that in the reaction of these singlet oxygen atoms with CO an association complex is formed with a lifetime of kinetic significance. Experimental Section The lamp used for these measurements was a lowpressure mercury-arc type Hanovia 24-in. straight cold cathode discharge lamp, emitting nearly exclusively the mercury resonance lines 2537 and 1850 A. The relative intensity of these lines was measured with a grating vacuum spectrograph evacuated to 10-5 torr and the light source against the 1 mm thick LiF window. The line intensity at 1850 ,4 was 19% of that a t 2537 A. The total flux of photons at 1850 A into the reaction vessel was 5 X lo1'quanta sec-l, and 3-23% of this was absorbed by NzO. The photolysis cell shown in Figure 1 consisted of three concentric cylindrical tubes. The innermost was constructed of Spectrosil synthetic fused silica, with very high transmission at 1850 A. This tube, i.d. 21 mm, length 65 cm, acted as the lamp holder. During operation Nz was passed to prevent reduction of light intensity by absorption in air and the attendant ozone formation. The second and third tubes were constructed of Vitreosil quartz with fairly high transmission a t 1850 A. The volume between the inner and the second tube contained the reaction mixture, while the The Journal of Physical Chemietry
r
"
kovar"
lampholder reaction vessel actinometer vessel
E E 0
m
"
discharge Lamp
Vi treos i I"
"spectrosil'
N,
--I--=
-
"kovar"
Figure 1. Photolysis cell and lamp position.
outer volume was filled with NzO that served as a relative actinometer. The volume of the reaction vessel was 560 cc, that of the actinometer vessel was 835 cc. The optical path length in both vessels was 11 mm. As the absorption coefficient of NzO a t 1850 A is 3.7 atm-l cm.-', it can be calculated that in the applied NzO partial pressure range (6-48 torr) light absorption occurred almost equally over the whole path length. The cell was cleaned with HNOI before mounting and then was baked a t 120" under vacuum. To avoid mercury-photosensitized reactions the reaction system was never exposed to mercury vapor and otherwise mercury was kept out of the reaction vessels very carefully. Before admitting a gas mixture the vessels were evacuated for about 2 hr to below 1 X lo-' (5) D.Katakis and H. Taube, J . Chem. Phys., 36, 416 (1962). (6)H.Yamaaaki and R. J. Cvetanovi6, ibid., 40, 582 (1964). (7) H.Yamazaki and R. J. Cvetanovib, ibid., 41, 3703 (1964). (8) H. Yamazaki and R. J. Cvetanovib, ibid., 39, 1902 (1963). (9) M. Zelikoff and L. M. Aschenbrand, ibid., 22, 1680,1685 (1964). (10) J. P. Doering and E. H.Mahan, ibid., 36, 1682 (1962).
ISOTOPIC EXCHANGE OF EXCITED OXYGENATOMSWITH CARBON MONOXIDE
torr, measured by an ionization gauge. Two traps both cooled by liquid nitrogen and separated by a stopcock were interposed between the pumping system and the vacuum line. The evacuated vessels stood overnight closed from the vacuum line, the background pressure never exceeding a few times torr. The gas pressures in the reaction vessel and in the actinometer vessel were ineasured by means of two glass Bourdon gauges as null indicators against a mercury manometer (accuracy 0.5 torr). Samples were stored for at least 20 hr in cylindrical reservoirs, i.d. 3 cm, volume 300 cc, continuously cooled by Dry Ice-acetone (- 78"), and subsequently admitted by expansion to the reaction vessel and to the actinometer vessel. Before mounting, the inlet tubes outside the reaction vessels were supplied with gold wires. The absence of mercury vapor was verified by measuring the absorption of the 2537-A line in the samples with the help of a grating spectrograph equipped with a photomultiplier. For that purpose the samples were admitted to a separate optical cell, length 35 cm, with parallel quartz windows. No detectable absorption was measured, indicating that the mercury partial pressure was below 3X torr. The gases N20, C02, and SF6 were purified by repeated bulb-to-bulb distillation; CO and 0 2 were condensed in a liquid nitrogen trap a t - 196" followed by slow evaporation, collecting a middle fraction. Purest available Ne, Kr, and Xe were used without further purification. Natural Oz was prepared by gentle heating of KMn04. Natural and enriched CO were obtained from purified COz by reaction on charcoal at 1000". Enriched COz was made by reaction of enriched 0 2 on charcoal a t 500". By electrolysis of H21s0 (90%) a small amount of lsOZ was prepared. The purity of the gases was checked by mass spectrometric analysis. Total impurities did not exceed O.l%, except for Kr that contained 0.8yoXe. To the NzO samples a small amount (1.4%) of Ne was added, which served as a standard for relative measurement of the product nitrogen. The l8O content of most of the samples of CO wtis about 2.4%. The products of reaction were separated by passing them through a liquid nitrogen trap at 77"K, retaining the N 2 0 and CO?, and through a solid nitrogen trap a t about 58"K, ret,aining NO. Thus Kr, Xe, and sF6 were condensed additionally. The CO in the noncondensable fraction was oxidized with iodine pentoxide" and subsequently condensed. The remaining fraction was N2, 02,Ne, and a small amount of CO. The amount of nitrogen formed was quantitatively measured by gas chromatographic analysis on a Molecular Sieves 5A column, length 4 m, a t 58" with He as
1769
the carrier gas. Isotope ratios in OZand in COz were determined mass spectrometrically. Product COzdetection in NzO was based on the m/e 22 peak of C02. The experiments were done a t room temperature, 23 2", and in the NzO partial pressure range 6 4 8 torr. To prevent photolysis of reaction products and interference of undesired secondary reactions, the reaction time was usually 240 sec or less and conversion of N20 did not exceed 5%.
*
Results The amount of the reaction products Nz and C02and the l80content of the product 0 2 were determined. The relative formation of Nzin N20-CO mixtures was compared with that in N20-CO2 mixtures. As shown in Figure 2 the nitrogen yield is decreased by addition of both CO and COz, but the effect of CO is considerably less marked. From the data in Table I it can be seen that, within the time limits involved, varying the irradiation or reaction time for a particular mixture has no influence on the l80content of the product 02,the differences being ascribed to experimental inaccuracy. Figure 3, showing l80contents of product 02 a t various mixing ratios [N20]/[CO] and different K20 partial pressures, illustrates two significant observations. First, a t a particular mixing ratio the I 8 0 content of the O2 is the higher the lower the pressure. Thus in the pressure range involved, the competition between N20 and CO for reaction with the primary excited oxygen atom is determined not only by the mixing ratio but also by the total pressure of the Relative formation of t
N2 [ N 2 0 ] = 4 6 torr
l
M-CO
OP8 Oa7
M= C 02
1 1 0
I
I
I
I
I
0.5
1.0
1.5
20
2.5 __c
Figure 2. Relative yields of nitrogen by addition of CO and by addition of COZ(irradiationtime, 180 sec). (11) C. A. Bank, E. A. Th. Verdurmen, A. E. de Vries, and F. L. Monterie, J . IWTQ. Nucl. Chena., 17, 295 (1961).
Volume 70.Number 6 June 1966
E. A. TH. VERDURMEN
1770
220
1
f=-
-g 2, - $0 z
wherein z is the l80content in the product 02. So from the data of Figure 3 corresponding exchange fractions were obtained and plotted in Figure 4. These plots suggest a linear relation between l/f and [N2OI/ [COI. The effect of total pressure on the l80content of the product O2 and on the attendant exchange fraction was studied separately, and the results are represented in Figure 5. There is obviously no linear relation between l/f and the total pressure.
Figure 3. content in product 0 2 as a function of the mixing ratio, [N20]/[CO], at various NzO partial pressures (irradiation time, 240 sec).
Table I: 1850-A Photolysis of NzO-CO Mixtures. 180 Contents in Product O2 a t Various Irradiation Times" Irradiation time. sec
180 210 240 270 300
Quantum dosageb
x
10-*o
0.79 0.99 1.19 1.39 1.59
%
180
in 01
0.475 0.480 0.471 0.467 0.466
a [NzO]/[CO] = 0.95;total pressure, 95 torr; l a 0 content of CO, 1.39%. Calculated assuming quantum yields $N, = 1.4 and 4 ~ = ~ 1.65 0 in pure NzO,g and a NzO absorption coefficient a t 1850 A of 3.7 atm-l cm-l.
mixture. Secondly, a t a particular partial pressure of N20 the l80 content of the product 0 2 tends to the value of the ' 8 0 content of the initial CO for [NzO[/ [CO] -t 0. I n such an extreme mixture practically every primary oxygen atom will exchange with CO. By photolysis of pure (natural) N2O evidently natural product O2 is formed. So from the various lSO contents exchange fractions can be obtained. Let the '*O content of the CO used be z,, and the l80 content of the (natural) N2O be zo. Then the fraction of the excited oxygen atoms that exchanges with CO is calculated from the equation The Journd of Physical Chmiatq/
Figure 4. Relation of exchange fraction and CO partial pressure at constant N20 partial pressure (180 content of CO,2.40%; irradiation time, 240 sec). 4-1
iI I
0
so
I
100 -Total
Figure 5. Relation of exchange fraction and total pressure a t constant mixing ratio [N20]/[CO] (180 content of CO, 2.34%; irradiation time, 240 sec).
I
150 torr pressure
ISOTOPIC EXCHANGE OF EXCITED OXYGEN ATOMSWITH CARBON MONOXIDE
By addition of Kr and of SFe the l80content of the product O2 and consequently the exchange fraction is decreased, Kr and SFe being completely inefficient collisional deactivators of electronic energy of the excited oxygen atoms.6-8 However, the exchange fraction is enhanced by addition of Xe, an efficient deactivator of the electronic energy of the excited oxygen In this connection it has to be considered that ground-state oxygen atoms too are supposed’ to show isotopic exchange with CO, while under the experimental conditions those atoms do not react with N20.12 The effect of added gases is reproduced in Figure 6. As is pointed out in the Discussion section, rapid isotopic exchange between the excited oxygen atoms and the product molecular oxygen is assumed. To investigate this phenomenon “ ~ c r a m b l i n g ”of~ ~oxygen isotopes in O2 was studied by 1850-A photolysis of NzO-02 mixtures (0.5% 0,). I n Table I1 the changing product of the mass ratios 34/32 and 34/36 is used as an indication of exchange. The limiting value of that product at equilibrium is 4.0. Just a few seconds irradiation time suffices to produce extensive scrambling of isotopes. The initial O2 is diluted by the photolysis product 0 2 so that the l80content of the oxygen is decreasing with increasing irradiation time. The attendant change in the product of the mass ratios 34/32 and 34/36 appears to be small and the corresponding product values have been reported in Table I1 under the heading “calculated.” According to the proposed mechanism discussed in the Discussion Table 11: The 1850-A Photolysis of N20-O2 Mixtures. Scrambling of Oxygen Isotopes in 0 Irradiation Times’ Irradiation time, Eec
...
...
20 30
0.36 0.60 0.87 6.3 9.4 12.6
0.0016 0.0016 0.0016 0.0018 0.0021 0.0026
40
a t Various
Product of ma88 ratioa 34/32 and 34/36 Calcd Found
...
150 200 250 a
Quantum dosage X 10-10
2
0.0015 0.035 0.074 0.103 0.266 0.307 0.362
%
I@
in Oa
3.87 3.57 3.29 3.07 1.19 0.90 0.73
[OZ]/[N~O] = 0.0041; total pressure, 25 torr.
section, the redistribution of isotopes in the oxygen ultimately is held out of balance by the continuous formation of product 0 2 . To what extent oxygen atom recombination a t the wall is involved was investigated by addition of an inert gas. From the data in Table I11
1771
-1 -1
it/I]
[#I:
0.5 0
[N 01:12 torr
20 -
1.5
1.0
-
I
0
I
I
I
10
20
30 __L
I
40 torr
[MI
Figure 6. Relation of exchange fraction and the pressure of foreign gases added to a particular mixture, [N20]/[CO] (180 content of CO, 2.27%; irradiation time, 240 see).
it is obvious that in a particular mixture the scrambling of isotopes in 0 2 is remarkably enhanced by addition of Kr. As the irradiation time, and hence the amount of product 02,is constant in these experiments, next to the product of mass ratios 34/32 and 34/36, now the exchange fraction F has been calculated and reported. The interpretation of the scrambling experiments, given in the next section, demonstrates that the measured redistribution of isotopes in the oxygen has to be ascribed to the isotopic exchange of ground-state oxygen atoms with 0 2 . So the scrambling appears to be unsuited for proving the supposed exchange of the excited oxygen atoms with 02. Product C02 had to be detected in a great excess of N20. By mass spectrometric analysis, by way of the m/e 22 peak of C02, just about 0.1% could be detected as a lower limit. In all performed experiments product C 0 2 was found in amounts little above the detection limit, ie., between 0.1 and 0.3%, subject to considerable experimental inaccuracy. Though these amounts are small, it should be remembered that the total number of excited oxygen atoms produced by photolysis did not exceed 3% of the number of N2O molecules available. Thus the amounts of C02 found indicate that in the experiments involved, apart from experimental inaccuracy, between 3 and 10% of the primary excited oxygen atoms was ultimately converted into C 0 2 . (12) F. Kaufman, P T O ~Reaction T. KindiC8, 1 , 27 (1961). (13) R. A. Ogg, Jr., and W. T. Sutphen, ~iseussionsFaraday SOC., 17, 47 (1954).
Volume 70,Number 6 June 1966
E. A. TH. VERDURMEN
1772
Discussion Mechanism. There is strong supportlo for the occurrence of two primary processes in the 1850-A photolysis of N 2 0: a principal (85%) oxygen atom primary process reaction (l),and a less probable (15%) nitrogen atom primary process reaction (2).
+ hv --+ A NZ(lZ) + O(lD) or O(lS) A NzO + hv +NO('II) + N(4S)
NzO
1850
(1)
1850
(2)
Next, the following secondary reactions have to be considered, to reaction 1
+ N2O('Z) N#Z) + O#Z) O(lD) or O(lS) + N20(lZ) + NO(TI) + NO(TI) O(lD) or O ( 9 )
--f
(3)
+ NO(Q) -+Nz(lZ) + O(aP) o(") + o(3~) + M + 02(32)+ M
(4)
(5)
(6)
I n the present study it is assumed that the excited singlet oxygen atom formed in reaction 1 is the important reactive species. The existing information on the nature other than the multiplicity of the primary excited oxygen atom is insufficient to exclude either O(lD) or O(%) as the product in reaction l . 1 4 7 1 6 Therefore, in the following discussion the excited singlet oxygen atom involved is referred to as O(sing1et). The main experimental results of the present work are summarized as follows : (i) The nitrogen quantum yield is only slightly decreased by addition of CO. Since ground-state oxygen atoms practically do not react with N20,12 scavenging as well as deactivation of the excited singlet oxygen atoms will suppress reaction 3 and consequently the Nz quantum yield. Thus CO appears to be a rather ineffective scavenger and deactivator of the electronic energy of the singlet oxygen atoms. (ii) Effective oxygen isotopic exchange between the excited singlet oxygen atoms and the CO is observed. (iii) The exchange fraction depends not only on the ratio of [KzO]and [CO] but also on the total pressure. (iv) Increasing the total pressure of a particular mixture results in a smaller exchange fraction. (v) The effect of added foreign gases strongly depends on its capability of collisionally deactivating the excited singlet oxygen atoms. (vi) Addition of an inefficient deactivator of the excited singlet oxygen atoms to a particular mixture results in a smaller exchange fraction. The Journal of Phyaical Chemistry
+ O(sing1et) (1) O(sing1et) + NzO Nz + (3) O(sing1et) + M +O(3P) + h1 (7) O(sing1et) + CO COz*(singlet) (8, 8') COz*(singlet) + M +COz**(singlet) + M (9) C02**(singlet)+CO + O(3P) (9') C02**(singlet) + M + COz + 11 (10) O(sing1et) + + O(sing1et) (11) + co co + (12) + co -+co2 (13) N2O
+ hv
1850 A d N2
0 2
-+
and to reaction 2 N(4S)
(vii) Small amounts of COzare detected. The rather ineffective scavenging and deactivating property of CO for singlet oxygen atoms and its effective isotopic exchange with these atoms are indications for an exchange reaction without attending deactivation of the singlet oxygen atoms. Observation iii is a strong argument for the occurrence of a complex mechanism. Observations iv and vi suggest that isotopic exchange is opposed by collisional vibrational deactivation. The results can be interpreted in terms of the following mechanism.
0 2 40 2
o(3~)
-+
o(3~)
o(3~)
where M, which can be any molecule in the mixture, is a deactivator. The definite reaction process between O(sing1et) and CO, reactions 8-10, is illustrated diagrammatically in Figure 7 showing COZpotential curves; for the sake of simplicity and for lack of information with relation to O(lS), only states corstates correlating with CO O(lD) and with CO 0 relating with CO (") are reproduced.16 CO and O(3P) interact to form repulsive states" which cross the ground electronic state and a number of excited states of C02. So there is a finite probability that a COzmolecule in one of those states will undergo radiationless transition to such a repulsive state, resulting in predissociation to CO O(3P). The reaction of O(sing1et) and CO is considered to be an association process in which the excited asso-
+
+
+
+
(14) P. Warneck, J. C h m . Phys., 43, 1849 (1965). (15) R. J. Cvetanovih, ibid., 43, 1850 (1965). (16)The excited singlet state of COZ represented is the 'Bz state with its vibrational zero level lying about 30 koa1 above that of the lowest triplet state, ~Bz.The 1Bz state of COz is supposed to be the radiating state in the CO-flame band emission. Cf. M. A. A. Clyne and B. A. Thrush, Proc.Roy. SOC.(London), A269, 404 (1962). (17) A repulsive ~ Bstate z correlating with CO O(3P) presumably O(8P) -P is the origin of the observed energy barrier in the GO COZreaction. Cf.ref 16.
+
+
ISOTOPIC EXCHANGE OF EXCITED OXYGENATOMSWITH CARBON MONOXIDE
by a third body, as there is no need for spin conservation by a third body. Exchange Fraction. By steady-state treatment of reactions 8-10 it is found that (a) the number of 0 atoms wjthdrawn from exchange (by predissociation and deactivation) per unit of time in the steady state is represented by
Potential energy Kcal/mole
A 2o01 I
1773
I ,
1-b
150
+';(kt
kn[MIn k"[M]" k[M])" + k[nll(k' k[M])
+
+
and (b) the number of 0 atoms exchanging per unit of time in the steady state is given by k's [C02*1. From these expressions it can be derived that the fraction of primary singlet oxygen atoms that exchanges with CO and the partial pressures of reactants are related by
-Reaction
}
kW1 k"[bIl" k'B (k' k[M])"
co-ordinate
+
Figure 7 . Schem:ttic diagram of COZpotential energy curves in the CO O(1D) reaction.
+
ciation complex C02*(singlet) may redissociate to the original reactants,ls or is deactivated by collision to lower vibrational states, COz**(singlet), that are unstable with respect to predissociation and subject to further vibrational deactivation. As the exchange fraction is decreased by increasing total pressure and adding foreign gases, isotopic exchange is ascribed to reaction 8', so that after exchange the oxygen atom still is in the singlet state. The complex COz*(singlet) actually is a C02 molecule in its electronic ground state or in one of its excited singlet electronic states, but very probably in a high degree of vibrational excitation. Collisional deactivation of these molecules occurs very readily.lg Thus the effect of increasing total pressure and adding foreign gases is the vibrational deactivation of the excited association complex COz*(singlet) resulting in predissociation to ground-state oxygen atoms, respectively, in formation of stable C02. Under the conditions of %he experiment, ground-state oxygen atoms are removed mainly by recombination to COZ (vide infra). Consequently, part of the singlet oxygen atoms reacting with CO are withdrawn from exchange and prevented from entering the product Oz, thus decreasing the '*O content of 02. The initial formation of C02* (singlet) from O(sing1et) CO has to be a bimolecular reaction. The full excitation energy of the bimolecular association complex is needed to allow its redissociation to CO O(sing1et) performing isotopic exchange. So in the initial reaction there is no need for energy stabilization
+
+
(14)
The integer n is the number of vibrational states, C02** (singlet), having a rate constant for predissociation k' # 0. For the sake of simplicity, the rate constants for vibrational deactivation k have been assumed to have the same value for every vibrational state C02** (singlet), ks = klo = k. A similar assumption has been made for the various predissociation rate constants k'a = k', much less rightly as the probability of predissociation is highest near the point of crossing of the potential curves. Additionally different vibrational deactivation rate constants k will appear for different species M. So the expression (14) is an approximation. The exchange fraction defined in the Results section and represented in Figures 4-6 is not simply identical with the fraction f of expression 14. According to the mechanism, excited oxygen atoms, whether exchanged or not, are removed by reaction 3 producing 02. Thus the concentration of l80in the atoms is diluted by the natural oxygen of N20 in reaction 3. Kevertheless, content in 0 2 equal to that of Figure 3 suggests an the reactant CO for the extreme mixture [NzO]/[CO] = 0. Therefore, it is assumed that rapid isotopic exchange occurs between the O(sing1et) atoms and the product OZ2O reaction (11). By this exchange the (18) The lowest excited state of CO is a state with its vibrational zero level near 6 ev. Available energy (2 or 4.2 ev) is insufficient for occurrence of the reaction COz*(singlet) -+ CO(3II) + O(SP). (19) V. N. Kondrat'ev, "Chemical Kinetics of Gas Reactions," translated by N. B. Slater, Pergamon Press Ltd., London, 1964, p 401.
(20) Katakis and Taube6 measured effective isotopic exchange between 01 and O(1D).
Volume 70,Number 6 June 1966
E. A. TH. VERDURMEN
1774
natural oxygen atom from the secondary reaction with NzO and the primary oxygen atom will have the same probability of reaction with CO. Then, consequently, both fractionsf are identical. The shape of the curves predicted by eq 14 strongly depends on the relative magnitude of the rate constants k' and k [ M ] . If predissociation completely predominates deactivation to stable COZ, as is expected for the gas phase reaction CO O(1D),4the last term in eq 14 is relatively small. Then for a particular mixture [N20]/[CO] a plot of (l/f - 1) against total pressure [MI would show a curve considerably increasing proportional to [MI" a t low [MI, with gradually decreasing slope approaching to zero a t higher values of [MI. This essentially is the shape of the curves in Figures 5 and 6 except for Rf = Xe. Figure 4 suggests a linear relation between (l/f) - 1 and 1/[CO] for [N20] = constant. Thus for CO the effect of [MI should be small or such that a linear relation between (l/f) - 1 and 1/[CO] is obtained. As can be seen from Figures 4-6, the effect of the varying mixing ratio [NzO]/[CO] relative to the effect of foreign gas addition is higher by a factor of 10-20. If the foreign gas effect of CO is comparable with that of Kr (Figure 6) hardly any deviation from linearity in Figure 4 is expected. The straight lines in Figure 4 practically intersect the origin as is expected from eq 14. Figure 5 clearly shows that if the foreign gas effect of CO is small that of NzO must be substantial. With some qualification concerning the small contribution of the M terms in (14) from the slopes in Figure 4 the ratio of the rate constants k3 and ks can be obtained. The lowest value of k3/ks = 1.5, deduced from the measurements a t [NzO] = 12 torr, has to be considered as an upper limit. I n the hypothetical case [MI = 0 the expression (14) reduces to
+
1 2k3 [Nzo1 1 = ks IC0 I f
(14a)
indicating that in this case not a single excited oxygen atom is deactivated. Accordingly,, for the particular mixture with [N20]/[CO] = 0.5, a t [MI = 0 the value of (l/f) - 1 equals that of k3/ks. Though in Figure 5 the shape of the curve below [MI = 18 torr is not known, it seems justified to conclude that the value of (l/j) - 1 a t [MI = 0 probably is smaller than 1.2. Hence, it appears that k3/ks < 1.2. Another argument to consider this value as an upper limit is adduced a t the end of the Discussion section, Fate of Ground-State Oxvaen Atoms. Recombination "" of ground-state oxygen atoms under the condition of the present experiments occurs by the reactions The Journal of Physical Chemistry
+ co --+co2 O(3P) + + SI + h1 + + w -+oz+ w o(3~)
0 2
o(3~) o(3~)
0 3
(13) (15)
(16)
The direct recombination in the gas phase is too slow to be of importance. Reaction 16 is wall recombination. With the knownz1 rate constants the relative probability of these reactions can be estimated. Depending on partial or total pressures, between 70 and 80% of the O(3P) atoms is converted into CO,. At low total pressures the remaining atoms are recombined mainly at the walls, but a t higher total pressures those atoms are converted into ozone. The ozone steady-state concentration is estimated to be between and torr. The ozone absorption coefficient a t 2537 A being 134 atm-I cm-', it is calculated that the number of quanta a t 2537 A absorbed by O3 will be not more than 1% of the number of quanta a t 1850 A absorbed by NzO. From the magnitude of the decrease in Nz quantum yield by addition of CO represented in Figure 2, and the data of Figure 5, the rate of formation of O(3P) atoms by deactivation at lower NZO partial pressures can be estimated. With this information the amounts of product C02 expected are calculated. Depending on total pressure, [COz]/[NzO]ratios between 0.3 and 0.7% should be found. The experimentally detected amounts are smaller by a factor of 2, but the experimental inaccuracy was considerable so that experiment and calculation probably are not in disagreement. Foreign Gas Efect. Collisional deactivation of the excited association complex COz*(singlet) will occur very readily.lg Except in the case of exact resonance the vibrational quantum is not transferred into vibrational energy of the colliding molecule 31, but is converted into energy of relative translational motion. A Kr atom, therefore, will not simply be a less effective deactivator than SF6 with its considerable number of vibrational degrees of freedom. Actually, the foreign gas effect of KY and probably that of CO ?vide supra)-is smaller than that of the heavier gas SF6. Therefore, the isotopic exchange is not due t,o reactions of O(sing1et) with excess translational energy. The primary effect of Xe is deactivating the excited singlet oxygen atoms to the ground state. Though isotopic exchange between CO and O(3P) has not been (21) kls (31 = CO) = 5 x 10-34 cme molecule-2 sec-1; kl5 (11 = ~ ~= 0 1.3 x ) 10-33 cm6 molecule-2 sec-1: S.w.Benson and A. E. Axworthy, J. Chem. Phys., 42, 2614 (1965). kla was calculated from the recombination coefficient on Vitreosil and the vessel dimensions: k l r = 5.5 sec-1: J. c. Greaves and J. W. Linnet, Trans. Fara&y SOC.,55, 1355 (1959). k13, the bimolecular rate constant, was used; k1g = 2.1 x 10-17 om3 molecule-' sec-1: B. H. Mahan and R. B. Solo, J . Chem. Phys., 37, 2669 (1962).
ISOTOPIC EXCHANGE OF EXCITED OXYGENATOMSWITH CARBON MONOXIDE
1775
~
~~
Table 111: 1850-A Photolysis of Nz0-02 Mixtures. Effect of Addition of an Inert Gas on Scrambling of Oxygen Isotopes in Oz4 Pressure of added Kr, torr
*.. 7 26 71
--34/32
Product of ma88 ratios and 34/36Calcd Found
0.0010 0.0010 0.0010 0.0010
Exchange fraction Fb
0.035 0.080 0.219 0.619
0.34 0.49 0.68 0.85
%
180
in Oz
3.51 3.50 3.48 3.46
Exchange rate, arbitrary units
1.0 1.6 2.8 4.6
l/Dc estimated, arbitrary unita
1.0 1.3 2.0 3.9
-
a [02]/[N20]= 0.0044; total pressure, 25 torr; irradiation time, 20 sec. F = ( y t y0)/(ym - yo), wherein yt, yo, and ym are the fractional contents of mms 36 in 02, respectively, at time t = t, t = 0, and 1 = m . D is the diffusion coefficient of oxygen atoms.
measured explicitly, there is strong indication’ for a rather efficient exchange. Reaction 12 presumably will have an activat,ion energy” of about 4 kcal/mole, but is not spin forbidden as is reaction 13 with its transmission coefficient of Therefore, before recombination into C02 the ground-state oxygen atom probably will have sufficient time to undergo isotopic exchange with CO. Part of the ground-state oxygen atoms is recombined into 02. Thus, oxygen atoms deactivated by addition of Xe undergo isotopic exchange with CO and subsequently recombine partly to 0 2 enhancing t’hel80content of the product 0 2 . Nature of C02*(Singlet). At the higher pressures in Figure 5 the variation of (l/f>- 1 with [MI is small. I n this range eq 14 can be modified to the approximated expression (last term in eq 14 omitted)
From a comparison of the value of (l/f>- 1 a t low pressures and the value a t higher pressures in Figure 5 it can be concluded that the rate constant for predissociation, k’, and the dissociation rate constant, k’g, are in the same order of magnitude. These rate constants are determined by the lifet,imesof, respectively, C02** and COz* that can be estimated using the Rice-Ramsperger-Kassel model for unimolecular decomposition.22 As the depth of the potential well of the triplet COz state involved in the predissociation probably is small, the lifetime of C02** will be determined by the singlettriplet radiationless transition. As this transition is forbidden probably by a factor of 10-4,2a the lifetime can be esOimated to be 10-9 sec with corresponding k’ = lo9 sec-l. If COz* is a vibrationally excited COZ molecule in its electronic ground state (depth of the potential well, 173 kcal), then a lifetime of 3.9 X sec is obtained and k’g = 2.6 X lo’ sec-l. A mean collision frequency a t 36 torr is 4.2 X lo8sec-’. Thus, a t that pressure on the average, the COz* with k’g =
2.6 X lo7 sec-’ endures about 20 collisions before redissociation. As mentioned above, the probability of conversion of vibrational quanta into translational energy for these collisions is high, between 0.1 and l.19 The observed effective isotopic exchange in the experimental pressure range and the deduced equality in order of magnitude of the rate constants k’ and k’8 therefore are indications that the C02*(singlet)is not in its electronic ground state. The depth of the potential well of the excited electronic state involved probably has to be considerably smaller than 173 kcal. If the primary oxygen atom is O(lS), the ground state of C02 is out of consideration since CO O(lS) do not correlate with that state. Oxygen Scrambling in 02. The enhancement of the scrambling rate by addition of an inert gas, as reported in Table 111, indicates that the oxygen atoms in the chain reaction are suffering wall recombination. The scrambling rate will be proportional to the number of encounters of a particular oxygen atom and O2 before the atom reaches the wall or is scavenged by a gas phase reaction. If only wall recombination is concerned, this number of collisions of an oxygen atom will be inversely proportional to the diffusion coeffi~ient.~~ The agreement between the measured relative exchange rates and the relative reciprocals of the diffusion coefficient represented in Table I11 suggests that the only effect of addition of an inert gas is preventing the oxygen atoms to reach the wall. I n the photolysis of pure N20 wall recombination of the excited singlet oxygen atoms is not an important reaction, as is demonstrated by the quantum yield of Nz formation that appearedg to be constant in the pressure range 1-34 torr. Evidently
+
(22) S. W. Benson, “The Foundations of Chemical Kinetics,” McGraw-Hill Book Co., Inc., New York, N. Y., 1960, p 231. (23) K. J. Laidler, “The Chemical Kinetics of Excited States,” Oxford at the Clarendon Press, Oxford, England, 1955,p 34. (24) W. Jost, “Diffusion in Solids Liquids Gases,” Academic Press Inc., New York, N . Y., 1952,p 419.
Volume 70,Number 6 June 1066
E. A. TH. VERDURMEN
1776
the observed scrambling is performed by ground-state oxygen atoms. The source of these atoms can be found in the nitrogen atom primary process, reaction 2, that is followed by reaction 5. Thus, under the experimental conditions the assumed isotopic exchange between the singlet oxygen atoms and the product 0 2 has to be considerably less than the exchange between O(3P) and the 02,probably due to a lower O(sing1et) partial pressure. Concluding Remarks. Oxygen exchange between together ground-state oxygen atoms O(3P) and the 02, with the supposed exchange between O(3P) and CO, would give another path for le0 transfer from CO to 02. For this over-all exchange at a particular 0 2 partial pressure, the half-life can be expressed as
Obviously in the present experiments isotopic equilibrium between CO and O2 has not been attained, and, therefore, the l8O content of the product 02 should vary with the O2 partial pressure. Table I demonstrates that the l80content of O2 does not depend on irradiation time and consequently on 0 2 partial pressure. So the indicated transfer is not important. The half-life of the over-all isotopic exchange between CO and O2 as a result of the combined O(sing1et) CO and O(sing1et) O2 exchanges, with the latter as the rate-determihing step, is given by
+
+
ty9 =
In 2
kll [O(singlet) ]
and no variation in l80content of O2 with irradiation time is expected.
The Journal of Physicd Chemistry
The absolute rate constant of reaction 3 is not known, but it is a fast reaction and so probably the exchange 02 will fall behind the O2 formation in O(sing1et) reaction 3. Figure 3, however, indicates that the difference cannot be considerable. Nevertheless, this is another argument to consider the measured value of k 3 / k 8 < 1.2 as an upper limit. The photochemically induced isotopic exchange in N2O-02 mixtures ascribed to the occurrence of groundstate oxygen atoms provides further indication for an additional nitrogen atom primary process in the 1850-A photolysis of N2O.l0 According to the proposed general mechanism, the decrease of the N2 quantum yield by addition of CO shown in Figure 2 should depend on the total pressure of the mixture. Accuracy and reproducibility of the gas chromatographic analyses for N2were too poor to demonstrate this effect. There is a striking resemblance between the present set of processes and the mechanism of the CO O(lD) r e a ~ t i o nbut , ~ that is insufficient argument for exclusion of O(%) as the primary product of 1850-A photolysis of ~ ~ 0 . 1 4 ~ 5
+
+
Acknowledgments. The author wishes to thank Professor Dr. J. Kistemaker and Professor Dr. J. A. A. Ketelaar and Dr. A. E. de Vries for helpful discussions and suggestions, and Mrs. J. de Gans and Miss A. Tom for assistance with the experimental work. This work is part of the research program of the Stichting voor Fundamenteel Onderzoek der RiIaterie (Foundation for Fundamental Research on Matter ) and has been made possible by financial support from the Nederlandse Organisatie voor Zuiver-Wetenschappelijk Onderzoek (Netherlands Organization for Pure Scientific Research).
