J. Phys. Chem. 1985,89, 845-849 mechanism is not known for either the k,, or the kbiscprocess.
Acknowledgment. This investigation was supported in Part by the National Science Foundation and by the U.S. Office of Naval Research. In addition, E. Zinato and P. Riccieri thank the Na-
845
tional Research Council of Italy (CNR). Registry No. Cr(NH3)5(CN)2t,42213-67-2; Cr(ND3)5(CN)2t, 68092-89-7; t r ~ n s - C r ( N H ~ ) ~ ( c76299-50-8; N ) ~ ~ , cis-Cr(NH34(CN)2t, 7798 1-98-7; truns-Cr(NH3)4(H20)(CN)2t, 74523-68-5; cis-Cr(NH,)477904-38-2. (H20)(CN)It, 74482-64-7; cis-Cr(NH3),(Me,SO)(CN)2t,
Photochemistry of Ketene in Oxygen/Argon Matrices and in Solid Oxygen at 11 K Hiroshi Bandow and Hajime Akimoto* Atmospheric Environment Division, The National Institute for Environmental Studies, P.O. Tsukuba-gakuen, Ibaraki 305,Japan (Received: August I , 1984)
Photolyses of matrix-isolated ketene in Ar, 02/Ar, and O2matrices at 11 K have been carried out by near-ultraviolet light in the 310-410-nm range. The product identification and semiquantitative measurements were made by Fourier transform infrared spectrometry. Ketene was not photodecomposed in the Ar matrix. The photooxidation products observed in solid O2 were HO2, CO, C02, HzO, and 03.The product identification was confirmed by the photolysis of CD2C0/02 and CHzC0/'80z samples. In addition to C'*02, C'60180was produced in the irradiation of CHzCO in solid Formic acid, formaldehyde, and CH202metastable species were not observed under our experimental conditions. The yield of H 0 2 against the reacted ketene decreased rapidly with the decrease of 02/Ar ratio whereas that of O3decreased more gradually and those of CO, C02, and HzO were insensitive to the ratio. Photooxidation of ketene in the OZ/Ar matrix and in solid 0, is suggested to occur via the reaction of the excited state of ketene surrounded by O2molecules rather than via the photolysis of ketene, giving CH2 + CO followed by the reaction of CH2 with 02.
Introduction Chemical reactions in low-temperature matrices have recently attracted considerable interest with the expectation that new reaction channels which are not observed in the gas phase might be opened in such media.'+ Photochemistry in reactive matrices such as O2could provide a type of reaction where the excitation of a reactant induces an oxidation pathway different from gasphase photooxidation. Lee and c o - w o r k e r ~have ~ . ~ reported such cases in the photooxidation of formaldehyde and SOzin solid 0, and Oz/Ar matrices. In the present study, near-UV (310-410 nm) photolyses of ketene in Ar, 02/Ar, and 0, matrices have been studied by means of Fourier transform infrared spectrometry so as to elucidate the reaction mechanism of matrix-assisted photochemistry of the CHZC0/Ozsystem and hopefully to isolate a C H 2 0 0 biradical
spectrum has not yet been observed. Photolysis of CH2O2/O2in the near-UV region in the gas phase is k n ~ w n ' ~to, ' involve ~ the reactions
+
CH2CO(g1A1) hv(1370 nm)
(1) Catalano, E.; Barletha, R. E.; Pearson, R. K. J. Chem. Phys. 1979,70, 3291. (2) Hauge, R. H.; Gransden, S. Wang, J. L. F.; Margrave, J. L. J . Am. Chem. SOC.1979, 101,6950. (3) Frei, H.; Pimentel, G. C. J . Chem. Phys. 1981, 74, 397. (4) Frei, H.; Pimentel, G. C. J. Phys. Chem. 1981, 85, 3355. ( 5 ) Smith, G. R.; Guillory, W. A. In?. J . Chem. Kinet. 1977, 68, 223. (6) Sodeau, J. R.; Lee, E. K. C. Chem. Phys. Left. 1978, 57, 71. (7) Diem, M.; Lee, E. K. C. Chem. Phys. 1979, 41, 373. (8) (a) Diem, M.;Lee, E. K. C. J . Phys. Chem. 1982,86,4507. (b) Tso, T.-L.; Diem, M.; Lee, E. K. C. Chem. Phys. Lett. 1982,91, 339. ( c ) Diem, M.;Tso, T.-L.; Lee, E. K. C. J . Chem. Phys. 1982, 76, 6452. (9) Scdeau, J. R.; Lee, E. K. C. J . Phys. Chem. 1980,84, 3358. (10) Herron, J. T.; Martinez, R. I.; Huie, R. E. Int. J. Chem. Kiner.1982, 14, 201 and references therein. (11) Harding, L. B.; Goddard 111, W. A. J . Am. Chem. SOC.1978,100, 7180 and references therein. ( 1 2 ) Sucnram, R. D.; Lovas, F.J. J. Am. Chem. SOC.1978, 100, 51 17. (13) Martinez, R. I.; Huie, R. E.; Herron, J. T. Chem. Phys. Left. 1977, 51, 457.
-
CH,CO('A") C H ~ C O * ( P A ~(1) )
CHz(5'AI) + C O
(2)
5 CH,CO(W~A~)
(3)
CH2CO*(g'Al)
-
CH2CO(g3A')
-
CH2(g3Bl) + C O
followed by the rea~tions'~J' CH,(R3Bl)
-0-
or CHz-0 (dioxirane). The former species has long been postulatedlOJ1as an intermediate of ozone-ethene reaction in both the gas and liquid phase but has never been detected spectroscopically. Dioxirane has been observed in the gas phase by microwave spectroscopy1zand mass ~pectrometry,'~ but its infrared
-
-
- + + -
-+
T O 1 [CH,O,*] [CHZ-01 [HCOOHt] C O H 2 0 , COz
+0 2
-+
CH2(H1A1) O2
+
-+
(4)
H2 etc. (5)
products
(6)
The relevance of reactions 5 to the ozone-ethene reaction
O3
C2H4
+
CH20z* H C H O
(7) has been pointed outla and studiedI9 from the standpoint of atmospheric chemistry. On the other hand, Lee and PimentelZohave observed the chemiluminescence of formic acid in the photolysis of the matrix samples of CH2Nz/O2/Ar at 8 K and attributed the emission to the cryogenic reaction CH2(g3Bl)
+ 0,
-
HCOOH*
(8) Although the present study was unsuccessful in detecting either (14) Okabe,H. "Photochemistry of Small Molecules"; Wiley-Intencience: New York, 1978; pp 309-314 and references therein. (15) Zebrausky, V.; Carr, Jr., R. W. J . Phys. Chem. 1975, 79, 1618. (16) Lin, M. C. Chem. Phys. 1975, 7, 442. (17) Hsu, D. S. Y.; Lin, M. C. Int. J . Chem. Kinet. 1977, 9, 507. (18) Martinez, R. I.; Huie, R. E.; Herron, J. T. J . Chem. Phys. 1981, 75, 5975. -. ..
(19) Hatakeyama, S.;Bandow, H.; Okuda, M.; Akimoto, H. J . Phys. Chem. 1981,85,2249. (20) Lee, Y . P.; Pimentel, G. C. J . Chem. Phys. 1981, 74, 4851.
0022-3654/85/2089-0845$01.50/00 1985 American Chemical Society
846
-
The Journal of Physical Chemistry, Vol. 89, No. 5. 1985
Bandow and Akimoto
L A -
Figure 1. Major parts of cryostat.
70,
CH,OO or CH,-0, we present the experimental results of the photooxidation of matrix samples of C H 2 C 0 / 0 2 / A r at 11 K and will discuss the possible occurrence of the matrix-assisted reactions. Experimental Section
The cryostat used was designed in our laboratory, and its major features are illustrated in Figure 1. It contains a CsI cold plate (25 X 35 mm, 2.0 mm in thickness) installed on a copper block which is mounted on the cold head of a commercial cryo-pump system (CTI-Cryogenics, Model 21). Indium metal sheet was used a t each contact surface in order to ensure good heat conductivity. The sample temperature was monitored by a silicon diode sensor connected to the edge of the CsI holder. The base of the copper block was fitted with a resistance heater to control the temperature of the cold window between 11 and 100 K within f l K by a temperature-control unit (Lakeshore Inc.). All cold parts of the system are enclosed in a cylindrical stainless steel vacuum vessel (102-mm i.d. and 265 mm long) which can be evacuated by an oil rotary pump with a liquid N2 trap. The vacuum vessel has a pair of KBr windows for the measurement of the IR absorption spectrum and a pair of fused quartz windows through which photolytic light irradiated the sample. The CsI window was set as 45O to both the KBr and quartz windows, allowing the measurement of the IR absorption spectrum during photolysis. Sample ketene diluted in Ar, Oz/Ar, or O2(( 1/200) - (1/5OO)) was introduced into the vacuum vessel through stainless steel tubing (1-mm 0.d.) and effusively jetted through one of the two nozzles located -20 mm from the CsI surface. The typical flow rates used was 4-6 mol/min, and the total amount of deposit was typically 0.5-1 mmol. The cold surface was kept at 11 K during the deposition, and the sample was annealed at 30 K for 15 min and recooled to 11 K before irradiation. Photolysis was done using a collimated beam (30-mm 0.d.) from a 500-W high-pressure Hg arc lamp (Ushio Electric Co. USH-5000) through a glass filter (Corning 7-51) transmitting light in the 310-410-nm range and a H 2 0 cell 150 mm in length to eliminate infrared radiation. Irradiation was made for 5-15 h against the deposited sample at 11 K. Ketene (CH2CO) was prepared2' by the pyrolysis of diketene at 550 OC and purified by trap-to-trap distillation. Deuterated ketene (CD2CO) was prepared22by the pyrolysis of (CD3)2C0 (C.E.A. 99.8%) and purified similarly. Research grade Ar and O2(Teikokusanso Co. Ltd.) were used after being passed through a molecular sieve trap at -70 to --90 "C. lSO2(Prochem, 99% isotope) was used without further purification. Infrared spectra were measured by a Fourier transform infrared spectrometer (Nicolet, FTS-7199) using liquid N2 cooled HgCdTe detector. Typically, 128 scannings were made (-8 min) to obtain a spectrum with a resolution of 0.5 cm-l. Results
Ketene in the O2 matrix ( R I M = 1/500) at 11 K displayed IR absorption bands at 3060 (m), 2142 (vs), 2087 (w), 1382 (w), 1145 (w), 1125 (m), 1001 (w), 995 (w), 978 (m), and 838 (w) cm-I where vs, m, and w stand for very strong, medium, and weak (21) Andeades, S.;Carlson, H. D. "Organic Synthesis"; Wiley: New York, 1973; Collect Vol. 5, p 679. (22) Hanford, W. E.; Sauer, H.C. In "Organic Reactions"; Adams, R., Bachmann, W. E., Fieser, L.F., Johnson, J. R., Snyder, H. R., Eds.; Wiley: New York, 1946; Vol. 3, pp 132-135.
-1.5
0
15
10
5 Irradiation
Time
20
(hour)
Figure 2. Decay of CH2C0 vs. irradiation time: (a) CH2CO/Ar ( I / 500); (b) CH2C0/02/Ar (1/50/450); (c) CH2C0/02(1/500). I
I
I
Ratio of 02 in Ar
I
V
I
A Matrix
(
r)
Figure 3. Relative decay rate of CH2C0vs. r (=0,/(02 + Ar)) in the photolysis of CH2C0/02/Ar matrices. Relative rates are normalized to that in the O2 matrix.
bands, respectively. Irradiation was first carried out for the CH2C0/02/Ar matrices with different 02/Ar matrix ratios while keeping the R I M ratio at 1/500. Figure 2 shows the decrease of C H 2 C 0 monitored at the C-H stretching band (3060 cm-I) as a function of irradiation time. As shown in Figure 2, no decrease of CHzCO was observed in the Ar matrix after 18 h of irradiation. Increase of the O2 ratio in the matrix increased the decay rate, and the maximum decrease rate was observed in the pure O2 matrix. The decay of CHzCO followed the first-order rate law as seen in Figure 2. Figure 3 shows the observed relative decay rate of CHzCOvs. the ratio of O2in the matrix (r = 02/(02 + Ar)). The decay rate increased distinctly at the relatively small O2 ratio ( r 5 0.2) and tended to saturate at r > 0.2. Figure 4 depicts the variation in the shape of the C-H stretching band of C H 2 C 0 at different Oz/Ar ratios before and after the irradiation. Figure 4a shows the band shape of the annealed sample before irradiation and reveals that as the 0 2 / A r ratio increases the width of the band increases from 1.0 cm-' in the Ar ( R I M = 1/500) matrix to 2.8 cm-I in the matrix of CH2CO/02/Ar = 1/100/400 ( r = 0.2). In solid O, the band width decreased to 1.4 cm-' and the peak position of the band shifted from 3063 to 3060 cm-l. These results reflect the fact that C H 2 C 0 molecules in the 0 2 / A r matrix are exerted more inhomogeneous perturbation as the 0 2 / A r ratio increases while in solid O2the perturbation is more homogeneous. It is of great interest to compare the band shape before and after the irradiation for the sample in solid O2and in the 0 2 / A r matrix. In solid 02, the bandwidth before and after the irradiation, and consequently that of the difference spectrum, is the same, 1.4 cm-I, as shown in Figure 4b. In contrast, in the 0 2 / A r matrices, the bandwidth after the irradiation is narrower than that before irradiation, implying that the band width of the spectrum attributed to the
The Journal of Physical Chemistry, Vol. 89, No. 5, 1985 847
Photochemistry of Ketene
TABLE I: IR Absorption Frequencies of Products in the Photolysis of Ketene/O, Matrices species
CH,CO/
CD,CO/
1602
0,
2344.4 2342.4a 2340.7
2344.5 2342' 2340.7
CH,CO/ 180,
remarks
.23446 2327' 2326' 2324.5 2309.3 2307' 2305.9
2154.3 2152.9
d d
C
2145 2141.6 2137.6 21 36.4
c
2137.6 2136.4
Figure 4. Spectral profile of the C-H stretching band of CH2C0in the CH2C0/02/Ar matrices: (a) before irradiation;(b) after 2-h irradiation and the difference spectrum before and after irradiation in solid 02j (c) after 2-h irradiation and the difference spectrum before and after irradiation in the CH2C0/02/Ar(1/50/450) matrix.
21100
23bo
-t
2zbo
2ib0
i6bo
i7bo
16b0
irbo
isbo
i3bo
i2bo
iibo
iobo
iiRVENUM0ERS
Figure 5. Product spectrumin the photolysis of the CH2C0/02(1/500) matrix for 13 h.
reacted C H 2 C 0 species is broader than that of the unreacted species for the CH2C0/OZ/Ar (1/50/450) matrix, as demonstrated in Figure 4c. Figure 5 displays a difference spectrum in the 2400-1000 cm-' range before and after irradiation of the CH2C0/I6Oz (1/500) sample for 13 h. In addition to C 0 2 , CO, HzO, and O3 which .'~ of the gas-phase photolysis have been r e p ~ r t e d ' ~ ~as' ~products of CH2C0 in the presence of 02,absorption bands of H 0 2 are distinct in the spectrum. In order to confirm the assignment, photolyses of C D 2 C 0 in the I6O2matrix and C H 2 C 0 in the I8O2 matrix were also carried out. Table I summarizes the observed frequencies of the product bands. The infrared absorption frequencies of the HOz, DOz, COz, CO, H 2 0 , DzO, and O3 bands in O2 matrix can be compared to the values reported by Diem and Lee8 and are generally in excellent agreement. The largest discrepancy was observed for the DOz v2 band for which the frequency observed in the present study, 1023.9 cm-I, is about 2 cm-' higher than the reported values of 1022 cm-'. The abwere sorption frequencies of the 180-labeled species in solid 180z not found in the literature, but the observed frequencies of the v2 and v3 bands of HI8O2, 1382.6 and 1040.4 cm-I, are close to the values in the Ar matrix, 1379.6 and 1039.7 cm-I, respectively, reported by Smith and A n d r e ~ s . The ~ ~ peaks of the v l band of H180, at 3388 cm-I appear shifted from the frequency in the Ar matrix21 (3401.5 cm-I) by -14 cm-', which is similar to the shift 2 cm-I) matrix from that of the HI6OZvl band in the 160(3400.5 in the Ar matrix (3414.0 cm-I) as noted by Diem and Lee.' The observed frequencies of C'60180,Cl8O CI8O, H2I80,and I8O3 are consistent with those observed in the gas phase.24 29
(23) Smith, D. W.; Andrews, L. J . Chem. Phys. 1974, 60, 81. Rnchas, S.;Laulicht, I. 'Infrared Spectra of Labelled Compounds"; Academic Press: New York, 1971. (24)
(H,l6o) 3732 3724 1600.5 1588.8 (HL60,) 3400.4 1391.6 1108.ge 1101.4
(D2l6 0 ) 2770 2761 1181.0 1173.3 (D'60,) 2520.8 1023.9
('603)
(l60,)
1037.9 1031.1
1038.0, 1031.1
C C
2137.9 2136.3 2103.3 2101.6 2085.0 2084' (H, 0 ) 3717 1594.2 1582.4 (H180,) 3388 1382.6 1047.5e 1040.4 ("03)
980.9 974.5
a Shoulder. C160, contaminant due to air leak of cryostat during irradiation. Corresponding bands were not determined due to large absorption of CH,CO. Corresponding bands were not determined due to large absorption of CD,CO. e Broad bands accompanying HO, monomer bands; could be HO, dimer bands according to a result of ref 7 .
Lo/;:& C02 deposit. 2$0
2320
wavenumbers
2280
Figure 6. Infrared absorption bands of carbon dioxide observed in the photolysis of (a) CH2C0/'60z(1/400), (b) CH2C0/I8O2(1/400), and (c) CHZC0/I8O2/Ar(1/67/334); (d) deposition of COz on Ar matrix. It should be noted in Table I that C160180 was produced in the photolysis of the CH2CL60/l8O2 sample. Parts a 4 of Figure 6 display the spectra of the vj region of carbon dioxide after the irradiation of CH2C0/I6O2 (1 /400), CH2CO/I8O2 (1/400), CH2CO/1802/Ar(1/67/334), respectively, and only after the deposition of C160z/1602.Among the bands observed in the photolysis of CH2CO/l8O2,CI6O2can obviously be ascribed to the deposition due to leakage from the atmosphere as confirmed by comparison with the band shape of Figure 6d. It is of interest to note the similarity of the band shapes of C1602 from
848
The Journal of Physical Chemistry, Vol. 89, No. T, 1985
Bandow and Akimoto TABLE III: Probability' of the Distribution of O2 Molecules Surrounding a CH2C0 Molecule in the CH2C0/02/Ar Matrices ( r = 02/(02 + Ad)' r x, x, x, x, 1 -x, 1 -x,-x,
15c
.
0 0.0385 0.1 0.2 0.3
0.00 0.30 0.38 0.21 0.07
1.00 0.62 0.28 0.07 0.01
0.00 0.06 0.23 0.28 0.17
0.00 0.01 0.09 0.24 0.24
0.00 0.38 0.72 0.93 0.99
0.00 0.08 0.34 0.73 0.92
" X i stands for a probability that the number of O2 molecules surrounding a CH2C0 molecule as a nearest neighbor is i , as defined by eq 13. bSee text. Irradiation
Time
(hour)
Figure 7. Time profile of the integrated absorption bands of CH2C0 (C-H stretching), C1*02(w3), and C160'802(w3) in the photolysis of CH2C0 in solid CI6O2(w3) is due to the leakage of the cryostat. RIM = 11400. TABLE II: Integrated Absorbance Ratio of Products in the Photolysis of the CH2C0/02/Ar Matrices ( AProduct/-ACHFO) r integrd
sDecies H02 0 2
H20
co co2
bands y3
w3 y2
fundam y3
0.2
0.4
0.6
1.o
nd" nd
0.06 0.27 0.37 0.5 13
0.05 0.34 0.30 0.4 12
0.27 0.40 0.43 1.4 17
0.60 0.8 22
'Not detected. CHzC0/1602and C's02 from CH2CO/1s02while that of C160180 from C H z C 0 / 1 8 0 2is somewhat different from the others, reflecting the different formation mechanism. Further, the yield of C160180in the Oz/Ar matrix is smaller than that in solid 0, (cf. Figure 6b,c). Formation curves of C160180 and C1802as a function of irradiation time in the photolysis of CH2C0/1802are shown in Figure 7, verifying that C ' 6 0 1 8 0as well as C1802are primary products of photooxidation. Other than the products summarized in Table I, no metastable
70, species which could be assigned to CHzOO or CHz-O were detected in the present study. Although the infrared absorption bands of these species are not known, a band due to the C-0 stretching mode is expected to lie in the 950-1 100-cm-l region. In the case of analogous species of C 0 3 in the C2, structure, the v2(al) and ~q(b1)modes mainly due to C-0 stretching are knownz5 to appear at 1073 and 972 cm-' in the C 0 2 matrix. No new band was observed in this frequency range. Formic acid (HCOOH) and formaldehyde (HCHO) are known7 to have absorption bands at 1762.8 ( C 4 stretch), 1100.3 (C-0 stretch) and 2874.2 (C-H asym stretch), 2810.2 (C-H sym stretch), 1740.0 (C=O stretch) in the 0, matrix. None of these bands were observed in the present study, indicating that neither HCOOH nor HCHO, both of which are known19 to be products in the gas-phase photooxidation of CH$O in the presence of 02,is produced in significant amounts in the photolysis of the C H 2 C 0 / 0 2 matrix. Table I1 shows the integrated absorbance ratio of products against the decrease of ketene after irradiation in matrices with different O,/Ar ratios. It should be noted that the yield of H0, decreases rapidly as the 0 2 / A r ratio decreases whereas the yields of H,O, CO, and CO, are relatively insensitive to the matrix ratio although the observed values are rather scattered. The yield of O3decreases more gradually than H02with the decrease of 0 2 / A r ratio. Discussion Our experiments showed that C H 2 C 0 is not photolyzed in the Ar matrix at wavelengths between 310 and 410 nm, where mercury lines at 366, 334, and 3 13 nm should be most effective (25) Jacox, M. E.; Milligan, D. E. J . Chem. Phys. 1971, 54, 919.
as photolytic light. In the gas phase, the primary quantum yield of the photodissociation of C H 2 C 0 at the low-pressure limit is known14to be unity at 313 nm, slightly less than unity at 334 nm, and about 0.04 at 366 nm. The CH, formed is almost all in the triplet state at 366 nm, and the singlet yield is 0.16 and 1.0 at 334 and 313 nm, respectively.26 The absence of photodecomposition of CHzCO in the Ar matrix means either that the excited state of CHzCO is totally quenched by matrix molecules or that the cage recombination of CH2 with CO occurs effectively after photolysis. In the 0 2 / A r or in solid 0, matrix, photooxidation of CHzCO did occur to yield HOz, CO, CO,, HzO, and O3(see Table I). It should be noted here, however, that neither HCOOH nor H C H O was detected in the present study, whereas these compounds have been observed as major products in the photolysis of CHzNzin the O,/Ar matrix studied by Lee and PimentelZ0 and ascribed to the reaction of the triplet methylene with 0,. CH2N2
-
+ M CH2(R3Bl)+ N, CH2(R3BI)+ 0, HCOOH CH2(R3Bl)+ 0, H C H O + O(3P)
+ hv
-+
CH2(Z1Al) Nz
-+
(9) (10)
(11) This evidence strongly suggests that the simple photolytic pathway of +
CH2C0
+ hu
-
CH2(Z1Alor g 3 B , )
+ CO
(12)
does not occur in the matrices in the near-ultraviolet region. Thus, the absence of photodecomposition of C H 2 C 0 in the Ar matrix should be due to the effective quenching of the CHzCO molecule either in the electronically excited state or in the vibrationally excited ground state (cf. reactions l-s), rather than from the cage-recombination effect. Therefore, the photochemistry of C H 2 C 0 in the 0 2 / A r matrix or in solid O2 as observed in the present study should be ascribed to the reaction of the excited C H 2 C 0 molecule perturbed by O2molecules. The spectroscopic evidence as shown in Figure 4 demonstrates the preferential photooxidation of the O,-perturbed CHzCO molecules. It is knownz7 that the rare-gas matrix normally has a cubic closed-packed (ccp) structure with a face-centered (fcc) unit cell. In crystals containing impurities, a hexagonal close-packed (hcp) structure results.,' In both structures, each atom has a total of 12 nearest neighbors. If we assume here as a first approximation that in the C H 2 C 0 / 0 2 / A r matrix ( R I M = 1/500, r 5 0.2) the impurity molecules, 0, and CH,CO, occupy substitutional sites in the Ar matrix giving a hcp structure and that 0, and C H 2 C 0 molecules are distributed in statistically uniformly, the probability that the number of O2molecules surrounding a C H 2 C 0molecule as nearest neighbors is i can be estimated by
xi = 12ciri(l- r)l2-i
(1 3) where r is the matrix ratio of O2 defined as r = O,/(O, Ar). Table 111 cites the probabilities X,,XI,X2, and X 3 for the values of r employed in the present study. Thus, it can be seen that the probability of having one, two, and three 0, molecules as nearest neighbors around a C H 2 C 0 molecule is highest for r = 0.0385,
+
(26) Kelley, P. M.; Hase, W. L. 1975, 35, 57 and references therein. (27) Meyer, B. "Low Temperature Spectroscopy"; Elsevier: New York, 1971; p 191.
The Journal of Physical Chemistry, Vol. 89, No. 5, 1985 849
Photochemistry of Ketene
Table I1 also shows that the formation of O3was observed at r = 0.4 but not at r = 0.2, which means that the 0,-forming step
01
0
02
"
03
10
R a t i o of 02 in Ar M a t r i x ( r )
Figure 8. Comparison of the probability function, 1 - X , -PI, with the experimental relative photooxidation rate of CHsCO in the Oz/Ar matrix. f = 0 ( o ) ; f = l (A);observed relative rate of photooxidation ( 0 ) . See text.
0.1, and 0.2, respectively. Table I1 also shows 1 - X o and 1 - Xo -XI, that is, the probability of having more than one or two Oz molecules around a CH2C0molecule as nearest neighbors. Figure 8 is a plot of 1 - X o -fxl vs. r compared with the experimental relative photooxidation rate of CHzCO. Figure 8 reveals that the experimental curve lies between the probability curves with f = 0 and 1, which means that not all of the CHzCOmolecules having only one O2 molecule as a nearest neighbor is subject to photooxidation but all of the CH2C0molecules having more than two Oz molecules do react. Although a more realistic model should rather take into consideration the preferential formationz8of (02)2 than the uniform distribution of O2molecules, the correlation of was not studied the photoreactivity with the concentration of (02)2 in the present work. From the above considerations, it is proposed that the photooxidation of CH2C0in the 02/Armatrices proceeds via an excited state of CHzCOperturbed by surrounding noz(n 1 1) molecules, which can be envisioned as a matrix-assisted photolytic process. Considering the stoichiometry of CHzCO photooxidation, the reaction pathways of such 02-matrix-assisted processes may be conceived as follows: CH2CO...02 + hu .--+ H2O + 2CO (14a)
-
+ - + + - + + +
CH+20..*202
CHZCO**.302 hu CHzCO.-nOz + hu
+ C02 + CO HzO + 2COz
H2
+ hu
H20
2H02
COZ + 03
CO
C02
CO
(14b) (15) (16)
( n - 2)O2 etc. (17)
The processes with more than two Oz niolecules would involve (Oz)zas reacting species although it is not specified here. Although the above steps are only schematic, several points can be discussed, referring to the observed product distributions shown in Table 11. If H atoms are ejected by a reaction path such as CH2CO.*.02 + hu 2H CO2 CO (14~)
-
+
+
which is analogous to the gas-phase reaction of CHz with O2
CH2 + 02
-+
[CH,O,*]
+
2H
+ COZ
(sa)
followed by the recombination of H atoms with 02,the drastic decrease of HOZyield at r 5 0.6 as shown in Table I1 cannot be explained, since the H atoms should be trapped by excess O2 to give HOZafter migration. Therefore, the reaction step giving HOZ should be envisioned as involving the excited state of CHzCO surrounded by a few O2molecules. ~~~
~
(28) Frei, H.;Pimentel, G. C. J . Chem. Phys. 1983, 79, 3307.
would require three or four surrounding Oz molecules. On the other hand, CO, COz, and HzO were formed regardless of the 02/Arratio in the matrix in the range of 0.2 Ir I1.0, suggesting that "gas-phase-like" reactions involving one to one pair of CHZCO-.O2cluster is responsible as a major forming step. Since the probability of the photooxidation for a CHzCO molecule surrounded by a single O2molecule was estimated less than unity, only a CH2C0molecule having a nearest-neighbor Ozmolecule at a favorable position to interact with the CHI group subjects the "gas-phase-like" reaction to give CO, C02, and H20. Formation of C160180 is more evidence of the direct interaction since it is not expected of an excited CHzCO molecule with to be formed after being photolyzed to CH2 CO. The formation could be envisioned as, for example where *O step of C160180
+
**
/o-o *o*
stands for an isotopically labeled oxygen atom. The result that decreased but was still substantial at r the yield of C160180 0.2 (see Figure 6) is consistent with this view. The photochemistry assisted by reactive matrices may also be observed for other molecules whose excited states are predissociative rather than repulsive. In this regard, it is interesting to compare the present results for CHzCO to the matrix photooxidation of HzCO studied by Diem and Lee8 under similar experimental conditions. In the case of HzCO,the isolated molecule in the Ar matrix was photodecomposed to give Hz CO, which is in contrast to the results of CHzCO where photolysis did not occur in the Ar matrix. Although quantitative results were not reported by Diem and Lee,8 HzCO in solid O2 was photolyzed more rapidly than in Ar, which is in accord with our results for CH2C0. They accounted for their result by stating that it was likely due to the reaction of Oz with the photofragments H and HCO to give H 0 2 and CO, and the fact that no HCO was detected after photolysis in the Ar matrix was due to matrix-cage recombination of H HCO to give HzCO or H2 CO. In view of the results of the present study, however, the Ozmatrix-assisted reaction might also be operating in the case of HCHO to give 2H02 CO directly. They interpreted the formation of COz and HzO as due to a secondary reaction of HzOz CO where HzOz is formed by the combination of two HOz radicals. If the matrix-assisted reactions proposed in the present study are occurring, a direct formation step HzCO...02 hu Hz0 + COZ (19)
+
+
+
+
+
+
-
could be possible analogous to reaction 15. On the other hand, since the excited state of CHINz related to the near-UV photolysis is repulsive, the photooxidation processes 9-1 1 are thought to be more probable than the matrix-assisted process. Although the results of the present study are only exploratory at this stage, the possibility that new photochemical reaction paths different from the corresponding gas-phase reactions could be opened is stressed and awaits further study.
Acknowledgment. We thank Dr. S.Hatakeyama for the supply of CH2C0 he prepared. Registry No. CH,CO, 463-51-4; CD2C0, 4789-21-3; CO, 630-08-0; COZ, 124-38-9; Hz0,7732-18-5; 0 3 , 10028-15-6; H02, 3170-83-0 Cl8O2, 2537-69-1; C'60180,18983-82-9; 0 2 , 7782-44-7; ' ' 0 2 , 14797-71-8.
