J . Phys. Chem. 1986, 90,4037-4043
1
' " 2.5
'
"
" " 3.0
'
"
~
3.5
4.0
1000/T (K)
+ CH3SH reaction. The solid line is obtained from a linear least-squares analysis. Figure 4. Arrhenius plot for the H
-
at 298 K'*J9 with kll 1 X 10-l2cm3molecule-' 8';if we assume this value for k, (rate constant for the competing reaction), then the results of Kuntzg and those of Steer and KnightSbagree with our 298 K rate constant within &20%. One measurement of Balla and Heicklen'O employed the competing reaction (18) Lee, J. H.; Michael, J. V.;Payne, W. A.; Stief, L. J. J. Chem. Phys. 1978, 68, 1817. (19) Nicovich, J. M.; Ravishankara, A. R., to be published.
4037
Under the assumption that O2has the same third-body efficiency as N2, then klz is known'5 to be 5.5 X [O,] cm3 molecule-' s-I. Using this value to convert Balla and Heicklen's relative measurement to an absolute rate constant gives k2 = 1.8 X cm3 molecule-I s-l-in excellent agreement with our 298 K result. The only temperature dependence data reported previously is that of Inaba and Darwent.4 They concluded that the activation energy for reaction 2 is slightly larger than the activation energy for reaction 11; i.e., the ratio of k2/kll increased slightly with increasing temperature over the range 323-493 K. Recent direct measurements suggest that, over the temperature and pressure ranges of Inaba and Darwent's study, reaction 11 is in the fall-off region between third and second order.lg Inaba and Darwent, however, observed k2/kll to be independent of total pressure a t both 323 and 493 K. Hence, quantitative comparison with Inaba and Darwent's results does not seem warranted. It is worth noting that Lee et a1.I8 report an activation energy for reaction 11 in 760 Torr Ar of 2.1 kcal/mol; their experiments covered the temperature range 198-320 K. We report an activation energy for reaction 2 of 1.7 kcal/mol (249-405 K). Hence, our direct measurements coupled with those of Lee et al. predict that k2/kll should decrease slightly with increasing temperature. Acknowledgment. This work was supported by NASA through Subcontract No. 9548 14 from the Jet Propulsion Laboratory and by NSF through Grant No. ATM-82-17232. Registry No. CH3SH, 74-93-1; 03,10028-15-6;H2, 1333-74-0; H, 12385-13-6.
Photofragmentation Dynamics of Carbdn Suboxide Brad R. Weiner and Robert N. Rosenfeld*' Department of Chemistry, University of California, Davis, California 95616 (Received: February 20, 1986)
Nascent CO photofragment vibrational energy distributions from the UV photolysis of C302 are measured by time-resolved carbon monoxide laser absorption spectroscopy. Photodissociation of C3O2 at 193 and 249 nm produces CO(u=O-5) and CO(v=0-3), respectively. The CO photofragment is rotationally excited to some extent, in both cases. A statistical model for energy disposal fits the experimentally determined CO vibrational energy distributions for 193- and 249-nm photolyses. A Franck-Condon model for vibrational energy partitioning cannot reproduce the experimental observations. Our data suggest that following abso tion of a W photon C3O2 internally converts to the electronic ground state and then dissociates, yielding C,O(E'A) and CO(%Z+). The photofragmentationdynamics of C302are compared with those of the "quasi-linear" polyatomics, CH2C0 and NCNO.
fntroduction Information about the dissociation dynamics of polyatomic molecules is implicit in the energy distribution of the nascent fragments? For molecules that undergo direct photodissociation, the dynamics of collisionless fragmentation are often dominated by the shape of the potential surface accessed by the absorption of a p h ~ t o n . ~Data obtained by monitoring energy disposal to the photofragments' degrees of freedom have proven to be a valuable dynamical With increasing molecular complexity, molecular rovibratiunal state densities increase, which can in turn greatly enhance the rate of nonradiative transitions. Various intramolecular relaxation processes can thus m r in competition with dissociation from optically prepared states. By probing the (1) Fellow of the Alfred P. Sloan Foundation (1985-1987). (2) Lcvine, R. D.; Bernstein, R. B. Molecular Reaction Dynamics; Oxford University Press: New York, 1974. (3) Bersohn, R. J . Phys. Chem. 1984,88, 5145. (4) Leone, S.R. Adu. Chem. Phys. 1982, 50,255. ( 5 ) Imre, D.; Kinsey, J. L.; Sinha, A.; Krenos, J. J. Phys. Chem. 1984,88,
3956.
(6) Simons, J. P. J. P h p . Chem. 1984.88, 1287. (7) Shapiro, M.; Bersohn, R. Annu. Reo. Phys. Chem. 1982, 33, 409.
partitioning of excess energy among the rovibrational modes of the photofragments, one can obtain information about the primary photophysical and photochemical events following photoactivation. Such data are important because currently available models for polyatomic photodissociation dynamics7 are limited in scope, at best. Carbon suboxide, C3O2, is an interesting test case in developing models for photodissociation dynamics, in part because it may represent an intermediate point between the small (nonstatistical) and large (statistical) molecule limits. The molecule has been shown to be linear.8-9 Useful comparisons can be drawn between the photodecarbonylation of C302 and similar "quasi-linear" species, e.g., ketene, nitrosyl cyanide, and isocyanic acid.1° In this paper, we report data on the energy disposal to the carbon monoxide fragment following excimer laser photolysis of carbon suboxide at 193 and 249 nm. Time-resolved C O laser (8) Livingston, R. L.;Rao, C. N. R. J . Am. Chem. SOC.1959, 81, 285. (9) Lafferty, W. J.; Maki, A. G.; Plyler, E. K. J . Chem. Phys. 1964, 40, 24. (10) Okabe, H. Photochemistry of Small Molecules; Wiley-Interscience: New York, 1978.
0022-3654/86/2090-4037$01.50/00 1986 American Chemical Society
4038
Weiner and Rosenfeld
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
absorption spectroscopy is used to measure the C O product's vibrational energy distribution. At 193 nm (photon energy = 148 kcal/mol), two processes are energetically possib1e:l C302 C 2 0 C O DHo 5 76 kcal/mol (1) C3O2
-
-
C(3P)
+
+ 2CO
AHO(2) = 138 kcal/mol (2)
If photolysis wavelength is 5207 nm, formation of C(3P) atoms is energetically feasible, but an accurate threshold for this process has not been experimentally determined." At wavelengths shorter than 170 nm, some work indicates the quantum yield for C O formation to be ac0= 2, suggesting that process 2 is dominant.I2 However, other studies done at 147 nm suggest that both C 2 0 and C atoms are formed as primary products.13 A recent communic_ation reports the observation of electronic fluorescence from C20(A311i)following excimer laser photolysis of C302 at 193 nm, but the origin of the emitting fragment has not been firmly established. l 4 If C3O2 is photolyzed at 249 nm (1 15 kcal/mol), only reaction 1 is thermochemically allowed. B a y s has measured product yields following the photolysis of C302/C2H4mixtures at 250 and 300 nm1.35-17Formation of allene (produced by the reaction of C 2 0 with C2H4) was found to be quenched by the triplet scavengers O2and NO at 300 nm but not at 250 nm. Bayes concluded that the shorter wavelength photolysis resulted almost exclusively in the formation of singlet C20, most probably the iXlA state.I6 The 300-nm_photolysis apparently generated C 2 0 in its ground triplet state (X3Z-). C 2 0 was first observed spectroscopically by Jacox et al. after the 147-nm photolysis of C302 trapped in an Ar matrix at 4.2 K.'* The fundamental vibrational frequencies were determined, and an absorption continuum beginning near 500 nm was observed for the matrix-isolated CCO. Devillers and Ramsay19 recpded well-resolved gas-phase absorption spectra for the C20(A311,X32-) transition following the flash photolysis of C302. The measured electronic spectra extended from 500 to 900 nm, with vibrational structure corroborating the matrix isolation assignments. McDonald and co-workers20*21 examined the same transiJion by using laser-induced fluorescence (LIF) to observe C 2 0 (X3Zr-) generated by the 266-nm photodissociation of C3O2. By varying the probe laser delay, they obtained a qualitative estimate of the C 2 0 internal energy distribution. At short delay times, the spectrum appeared congested and was largely unassigned. Simplification of the spectrum was observed with increasing probe laser delay. The authors attributed this to cooling of the hot nascent distribution. The C 2 0 was also found to be translationally excited with an average velocity ca. 3 times greater than the 300 K thermal velocity. A search for the low-lying singlet states of C20, ZIA and b'Z+, was conducted by tuning the probe laser over a broad spectral region and looking for LIF signals. No emission was observed for probe laser wavelengths of 465-539 nm. Becker and co-workers22 have used LIF to monitor C20(X3z-) formed in the 249-nm laser photolysis of C3O2. The similarity of their results with those of McDonald and co-workers2' at 266 nm led these workers to conclude (in contrast to Bayesl') that the quantum yields for C20(X32-)production are comparable at these two wavelengths. ( 1 1 ) Filseth, S. V. Ado. Photochem. 1977, 10, 1 . (12) Brown, W.; Bass, A. M.; Davis, D. D.; Simmons, J. D. Proc. R. SOC. London, A 1969, 312, 417. (13) Forchioni, A.; Willis, C. J . Phys. Chem. 1968, 72, 3105. (14) Bauer, K.; Meuser, R.; Becker, K. H. J . Photochem. 1984, 24, 99. (15) Bayes, K. D. J . Am. Chem. SOC.1961,83, 3712. (16) Bayes, K. D. J . Am. Chem. SOC.1962, 84, 4077. (17) Bayes, K. D. J . Am. Chem. SOC.1963, 85, 1730. (18) Jacox, M. E.; Milligan, D. E.; Moll, N . G.; Thompson, W. E. J . Chem. Phys. 1965.43, 3734. (19) Devillers, C.; Ramsay, D. A. Can. J . Phys. 1971, 49, 2839. (20) Donnelly, V. M.; Pitts, W. M.; McDonald, J. R. Chem. Phys. 1980, 49, 289.
(21) Pitts, W. M.; Donnelly, V. M.; Baronovski, A. P.; McDonald, J. R. Chem. Phvs. 1981. 61. 451. 456. (22) Bicker, K.'H.; Horie, 0.;Schmidt, V. H.; Wiesen, P. Chem. Phys. Lett. 1982, 90, 64
It is apparent that the primary photochemistry of carbon suboxide has not been well established. The limited spectroscopic data on C3O2 and C 2 0 have hampered the development of a detailed understanding of the photodissociation dynamics of C302. The nature of the photodissociation dynamics can be studied via measurements of energy disposal to a well-characterized species, the C O photofragment. The dynamical model that emerges is considered in light of data available on the photodissociation of other four- and five-atom system^.^^-*^ Experimental Section The experimental apparatus has been previously d e s ~ r i b e d . ~ ~ ? ~ ~ Briefly, C302(0.002-0. 100 Torr) diluted in buffer gas (He, Ar, or SF6) is introduced into a 1-m Pyrex absorption cell fitted with CaF2 windows. Pressures are measured using a 0-10-Torr capacitance manometer (MKS Instruments Baratron 220B). The output of a rare gas halide excimer laser (Lambda Physik EMG-101; 15-11s pulse width) operating on transitions at 193 (ArF*) or 249 nm (KrF*) is directed through the cell. UV fluences are typically 1-10 mJ/cm2 at 193 nm and 5-20 mJ/cm2 at 249 nm. A continuous-wave, grating-tuned, carbon monoxide laser is propagated through the absorption cell coaxially with respect to the UV laser beam. The CO laser operates on P,,,(J) transitions, where u = 0-12 and J = 8-15. The infrared intensity is monitored with an InSb detector (time constant I 100 ns). Output from the detector is amplified, digitized (LeCroy TR8837 transient recorder), and finally signal-averaged in a microcomputer. Typically, 100-200 transients are averaged. Control experiments showed that absorption due to the buildup of reaction products in the absorption cell and on the windows was negligible. C3O2 was prepared by the dehydration of malonic acid with P2OSby using the procedure of Long, Murfin, and Williams.30 Purification was accomplished in two steps following the method of Miller and fat el^.^^ Purity was checked (299%) by infrared absorption spectroscopy. Results Time-Dependent CO Production. The C O laser employed in our experiments runs only on the lower P-branch transitions, so that any CO photoproduct formed with J >> 15 will not be detected until it is collisionally quenched to J = 8-15. Irradiation of C3O2 at either 193 or 249 nm results in transient CO laser absorption due to CO(u=0-5) or CO(u=0-3), respectively. Peak amplitudes of the transient signals were measured as a function of UV laser fluence. For photolysis at either 193 or 249 nm, a linear dependence on fluence [0.3-2.0 mJ/cm2 (193 nm), 4.5-19 mJ/cm2 (249 nm)] was observed. This result suggests that C O is produced via a one-photon-absorption process. The temporal profiles of the C O photoproduct fall into three distinct groups. Examples of the three types of transient absorptions are shown in Figure 1. With the C O laser operating on the PI,,,(11) transition, irradiation of C3O2 at 193 or 249 nm produces transient CO absorption signals such as that shown in Figure la. The rising part of the absorption curve cannot be fit to a single or biexponential function, indicating that there is more than one pathway for CO production. Plots of log (intensity) vs. time indicate at least three (23) Nesbitt, D. J.; Petek, H.; Foltz, M. F.; Filseth, S. V.; Bamford, D. J.; Moore, C. B. J . Chem. Phys. 1985, 83, 223. (24) Nadler, I.; Noble, M.; Reisler, H.; Wittig, C. J . Chem. Phys. 1985, 82, 2608.
(25) Moore, C. B.; Weisshaar, J. C. Annu. Reu. Phys. Chem. 1983, 34, 525. (26) Hayden, C. D.; Neumark, D. M.; Shobatake, K.; Sparks, R. K.; Lee, Y. T.J . Chem. Phys. 1982, 76, 3607. (27) Fujimoto, G. T.; Umstead, M. E.; Lin, M. C. Chem. Phys. 1982,65, 197. (28) Sonobe, B. I.; Fletcher, T. R.; Rosenfeld, R. N . J . Am. Chem. SOC. 1984, 106, 4352. (29) Sonobe, B. I.; Fletcher, T. R.; Rosenfeld, R. N . Chem. Phys. Letf. 1984, 105, 322. (30) Long, D. A,; Murfin, F. S.; Williams, R. L.Proc. R. SOC.London A 1954, 223, 251. (31) Miller, F. A.; Fately, W. G. Spectrochim. Acta 1964, 20, 253.
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 A039
Photofragmentation Dynamics of Carbon Suboxide 0
0.30 0.28
40
I p
h
8
-
0.24
-
0.22
-
0.26
0.20
-
0.180.16-
Ln
80
:,E'
0.14 0.12
f
0.10
v
-
-
m
0.08 1
~
0
>
0.04 0.06
b cn
0 02
000 , 0 00
z
w
20
I-
w
L
4
0 02
0 04
0 06
0 08
0 10
Figure 3. Plot of the pseudo-first-order rate constant, kII,for production of CO(v=O) in region I1 (see Figure 2) as a function of COO2 pressure. Experiments were carried out at a constant total pressure of 5 Torr, using SF6 as the buffer gas. The photolysis wavelength was 193 nm and the fluence was 1.8 mJ/cmz. Data are shown as ( O ) , while the straight line is a linear least-squares fit to the data, which yields a bimolecular rate constant k8 = 2.24 (f0.56)X lo6 Torr-' s-l.
I-
L1:
-
CARBON SUBOXIDE PRESSURE (torr)
z
w
L
o
10
10
,
t 0
10
20
30
-...- ,
50
40
60
1
l l M t (/LSJ
Figure 1. Transient absorption observed following the 193-nmphotolysis of COO,at a fluence of 2.0 mJ/cm2. Parts a, b, and c were obtained with the Pl,o(ll), Pz,l(ll), and P4,Jl 1) CO laser transitions, respectively. All three transients were recorded under similar conditions: COO2pressure = 44 mTorr, total pressure = 2 Torr (balance SF6).
0.0
TIME I f , o
,
0.0
100
20.0
30.0
40.0
TIME (ps)
Figure 2. Semilog plot of the transient displayed in Figure la. Regions I (O), I1 (+), and 111 (0)[see text] are shown for the 40 ps immediately following the laser pulse. The insert is an expression of the first 5 ps.
distinct components (see Figure 2), which can be differentiated by their dependence on [C302]and [SF,]. Pseudo-first-order rate constants for CO production in each region are obtained by nonlinear regression analysis. The first process (shown as region I in Figure 2) corresponds to the direct photochemical production of CO from C302. If the buffer gas (SF6) pressure is 1 2 Torr, CO production in region I occurs a t a detector-limited rate. Lowering the buffer gas pressure (0-2 Torr of sF6)slows the rate. This observation can be. attributed to rotational excitation in the nascent CO photofragment. Thus, ca. 2 Torr of SF6rotationally
thermalizes the CO photofragment within ca. 100 ns. When no buffer gas is added, no CO is detected within 100 ns of the laser pulse, indicating that virtually all the CO product is produced with J > 11 or has moved out of the probe beam due to translational excitation. Similar behavior is observed when argon or helium is added, but higher pressures (15 Torr) are needed to collisionally quench the rotationally excited C O in this case. The second component of the rise time (region I1 in Figure 2) was found to be dependent on C302concentration but independent of He, Ar,and SF6pressure. A plot of the pseudo-first-order rate constant for region I1 vs. C302pressure is linear and yields a bimolecular rate constant of 2.24 (f0.56)X 10, Torr-' s-*. This plot is shown in Figure 3. Qualitatively similar behavior is observed at both 193 and 249 nm. This rate constant corresponds to the reaction of some reactive species, e.g., C20, with C302to give one or more carbon monoxide molecules. The slowest component of the rise time, region I11 (cf. Figure 2), is due to vibrational relaxation of CO(u11). This component of the signal is only seen when buffer gas pressures are large enough to prevent diffusion of the vibrationally excited products out of the probe beam (diameter of 1-2 mm) prior to vibrational relaxation. The relaxation rate is found to vary with the pressure of C302and SF,. The correspondingpr's are 5.88 (f1.28) X lod and 1.47 (k0.15) X Torr s, respectively. Similar values are obtained by monitoring the decay of vibrationally excited CO (vide infra). C O vibrational deactivation rate constants have been reported for several carbonyl-containing p o l y a t o m i c ~ ,e.g., ~~ HCOOH, CH3COOH, and CH3CH0. Our measured p~ values show C302to be a better vibrational relaxer of CO than these organic compounds by a factor of ca. 5-10. The order of the efficiency of the carbon monoxide vibrational relaxers used in this experiment is C302 >> SF, > Ar > He. The observed exponential decay of the signal obtained with the 11) laser transition is due to diffusion of CO out of the probe beam and is characterized by a decrease in the decay rate constant as buffer gas pressure increases. Tuning the CO laser to the Pzl(l 1) or P3Jl 1) transitions results in transient absorptions similar to that shown in Figure 1b. Again, the data indicate more than one pathway for the formation of CO(u=1,2) and the rise times were analyzed by nonlinear regression methods in analogy to the Pl,o(ll) data. The faster channel can be assigned to the direct photochemical production of CO from C302. The CO product is rotationally hot, and ca. 2 Torr of SF6 is needed for collisional quenching. The slower component of the rise time is found to be dependent upon C3O2 pressure corresponding to a rate constant of ca. 2 X 10, Torr-' (32) Stephenson, J. C.; Mosburg, E.R.,Jr. J. Chem. Phys. 1974,60,3562.
Weiner and Rosenfeld
4040 The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 s-l. By analogy with the Pl,o(l1) data above, the vibrationally excited CO here is produced by the same bimolecular reaction channel that generated ground-state CO. The decay rate of the P2,'(11) signal was measured and found to coincide with the rate of CO production attributed to vibrational relaxation in the 11) data (region I11 in Figure 2). CO produced in higher vibrational states was detected with the P4,3(1 l), P5,J 1l), and P6,s(11) transitions. The highest CO vibrational state populated was u = 5 in the case of 193-nm photolyses and u = 3 in the case of 249-nm photolyses. The transient absorptions observed with the indicated CO laser lines are all qualitatively similar to that shown in Figure IC. When buffer gas (SF,) pressures are 1 2 Torr, C o is produced at a detectorlimited rate. No evidence for the bimolecular reaction pathway discussed above was seen. Detector-limited rise times were followed by single-exponential decays in all cases, corresponding to the vibrational relaxation of CO(uL3). Vibrational Distribution. The relative peak absoprtion intensities corresponding to the direct photochemical production of C O from C302were measured for all PV+',J11) up to the highest vibrational levels where absorption could be detected, i.e., u = 0-5. Vibrational level populations are obtained with the r e l a t i ~ n s h i p ~ ~
0.0
a -1.0
-
-2.0-
A
L -4.0
-5.0
-7.0
(3)
CO VIBRATIONAL OUANTA (v)
0.0
where u, is the CO vibrational level, beyond which no absorption can be observed,and H,, is the absorption intensity of the P,,,(J) CO laser line. The results are shown graphically in Figure 4. The In [N(u)]vs. u plots for both 193- and 249-nm photolyses are nearly linear and yield vibrational temperatures, T,, of ca. 2600 and 1550 K, respectively. The nascent distributions were measured at SF6pressures of 2 and 5 Torr, both yielding the same results.
b -1.0
-2.0
a\
Discussion -3.0 Correlation of Electronic States. Much of the uncertainty in \ I "> \ Z the primary photochemistry of C3O2 stems from a lack of highresolution spectroscopic data for the compound in the near-UV. 5 -4.0 The most recent and thorough investigation is that of R ~ e b b e r . ~ ~ Following his assignments, irradiation of C302at the two excimer laser transitions used in our experiments can populate different -5.c electronic states. Absorption of a 249-nm photon populates the degenerate l2,,-and 'A,, electronic states of C3O2, while excitation at 193 nm can populate high vibrational levels of the same states -6.C or the degenerate IZ; and I$ pair of states. All of these electronic transitions are zeroth-order forbidden, but become first-order allowed through vibronic interactions. State correlation diagrams -7s I for the linear (C-J and nonlinear (C,) decarbonylation of C3O2 0 2 4 are shown in Figure 5. The l2,- and 'Ag states do not correlate CO VIBRATIONAL OUANTA (v) with any of the energetically accessible states of C 2 0 for either case. Molecules with large rovibrational state densities can unFigure 4. Experimental and calculated vibrational energy distributions for C O formed in the photodissociation of C 3 0 2at 193 nm (a) and 249 dergo rapid radiationless transitions to low-lying or ground nm (b). Calculated distributions are from the solution of eq 5 for the electronic states prior to dis~ociation.~~ Carbon suboxide has a * stated available energies, E: (a) (O), experimental data; (+) E = 72 large rovibrational level density in its ground electronic state, the kcal/mol; (A) E = 46 kcal/mol; ( 0 ) E = 30 kcal/mol. (b) (0)exresult of a low-frequency bending mode (u, = 23 ~ m - l ) . ~ ,On perimental data; (+) E = 39 kcal/mol; (A) E = 22 kcal/mol; ( 0 )E = the basis of the correlation diagram, if C 3 0 2is prepared in the 16 kcal/mol. IZ, or 'Ag state, it must undergo a radiationless transition in order to dissociate to thermochemically feasible products. In the case channel.37 Our observation that the nascent CO produced by of a linear fragmentation, ground-state C 3 0 2correlates with the photodissociation of C3O2 is rotationally excited is consistent C20(b'Z+), whereas ground-state C3O2 correlates with C20(n1A) with a nonlinear fragmentation path; Le., the reaction proceeds for a nonlinear fragmentation. The relatively low-lying 'A,, state via a bent transition state. of C30z correlates with C20(Z'A) and C2(b'Z+) for the linear Energy Partitioning and Dynamics. The CO product vibraand nonlinear decarbonylation reactions, respectively. If electional distributions shown in Figure 4 are best interpreted in light tronically excited C3O2 were to undergo ittersystem cros.sing, of some simple models for energy partitioning. Statistical models subsequent fragmentation would yield C20(X32-) or C20(A3ni) have proven to be useful at predicting nascent product energy as shown in Figure 5. Ab initio calculations indicate that a distributions following the photodissociation of polyatomic molnonlinear fragmentation is the minimum energy decarbonylation ecules in several cases, particularly for reactions with no energy barrier beyond that of the endothermi~ity.~~ CHzC0,23-26J7 Y
I
(33) Houston, P.L.; Moore, C. B. J . Chem. Phys. 1976, 65, 757. (34) Roebber, J. L. J . Chem. Phy. 1971, 54,4001 and references therein. (35) Jortner, J.; Rice, S . A.; Hochstrasser, R. M. Adu. Photochem. 1969, 7, 149. (36) Duckett, J. A,; Mills, I. M.; Robiette, A. G. J . Mol. Spectrosc. 1976, 63, 249.
(37) Osamura, Y . ;Nishimoto, K.; Yamabe, S.; Minato, T. Theoret. Chim. Acta 1979, 52, 251. (38) Wittig, C.; Nadler, I.; Reisler, H.; Noble, M.; Catanzarite, J.; Radhaknshnan, G. J . Chem. Phys. 1985, 83, 5581.
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 4041
Photofragmentation Dynamics of Carbon Suboxide
l
8-
l
-C,O(b'Z,)
E"
I
-
C,OIA'n,l
7-
-
6-
>
B
5-
w 42
C,02
5-I
z
Figure 6. Schematic representation of the energetics of the dissociation is activated by a photon of energy hu. of C302. The reactant (C302) Four or five product states are energetically accessible, depending on the excitation wavelength (see text). E, is the reaction exoergicity. DHo is the endothermicity for the dissociation of C302to give ground-electronic-state products. See text and Table I.
3-
9 -
-PRODUCTS
"1
NCN0,z4 and HZCOz5have all been found to dissociate on the ground-state surface following photoactivation and fast internal conversion to give vibrationally excited products. Photodissociation of CH2C023and NCNO% resulted in product vibrational energy distributions that could be accurately described by a simple statistical model. Conversely, the photodissociation of H2C0,yielding Hzand CO, resulted in a markedly nonstatistical vibrational distribution in the nascent products.3g This is apparently a result of the substantial barrier (in excess of the endothermicity) for the H 2 C 0 dissociation reaction. Potential energy release in the exit channel can dominate the reaction's energy disposal dynamics. There is no appfeciable barrier to the dissociation of C H 2 C 0 or NCNO. C20(X32-), generated in a cryogenic C O matrix, has been shown to recombine with CO to reform C302with little or no activation barrier.18 By analogy with the ketene and N C N O examples, the fragmentation of C302should yield products with a statistical distribution of energy. Our experimental results corroborate these expectations. We have previously shown that statistical models for energy partitioning in the decarbonylation of cyclic ketones28-a provide some insight regarding the degree of interfragment "coupling" a t the transition state and in the exit channel. Nascent product energy distributions are computed as a function of the magnitude of the energy available to be partitioned among the degrees of freedom of the separating photofragments. The maximum available energy, or exoergicity, for reaction 1 is shown schematically as E, in Figure 6 . The exoergicity is defined as E, = hv - DHo (4) where hv is the photon energy and DHo is the bond dissociation energy for (1). The five channels shown in Figure 6 represent the thermochemically feasible product states for 193- and 249-nm photolyses of C3Oz (see below). The probability for forming CO with vibrational energy, t, for a given available energy, E I E,, is given by
1 I Figure 5. State correlation diagrams for the linear (a) and nonlinear (b) dissociation of carbon suboxide yielding C 2 0and CO. These diagrams have been adapted from Minato, et. al.49 Relative energies are taken from ref 34, 37, and 43. CO(%'Z+)is assumed for each of the indicated product channels.
where Nco(e) is the vibrational density of states for CO at energy t, Pr(E-e-Et) is the total number of states corresponding to C20 vibrational and all relative rotational degrees of freedom at energy (39) Debarre, D.; Lefebvre, M.; PEalat, M.; Taran, J. P. E.; Barnford, D. J.; Moore, C. B. J. Chem. Phys. 1985,83, 4416. (40) Sonobe, B. I.; Fletcher, T. R.; Rosenfeld, R. N. J . Am. Chem. SOC. 1984, 206, 5800.
4042 The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 TABLE I: Maximum Available Energies, E , (kcal/mol), in the Photodissociation of Carbon Suboxidea c20 E, at E , at electronic state Tn(C,O) 193 nm 249 nm R3z-
5lA
A3n, LIZ+
0 226 33b
39 16
72 50 39 35
5 2
"Assuming DH"(OCC-CO) = 76 kcal/mol (ref 42). *Reference 43.
( E - e - Et), and is proportional to the one-dimensional translational-state density at translational energy E,. The denominator in ( 5 ) normalizes the probability function. Rovibrational state counting is done with the semiclassical WhittenRabinovitch appro~imation.~'Equation 5 can be used to compute fle;E) for a range of available energies, E , in order to generate a "best fit" to the experimental data. C 2 0 vibrational frequencies and moments of inertia are fixed at those of the ground state, %Z-, (see ref 19). Some results are shown in Figure 4. We find that the nascent CO photoproduct vibrational energy distributions are best fit by choosing E = 46 kcal/mol for 193-nm photolyses and E 22 kcal/mol for 249-nm photolyses. These values can be compared with values computed on the basis of thresholds reported in the literature for the five channels shown in Figure 6. In this way, we can begin to assess the relative importance of these various channels in the primary photochemistry of C302. First, to what extent are C(3P) atoms produced from the 193-nm photolysis of C302? The endothermicity of reaction 2 is W ( 2 ) = 138 kcal/mol. Thus, the maximum energy available to the products, C(3P) + 2C0, is ca. 10 kcal/mol for photolysis at 193 nm. This assumes that the two carbon monoxide molecules are formed in concert. The available energy, in this case, is insufficient to populate CO(u=2), much less the CO(ue5) observed here. Thus, the concerted formation of two CO molecules can be ruled out as the primary dissociation channel at 193 nm. It is possible that C20 is a primary product of C3O2 at 193 nm but is formed with enough internal energy to undergo a secondary dissociation. The observed CO product vibrational energy distribution (Figure 4a) is difficult to reconcile with this hypothesis. Additionally, if C(3P)atoms are formed a t 193 nm, but not at 249 nm, the transient absorption feature due to the bimolecular reaction (region 11, Figure 2) might be expected to exhibit different kinetic behavior. The consistency of the observed region I1 kinetics for both photolysis wavelengths suggests that the same chemistry is occurring in both cases. If the secondary fragmentation occurred at a rate 110' s-l, the production of CO due to this process could be directly observed. Our experiments clearly indicate the formation of C O in region I1 to be bimolecular in origin. We conclude that C20is the primary product of C302photolysis at both 193 and 249 nm and that this product is stable with respect to unimolecular decay under our experimental conditions. As noted above, some uncertainty exists regarding the nascent electronic state(s) of C 2 0 formed upon photolysis of C302. A lack of accurate thermochemical and spectroscopic data for C20 complicates this problem. Meyer and S e t ~ e used r ~ ~collisions of metastable rare gas atoms with C302to measure D P ( 0 C C - C O ) . They determined upper and lower limits of 76 and 54 kcal/mol, respectively, for the bond dissociation energy. Estimates of DH" based on kinetic measurements" and a b initio calculation^^^ are most consistent with the larger value. D P ( 0 C C - C O ) is indicated schematically in Figure 6. Threshold values for the other channels, corresponding to the formation of electronically excited CzO, are estimated by adding the appropriate electronic energy term, T o ( C ~ O )The . ~ ~maximum energy available to the products of the photofragmentation of C302,E,,,, depends on the excitation frequency, hv, the value of DHO(0CC-CO), and the electronic state in which the C 2 0 product is formed: E , = hv - DH" ~
~~~
(41) Whitten, G. Z.; Rabinovitch, B. S . J . Chem. Phys. 1964, 41, 1883 (42) Meyer, J. A.; Setser, D.W. J . Phys. Chem. 1970, 74, 3452.
(43) Walch, S. P. J Chem Phys 1980, 72, 5679
Weiner and Rosenfeld T0(C20). Some values of E,,, are listed in Table I. Consider the 193-nm photodissociation of C302 to give C,O(R'Z-) and CO. If we taken DHo(OCC-CO) = 76 kcal/mol and assume the full reaction exoergicity ( E = E, = 72 kcal/mol) is available to be statistically partitioned among the products' degrees of freedom, eq 5 predicts a C O product vibrational energy distribution substantially hotter than what is experimentally observed (see Figure 4a). In order to obtain agreement with the experimental data, the available energy in eq 5 must be reduced to E = 46 kcal/mol. Thus, if energy disposal in reaction 1 is statistical, the available energy is ca. 26 kcal/mol less than the reaction exoergicity for C20(X32-) production. This result might obtain if there was a 26 kcal/mol barrier to (1) in excess of D P ( O C C 4 2 0 ) . However, it has been shown that there is a negligible barrier to the recombination reactioni8
This suggests either that energy partitioning in (1) is nonstatistical or that C20(X3Z-) is nor a primary product of the 193-nm photodissociation of C302. Energy disposal data for the 249-nm photodissociation of C302 can be analyzed in analogy to that for the 193-nm experiments. The CO product vibrational energy distributions calculated with eq 5 are shown in Figure 4b. If we assume the primary product is C20(jc3Z-) with DHo(OCC-CO) = 76 kcal/mol, the full reaction exoergicity is 39 kcal/mol. The calculated distribution function, f(t;E,), is again significantly hotter than that observed experimentally. To obtain agreement between the statistical model and the observed C O vibrational distributions requires that the available energy in the former case be reduced to ca. 22 kcal/mol. This result is qualitatively consistent with our findings in the case of the 193-nm photodissociation; Le., in both cases, a statistical energy disposal model indicates that ca. 20 kcal/mol less than the full reaction exoergicity is available. Since the two excitation wavelengths employed here can populate different electronic states34of C3O2, this finding argues against the importance of nonstatistical effects on the energy-disposal dynamics in our experiments. We conclude that C20(X3Z-) is nor a primary product of the 193- and 249-nm photodissociation of C3O2. If the primary product was C20(giA),the maximum available energy would be 50 and 17 kcal/mol at 193 and 249 nm, respectively. Comparison with the energies required to fit the experimental data with the statistical model, eq 5, Le., 46 kcal/mol(l93 nm) and 22 kcal/mol (249 nm), suggests that nascent C 2 0 from (1) may be formed in th_eg'A state. Available energy in (1) is sufficient to populate the A3ni st_ateof-C20 for either 193- and 249-nm photolyses. However, (A3ni-X3Z-) visible fluorescence was not observed in our experiments, suggesting that triplet-state products are not formed to an appreciable extent in the photodissociation of C 3 0 2 Finally, although it is energetically possible to form C20(biZ+) in the 193- or 249-nm photodissociationof C302, the corresponding maximum available energies (35 and 2 kcal/mol) are substantially less than the values determined by fitting our data with a statistical mode! (i.e., 46 and 22 kcal/mol). Although the formation of C20(b'Z+) cannot be rigorously ruled out, our results argue that C20(ZiA)is the major product of the 193- and 249-nm photolyses of C3O2. It should be noted that this conclusion depends to some extent on the values assumed for DHo(OCC-CO) and the C 2 0 electronic terms, none of which have been precisely determined. The photophysical processes that occur prior to the dissociation of C3O2 cannot be directly inferred from our data. However, some observations may be made. Our observation that the CO product is rotationally excited is consistent with a nonlinear fragmentation mechanism. Thus, the symmetry constraints on the dissociation process are as illustrated in Figure 5b. Excitation of carbon suboxide at 249 nm populates C3O2('AUt1&,-). Excitation at 193 nm populates the same set of states or C302('4,'Z;). Our energy disposal data suggest that C20(H'A) is the principal product in bot? cases. Thus, chemistry must originate from the states of C 3 0 2 .The "quasi-linear" species, C H 2 C 0 and or XIZg+ NCNO, are both found to dissociate on the gound-electronic-state surface following photoa~tivation.~~,~~ Thus by analogy, we might
Photofragmentation Dynamics of Carbon Suboxide
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 4043
anticipate that in the case of C302internal conversion to the state precedes fragmentation. Although statistical energy disposal models have proven useful in a variety of cases, they are not necessarily a unique description of the reaction dynamics. For example, Franck-Condon modelshave been used to characterize energy disposal to a diatomic fragment in the photodissociation of some triatomic and polyatomic species. Berry44*45has developed an approach for computing diatomic product vibrational energy distributions based on a golden rule type expression, (7). Here the product states, {If)],correspond
wi-f = 2a/hl(9i)12p(e)
(7)
to the various vibrational states of CO, the initial state, li), is taken to be the ("dressed") CO oscillator component of the reactant and p ( e ) is the products' density of states. We have investigated the application of (7) in characterizing our experimental data (cf. Figure 4). Morse oscillator functions are used for (If)) and li). We find there is no reasonable choice of parameters of li), e.g., force constants and bond lengths, that reproduce the experimentally observed CO product vibrational energy distributions. We conclude that a Franck-Condon model, (7), is an inappropriate description of the dissociation dynamics of C302. The results reported here can be compared with existing data on C302. Recently, B e k e r and c o - ~ o r k e r s 'reported ~ the observation of C20(A311i-X3Z-) fluorescence following the 193-nm photolysis of C302.This emission was observed to grew in over a period of 5-10 1 s . The mechanism by which C20(A311i)was formed was not established. In our experiments, no visible emission is observed when C302is photolyzed at 193 nm. We find that C O is formed at a detector-limited rate, an! therefore C20 must be produced at this same rate. Thus, C20(A3ni)is not a primary product of the photolysis of C302a t 193 or 249 nm. Analysis of energy disposal to the CO product icdicates the most likely primary products are C,O(Z'A) CO(X'Z+). This conclusion is consistent with the recent results4' of Yamada et al. who have used absorption spectroscopy to detect C20formed following the 193- and 248-nm photolyses of C3O2. Kinetics. Our results indicate that following the UV photoactivation of C302CO is produced both directly and as the result of a bimolecular reaction whose rate is first order with respect to [ c 3 o d , e&
+
C 2 0 + C302
ks
n(C0)
+ products
(8)
where n = 1-3. We find k8 = 2.24 (f0.56) X lo6 Torr-' s-l, assuming n = 1. Forchioni and W i l l i ~ reported '~ that in the (44) Berry, M. J. Chem. Phys. 1974, 29, 323, 329. (45) Berry, M. J. J . Chem. Phys. 1974, 61, 3114. (46) Freed. K. F.: Band. Y. E. In Excited Stutes. Lim. E. C..,~ Ed.:, Academic Press: New York, 1977; Vol. 3 , p 109. (47) Yamada, C.; Kanamori, H.; Horiguchi, H.; Tsuchiya, S.;Hirota, E. J. Chem. Phys. 1986, 84, 2573. ,
I
253.7-nm photolysis of C302,CO formation was quenched by a factor of 160 f 30 in the presence of HZ. The measured48rate constant for (9) is k9 = 1.24 (f0.58) X lo4 Torr-' s-I. This C20(g3Z-) + H2 -% HC20
+H
(9) suggests that C20(w3Z-) reacts with C302with a rate constant of ca. 2 X lo6 Torr-' s-*. The rate constant for (8) deduced from literature data is thus in good agreement with our measured value for ks, again assuming n = 1. Since the primary product of the photodissociation of C302 is C20(Z'A), we conclude t h g either C20(Z1A) reacts with C 3 0 2at the same rate 5s C20(X3z-) or C20(5'A) undergoes rapid relaxation to C20(X38-) prior to reaction.
Conclusions We have used time-resolved CO laser absorption spectroscopy to study the dissociation dynamics of C3O2. The following results have been obtained: 1. Absorption of a 193- or 249-nm photon may initially populate different electronic states of C302, but dissociation to CO occurs from the same electronic state, most probably the ground electronic state following internal conversion. The fragmentation of C3O2 occurs from an electronic state of singlet multiplicity. 2. Fragmentation of C302 into C 2 0 and C O is the primary photochemical event following absorption of a 193- or 249-nm photon. Formation of C(3P) atoms at 193 nm does n,ot occur to a significant extent. The primary products are CO(X1zI+)and, most likely, C20(?i'A). 3. The vibrational energy distribution of the nascent C O product has been measured following the 193- and 249-nm photolyses of C3O2. In both cases, this distribution can be approximated by a Boltzmann function. The CO photofragment is rotationally excited to some extent. 4. Two models for energy partitioning in the dissociation of C302were investigated. A statistical model, which treats all the separating fragment degrees of freedom as possible modes for energy deposition, fits our data assuming an available energy of 46 kcal/mol a t 193 nm and 22 kcal/mol at 249 nm. A Franck-Condon photodissociation model is unable to fit the observed vibrational energy distributions. 5. C 2 0 reacts rapidly with C302 to form CO. The bimolecular rate constant, k8, is found to be 2.24 (f0.56) X lo6 Torr-' s-'. CO(uL 1) undergoes vibrational relaxation upon collisions with C3O2 with a rate constant of 1.70 (f0.37) X IO5 Torr-' s-', Acknowledgment. Acknowledgment is made to the National Science Foundation (CHE-85007 13) for support of this research. Registry No. C,02, 504-64-3; CO, 630-08-0. (48) Horie, 0.; Bauer, W.; Meuser, R.; Schmidt, V. H.; Becker, K. H. Chem. Phys. Lett. 1983, 100, 251. (49) Minato, T.; Osarnura, Y.; Yarnabe, S.; Fukui, K. J. Am. Chem. Soc. 1980, 102, 581.
