6337
J . Phys. Chem. 1990, 94. 6337-6341
it is probably also too slow. Reaction 33 is endothermic by only 59 kJ/mol. It is conceivable that the activation energy is not much higher. Therefore, we suggest it as a candidate to explain the exchange of CF2 groups. The CF2 exchange is slow compared to the chain growth and chain transfer. Therefore, the n dependence of the yields are parallel for odd and even n. For the n dependence of the even n products, a Poisson distribution is expected, if each iodide product can enter again into the reaction sequence just like the starting iodide CF31.I Such a distribution is also indicated in the figure. But similar product distributions can also be expected, if log (chain-transfer rate/chain-growth rate) decreases as ( n + 1)-2.1 Such behavior is often observed and is interpreted as a consequence of electrostatic effects on the rate constants. Our data are not sufficient to distinguish the two mechanisms. 7. Conclusion We obtained quantum yields Q, (produced CF3(CF2),I molecules/absorbed photons) of up to 0.2 with selectivities S (produced iodide/converted CF3J) up to 0.8. These results are much better than the previous ones (a = S = O.l), attained at low pressures. The success at high pressure is due to the fact that the radicals are consumed already while they are generated. So their concentration never grows large enough for dimerization. The high S is surprising in view of the high temperatures involved (more than 1000 K). S begins to drop again only at temperatures where a side reaction sets in. This is the reverse of the C2F4 addition, in our case. Therefore, the optimum temperature rises with pressure (Figure 4). That both S and 0 also rise with pressure is a consequence of the steady-state kinetics with propagation of first order and termination of second order (section 4). This is the most common type of chain reaction. If propagation and termination have the same kinetic order, S and Q, will not depend on pressure.
Although the application of kinetic principles allowed us to increase the quantum yield already by 2 orders of magnitude? an even higher Q, could be attained if the ignition condition -AH I Q were reached. The key problem consists in that the penetration depth became too small at the pressures of interest. The thin heated layer has a surface-to-volume ratio that favors heat conduction. In order to reach the ignition threshold, one should obviously use a wavelength that can penetrate 10 cm or more at superatmospheric pressures. This is possible in the IO-pm range, especially in the presence of a sensitizer (e.g., cyclo-C4FBwhich is a common impurity in C2F4). However, this is more difficult than it seems. To heat more molecules requires more energy per cm2. Above about 5 J/cm2 of a microsecond C 0 2 laser pulse, windows begin to be damaged. A way out of this difficulty is to use pulses of about 1 ms length. The energy load (which is basically a thermal load) that a window can stand grows as the square root of the pulse length. Heat conduction (Figure 1) as well as convection takes place on a longer time scale. C 0 2 laser pulses with 1 ms length are often applied in material processing. On the other hand, it should be noted that it is hardly worthwhile to improve our quantum yields. At a cost of about 0.02 DM/mol of C 0 2 laser photons (100-ps pulses),21 the laser’s contribution to the product cost is only 0.10 DM/mol. Acknowledgment. For the detailed kinetic calculation, we used a computer program of Dr. F. Rebentrost, which he kindly made available to us. To Prof. Zhang Linyang we are indebted for many helpful and stimulating discussions and for his help in the initial experiments. For his helpful technical support we thank Mr. J. Hartmann. Registry No. C2F4,116-14-3; CFJ, 2314-97-8. (21) Fuss, W.; Schmid, W. E. “Kohlenstoffisotopentrennung mit einem giitegeschalteten C02-Laser”; Report MPQ 138, Max-Planck-Institut fiir Quantenoptik, Garching, 1988.
An Examination of Pathways for the Reaction of O(’P) Atoms with FC0(2A‘) Radicals J . S . Francisco* and A. Ostafin Department of Chemistry, Wayne State University, Detroit, Michigan 48202 (Received: November 6, 1989; In Final Form: March I , 1990)
The reaction mechanism for the reaction of O(’P) atoms with FC0(2A’) radicals has been studied by means of ab initio quantum chemical techniques using polarized basis sets and including the effects of electron correlation and zero-point corrections. The mechanism involves the initial addition of O(3P) to the FC0(2A’) radical to form FC(0)O radicals, subsequently followed by fluorine elimination to form CO2(I2 +). These results are consistent with experimental observations of Ryan and Plumb. The previously measured rate constant for the O(3P) + FC0(2A’) reaction is also suggested to be due solely to the following [FC(O)O]’ F(2P) + C02(1Z,+). reaction which accounts for the loss of FC0(2A’) radicals: O(’P) + FC0(2A’)
-
-
+
Introduction
The increased use of fluorine-containing compounds in industrial processing has created the need for understanding the chemical processes taking place in these systems. One particular application has been their use in plasma etching of semiconductor devices. It has been observed that the addition of O2to CF, plasmas leads to enhanced rates of etching in single and polycrystalline silicon, and it has been concluded that this rate change is coupled with increases in excited fluorine atom concentration^,'-^ which are considered to be the principal etchant species. ( I ) Flamm, D. L.; Donnelly, V. M. Plasma Chem. Plasma Process 1981, 1317.
( 2 ) Jacob, A . US.Patent 3, 795, 597, 1974. (3) Harshbarger, W. R.; Miller, T.A.; Norton, P.; Porter, R. H. Appl. Spectrosc. 1977, 31. 1977.
0022-3654/90/2094-6337$02.50/0
Detection of fluorine atoms, in the O(’P) CF3 reaction as well as stable species such as CO, C 0 2 , COF2, and SiF4 has suggested the existence of a number of mechanisms for their formation in the plasma gas discharge. It has been proposed4 that the addition of O2results in the formation of fluorine, viz. 0 + CF2 FCO F (1)
+ 0 + CF2 * C O + 2F -+
(2) In the reaction between CF2 and O(’P) studied at 295 K in a gas flow reactor, the major by-product is found to be C02, with some C O and COF2 present in minor quantities. The possibility of COz formation, VIZ. (4) Flamm, D. L.; Donnelly, V. M.; Ibbotson, D. E. J. Vac. Sci. Technol. 1983, B I , 23.
0 1990 American Chemical Society
6338 The Journal of Physical Chemistry, Vol. 94, No. 16, 1990
0, + FCO
-
CO,
Francisco and Ostafin
+ FO
F(’P) + COz(’B,)
(3) has been investigated and concluded to be not significant under the given conditions, as the FCO signal remained unchanged with the addition of increased amounts of 02.Since the major product is consistently C02, it has been suggested that the major occurring reactions that could account for it are 0 + FCO C02 + F (4)
I
s
+
0 + FCO
-.C O + FO
(5)
with the principal route to CO, formation posited as reaction 4. A combined rate coefficient for reactions 4 and 5 is reported5 as (9.3 f 2.1) X IO-” cm3 molecule-] s-I. CO, observed in studies of the 0 + H2C0reaction system is consistent with the observation of CO, in the 0 + CF2 reaction. However, abstraction of hydrogen from HCO is competitive with the route which yields C026,7 0 + HCO - + O H + CO ---+
H
+ C02
I
i
I
(6) (7)
More recent studies of the reaction of O(3P) with CF, using time-resolved FT-IR suggest that reaction 5 is not significant, since no CO is detected. Only ground state CO, is observed as the principle reaction product in the 0 + FCO reaction.* The precise mechanism for CO, production has not yet been elucidated. However, a potential pathway is via a fluoroformyloxy intermediate FC(0)O. Studies using ab initio molecular orbital techniques have indicated that the most favorable dissociation pathway for FC(0)O is extrusion of fluorine to form C02,9 consistent with experimental findings which conclude CO, as the resulting product. I n the present study, an analysis of the 0 + FCO reaction system is undertaken using ab initio molecular orbital theory in order to estimate the energetic feasibility of various possible reaction pathways of potential importance in the 0 + FCO reaction, and to assess the extent of involvement of the fluoroformyloxyl radical. Computation Methods All calculations were performed using the G A U S S I A N E pro~ gramlo implemented on a Stellar GS1000. Geometry optimizations were carried out for all structures using Schlegel’s method’] to better than 0.001 8, for bond lengths and O s l o for angles; with a S C F convergence of at least on the density matrix, the residual rms force was always less than IOd4 au. To find the transition structure for the reaction of FCO with 0 atoms, several points were calculated along the reaction path by fixing the oxygen reactant distance and minimizing the energy with respect to all of the other parameters. With this procedure a preliminary structure was obtained. Full optimization for the transition state was done at the MP2/3-21G level of theory in order to provide an adequate description of the FO bond distance which is found to be poorly described at the UHF/3-21G and UHF/6-31G* level of theory. Optimizations at the second-order unrestricted Maller-Plesset (UMPZ) level of theory were performed with all orbitals active. Refinement of the structure of both products and reactants as well as the transition structure was done at the MP2/6-31G* level of a b initio M O theory. Higher order electron correlation was included by using fourth-order M~ller-Plesset perturbation theory in the space of single, double, quadruple, and triple excitations ( 5 ) Ryan, K. K.; Plumb, 1. C. Plosmo Cfiem.Plasmo Process 1984,4, 271. (6) Niki, H. J. Cfiem. Pfiys. 1966, 45, 2330. ( 7 ) Westenberg, A . A,; deHaas, N. J. Chem. Pfiys. 1972, 76, 2215. ( 8 ) Hancock, G.; Heard, D. E. Private communication. (9) Francisco, J. S.; Goldstein, A. N. Cfiem. Pfiys. 1988, 127, 73.
(IO) Frisch, M. J.; Binkley, J. S.; DeFrees, D. J.; Raghavachari, K.; Schlegel, H. B.; Whiteside. R. A.; Fox, D. J.; Martin, R. L.; Fluder, E. M.: Melius, C. F.; Kahn, L. R.; Stewart, J. J. P.; Bobrowicz, F. W.; Pople, J. A. GAUSSIANB~;Carnegie-Mellon Quantum Chemistry Publishing Unit, Pittsburgh, PA, 1984. ( I I ) Schlegel, H . B. J . Comput. Cfiem. 1982. 3, 214.
Figure 1. Correlation diagram for the O(’P)
+ FCO(*A’) system.
A
-
Figure 2. Transition state and transition vectors for the O(’P) FC0(2A’) FO(*IIt) C0(’Zt) abstraction reaction.
+
+
(MP4SDTQ).I2 Spin contamination corrections were performed by annihilating the highest spin contaminant of the unrestricted wave function.I3 Vibrational frequencies for products, transition structures, and reactants were estimated at the MP2/6-3 1G* level of theory.I4 Results and Discussion A . Application of the Adiabatic Correlation Rules. The adiabatic correlation r u l e ~ ’ may ~ J ~be used in order to understand what molecular states and their corresponding products arise as a consequence of collision with reactants of known electronic character. The adiabatic correlation rules for spin and orbital symmetry predict that the interaction of O(3P) and FCO(,A’) yields complexes in both ,A’ and ZA” states. Using orbital symmetry considerations,” it is found that the ,A’’ hypersurface correlates to FO(,Il+) and CO(’Bg+) through an OFCO complex in the F atom abstraction process (see Figure 1). The addition reaction of O(3P) with COz(’Z,+) proceeds through a fluoroformyloxyl intermediate, FC(0)O. The ground state of FC(0)O has 2B2symmetry, but approach of oxygen atom toward FCO does not permit this symmetry constraint, and consequently, the addition route proceeds only along a C, surface of either A’ or A“ components. The 2A” surface connects the O(3P) and FCO(,A’) reactant to the lowest energy state of the FC(0)O radical. A ,A‘ surface also connects the O(3P) and FCO(,A’) reactants to the lowest energy state of FC(0)O on the C, surface. Dissociation of FC(0)O in the ,A’ state proceeds only on the C , ,A’ surface to form the products F(zP) and C02(IZg+). B. Geometries. Optimized geometries for the reactants, transition structures, and products of the O(3P) FC0(2A’) reaction system are given in Table I. Preliminary calculations at the MP2/3-21G level of theory indicate that the most energetically preferred approach of the 0 atom is along the symmetry plane of the molecule toward the carbon, along the ,A‘‘ surface, resulting in the formation of FC(0)O in the ,B2 ground state. Several points were optimized along the reaction path at this level
+
( I 2) Kirshnan, R.; Pople, J. A. Int. J . Quantum Cfiem.,Quanrum Cfiem.
Symp. 1980, 14, 91.
( 1 3 ) Schlegel, H. B. J. Chem. Pfiys. 1986, 84, 4530. (14) Pople, J. A.; Kirshnan, R.; Schlegel, H. 9.; Binkley, J. Quantum Chem., Quonrum Chem. Symp. 1979, 13. 225. (IS) Shuler, K . E. J. Chem. Pfiys. 1953, 21, 624. (16) Donovan, R. J.; Husain, D. Cfiem. Reu. 1970, 70,489. ( 1 7 ) Walsh, A . D. J. Cfiem. SOC.1953, 2266.
S. fnr. J .
Reaction of O(3P) Atoms with FCO(*A’) Radicals TABLE I: Optimized Geometries of Species Involved in the O(3P) and FCO(*A’) Reaction“ MP2/6species state parameter MP2/3-21G 31G* exp 1.358b I .367 1.344 FO 211+ R(F0) 1.12gC 1.123 1.151 co 1.16Ic 1.197 1.179 co2 1.277 1.250 CO2 118.5 116.3 1.169d 1.185a 1.182 FCO 1.334 1.383 1.342 127.3 128.8 128.0 1.254 1.244 FC(0)O 1.342 1.342 121.7 121.3 116.6 117.4 1.210 1.194 FC(0)O 1.382 1.329 1.374 1.339 125.6 125.6 127.1 126.5 2.21 1 2.409 [0-FCO]’ 1.169 1.148 1.878 2.01 5 130.9 137.7 169.7 167.8 1.187 1.170 [ F-C02] 1.250 1.225 1.660 1.662 119.4 118.2 156.2 158.9
’
aBond lengths i n units of angstroms and angles in degrees. *McKellar, J. A. R. W. Can. J . Phys. 1979, 57, 2106. CStull,D. R.; Prophet, H. JANAF Thermodynamical Tables, 2nd ed.;NSRDS-NBS 37; US. Government Printing Office: Washington, DC, 1971. dNagai, K.; Yamada, C.; Endo, Y.; Hirota, E. J . Mol. Spectrosc. 1981, 90, 249. TABLE 11: UMP2/6-31G* Vibrational Frequencies for Species in the O(jP) and FC0(2A’) Reaction
species FO
co CO2 CO2 FCO FC(0)O FC(0)O [0-FCO] [ F-CO,]
state
frequencies,”cm-l
211+
1542 (1033)b 2125 (2146)c 2454 (2340). 1336 (1970). 642 (658)d 1524, 1 197, 648 2006 (1862). 1098 (1026). 629 (628)e 3401, 1626, 1016, 880, 574, 557 1923, 1306, 958, 752, 582, 449 2141, 185, 81, 58, 19, 174i 2245, 126 I , 68 1, 640, 444, 1 153i
lE+
’ ’
)Ea+ IB2 2A‘ 2B2 2A’ 2Ajt
ZA‘
a Experimental values where available in parentheses. McKellar, A. R. W. Can. J . Phys. 1979, 57, 2106. cLagemann, R. T. Phys. Rev. 1942,61, 729. dEgger, D. F.; Arends, C. B. J. Chem. Phys. 1957,27, 1405. eNagai, K.; Yamada, C.; Endo, Y.; and Hirota, E. J . Mol. Spectrosc. 1981, 90, 249.
of calculation and revealed a smooth energy profile with no maximum; no transition state was found. Abstraction of fluorine to yield ground-state FO and carbon monoxide was found to be exothermic as well. A search for the transition state did show that the energy profile for this reaction exhibits a maximum. The transition structure, as shown in Figure 2, shows the breaking C F bond distance is 2.018 A at the MP2/6-31G* level of ab initio molecular orbital theory. The C O bond length in the transition structure is 1.148 A, compared to that in the reactant FCO of 1.182 A. MP2/6-31G* vibration frequencies are collected in Table I1 for all species involved in the O(’P) + FCO(ZA’) reaction system and are compared with experimental frequencies for those that are available. Examination of frequencies for the [O-FCO]’ and [F-COZ] * transition structures reveals the potential maxima possess a single negative eigenvalue, which is consistent with what is expected for a true transition state on the potential surface. C. Energies. The total energies for reactants, transition structures, and products are collected in supplementary Table 1
The Journal of Physical Chemistry, Vol. 94, No. 16, 1990 6339
Figure 3. Summary of the energetics of reaction pathways for the O(3P) FC0(2A’) system calculated at the PMP4SDTQ/6-31G*//UMP2/ 6-31G* level of theory. Energies are given in kJ/mol.
+
(see paragraph at the end of this article), and the heats of reaction are summarized in Table 111. Calculated energetic barriers for the pertinent reactions are listed in Table IV. The calculated heats of reaction for both reactions 3 and 4 were discussed previously e l s e ~ h e r e . ~ MP2 + ’ ~ calculations were found to be more reliable for the description of the ground-state FC(0)O radical and FO. Examination of the potential surface profile for the addition of O(’P) + FC0(2A’) to form FC(0)O radicals in the 2B2state reveals a smooth transition to the product with no discernible maximum; no transition state was found. Enlargement of basis set (3-21G > 6-31G*) enhances the exothermicity of the FC(0)O formation reaction. Inclusion of higher order electron correlation and spin contamination effects decreases the heat of reaction, at the inclusion of fourth-order electron correlation the value is -384 kJ/mol (PMP4SDTQ/6-31G*//UMP2/6-3 lG*). Examination of the energies using spin projection resulted in little change for the heat of reaction for the formation of FC(0)O in the *A’ state from FCO(*A’) and O(’P). Preliminary examinations performed using yet a larger basis set (UMP4/6-31G*) revealed no significant differences in the relative energies for species relevant to the reaction system under consideration. Abstraction of fluorine by O(’P) atom from the FC(0)O radical to form FO and C O is predicted to be exothermic. Inclusion of spin projection appeared to have little effect; values for the calculated heat of reaction differ by less than 3%. The current estimate for the heat of reaction for the process of 0 + FCO FO C O is -48 kJ/mol (PMP4SDTQ/6-3 IG*//UMP2/631G*). The energy profile along the approximate reaction path for 0 + FCO reaction is given in Figure 3. Optimization of several points along the 0 + FCO FO CO reaction coordinate revealed a maximum, corresponding to an [O-FCO] * transition structure. At the UMP2/6-31G* level the barrier height for this reaction is 15 1 kJ/mol. Expansion of basis set and inclusion of higher order electron correlation predict the barrier height to be 125 kJ/mol (UMP4SDTQ/6-3 IG*//UMP2/6-3 1G*). Inclusion of spin projection lowers the barrier height somewhat. The best current estimate is at the PMP4SDTQ/6-31G*//UMP2/6-31G* level is 91 kJ/mol with zero-point-energy correction. D. Mechanism of t h e O(’P) and F C 0 ( z A 3 Reaction. With the information about the potential energy surface, as summarized
-
+
-
(18) Francisco, J.
+
S.;Zhao, Y.Chem. Phys. Lett. 1988, 153, 296.
6340
The Journal of Physical Chemistry, Vol. 94, No. 16, 1990
Francisco and Ostafin
-
-
TABLE 111: Calculated Heats of Reaction for Species Involved in the O(3P) and FCO(’A’) Reaction‘ o ( 3 ~+ ) FCO(*A’) FC(O)O(~A’) FC(0)O FC(0)O F0(211+) F(2P) level of theory ?A‘) (2Bz) CO(’Z+) cod’Zg+) UMP2/3-21 -G -319 -318 -32 -28 -407 -428 UMP2/6-31G* -37 24 UMP2/6-3 IG*//UMP2/6-3 1G* UMP3/6-31G*//UMP2/6-3 IC* UMP4SDQ/6-3 IG*//UMP2/6-3 1G* UMP4SDTQ/6-3 1 G*//UMP2/6-3 1 G*
-404 -389 -456 -392
PMP2/6-3 I G * / / U MP2/6-3 I G* PMP3/6-3 1 G*//UMP2/6-3 1G* PMP4SDQ/6-3 IG*//UMP2/6-3 1G* PMP4SDTQ/6-3 1 G*//UMP2/6-3 IC*
+
Without Spin Projection -424 -38 -384 -29 -388 -42 -409 -5 1
-
FC(0)0(2B2) F(2P) + F(2P) + COz(’Z8+) COA’B2) -29 490 45 563
33 26 14
36 29 30 31
560 536 527 529
After Annihilation of the Highest Suin Contaminant -397 -430 -j4 I2 -384 -386 -27 34 -379 -388 -60 28 -388 -410 -49 15
46 35 37 37
590 559 550 499
-22
-49
PZPEIU MP2/6-3 I G * a
+
26
13
10
I
-9
Values in kJ/mol.
TABLE IV: Calculated Barrier Heights in the O(’P) and FCO(’A’) Systema transition state o(3~+ ) FCO(~A’) FC(O)O(~A’) FO(’II+) CO(’Zt) F(’P) + COz(’Z8+) UMP2/3-2 1G 88 99 UMP216-3 I G* 151 102
+
-
Without Spin Projection 150
U MP2/6-3 1G * / /
UMP2/6-3 1G’ UMP3/6-3IG*// UMP2/6-31G1 UMP4SDQ/6-3 1 G * / / UMP2/6-3 lG* UMP4SDTQ/6-3 1 G * / / UMP2/6-3 1 G*
-
101
123
119
I20
109
I25
97
After Annihilation of the Highest Spin Contaminant PMP2/6-31G*// 127 78 ti MP2/6-3 1G* PMP3/6-3 IC*// 96 I04 UMP2/6-3 IG* PMP4SDQ/6-3 I G * / / 95 95 UMP2/6-3 1G* PMP4SDTQ/6-3 l G * / / 98 83 UMP2/6-31G* AZPE//UMP2/6-31 G*
-7
-1 3
Values in kJ/mol
surface has an activation energy of 70 kJ/mol to yield F(2P) and C02(’Z,+). If dissociation occurred on the 2A’ surface, considering the exothermicity of the FC(0)O formation reaction, there would be 305 kJ/mol excess energy remaining in the FC(0)O radical. Since this energy content is in considerable excess of the dissociation barrier, fluoroformyl radicals would be expected to exist only transiently. If dissociation of the fluoroformyl radical occurred on the 2B2surface, there are two possible reaction pathways: (1) F(2P) and C02(’Z2);and (2) F(2P) and CO2(IB2). The latter process is highly endothermic (about 435 kJ/mol above the F(2P) and C02(IZg+) products at the PMP4SDTQ/6-31G*// UMP2/6-31G* level) and would not be of significance. The former process would be a route forming ground-state C02, which has been observed experimentally. (Several attempts to determine a transition state on the 2B2 surface at the UMP2/3-21G level of theory were not successful in finding a saddle point on the potential surface.) In proposing a dynamical mechanism for the formation of C 0 2 within this reaction framework, it is quite possible that dissociation occurs first by the addition of O(3P) to FCO(2A’) on the 2A” surface leading to the formation of FC(0)0(2B2) which subsequently dissociates to F(2P) and C02(IZ +). It is also a possibility that states of the 2A’’ surface couple to 3A’ background states which lead to dissociation on the 2A’ surface. In either scheme, the reaction of O(3P) and FCO(2A’) radicals leads to the formation of F(2P) atoms and C02(IFg+)through the transient FC(0)O intermediate. The implications are that the reaction of O(3P) + FCO(2A’) [FC(O)O]* -+ F(2P) + CO2(’ZB+)(8) +
in Figure 3, we may now address possible pathways for the reaction of OpP) and FCO(2A’). On energetic grounds, fluorine extrusion from the fluoroformyl radical via collisional reaction with oxygen atoms is an exothermic process by 48 kJ/mol with an additional barrier of approximately 91 kJ/mol; because of the significantly high barrier this reaction is unlikely to occur at room temperature. However, the favored pathway is the addition of oxygen atoms to the fluoroformyl radical resulting in the formation of the fluoroformyl radical, FC(0)O. This process is exothermic by 375 kJ/mol, provided the reaction takes place on the 2A’ surface to yield the FC(0)O radical in its 2A’ electronic state or 384 kJ/mol if the reaction follows the 2A’r surface to produce the 2B2 ground state of the FC(0)O radical. Dissociation of FC(0)O on the 2A’
is an important source of fluorine atoms in CF4/02 plasmas. E . Calculation of Reaction Rates and Experimental Comparisons. Ryan and Plumb5 reported a total rate constant of (9.3 f 2.1) X IO-’’cm3 molecule-l s-I for all processes leading to the removal of FCO radicals by reaction with O(3P) atoms. The major reactions are represented by reactions 4 and 5. Although it has been suggested that reaction 4 is the major route, its contribution to the total rate constant could not be determined. Previously we used a qualitative transition state theory treatment of the kinetics of the F HFCO reaction to determine the temperature dependence of the rate coefficient using potential energy surface parameters calculated from ab initio molecular orbital theory. Theoretical rate coefficient for the F + HFCO reactionIg were
+
TABLE V: Summary of Thermodynamic Analysis and Arrhenius Parameters for the O(3P) + FCO(’A’) Temperatures 295, 700, and 1000 Kasb AE’, AH’. Ea * T, K kJ/mol kJ/mol kJ/mol Eckart Wigner 295 94 92 97 1.077 1.075 93 I04 1.014 1.013 700 99 IO00 IO0 92 109 1.007 1.006 k in cm3 mol-‘
5-l
-
FO(*II+) + CO2(’Zgt) for log ka -26.68 -16.39 -13.92
A, cm3 mol-’ s-l 7.59 x 10-9 6.81 x 10-9 1.55 x 10-8
J . Phys. Chem. 1990, 94, 6341-6350 found to be close to the experimental values by a factor of 4. Similar theoretical treatment to describe the CH, + OH reaction obtained reasonable agreement.20 Consequently, the rate coefficient for the abstraction of fluorine atoms from the fluoroformyl radical by oxygen atoms is computed by using transition-state theory2' kBT - e - EQ* k(T) = o/bT h
QO~QFCO
(9)
where Qo, QFC0,and Q* are total partition functions for the reactants (0and FCO) and transition state ([O-FCO]'). These quantities can be evaluated by using statistical mechanical methods for the structures, the total energies, and the vibrational frequencies for the reactant and transition state. In our evaluation of the rate coefficient, UMP2/6-3 IC* results for the structure (Table I), vibrational frequencies (Table II), and total energies calculated in the PMP4SDTQ/6-31G*//UMP2/6-31G* level (Supplementary Table 1) were used. Rates are calculated from the classical transition state theory expression and corrections due to tunneling using Wigner22and &kartU potential procedure were incorporated. Rates calculated by using the Eckart potential have shown better consistency with experimental data in that they can usually reproduce the shape of the Arrhenius plot within experimental uncertainty. A summary of the calculation for three different temperatures (295, 700,and 1000 K) is shown in Table V. The reaction barrier, AE* (including thermal and zeropoint-energy corrections), and the enthalpy of activation, AH*, are listed along with the Eckart and Wigner tunneling corrections (19) Francisco, J. S.;Zhao, Y.J . Chem. Phys. 1990, 93, 276. (20) Gonzalez, C.; McDouall, J. J. W.; Schlegel, H. 9. J . Phys. Chem., in press. (21) Steinfeld, J. 1.; Francisco, J. S.;Hase, W. L. Chemical Kinetics and Dynamics; Prentice-Hall, Englewood Cliffs, NJ, 1989. (22) Wigner, E. P. 2.Phys. Chem. 1932, 819, 203. (23) Johnston, H. S.;Heicklen, J. J . Chem. Phys. 1966, 66, 532.
6341
factor, the calculated Arrhenius parameters A and E,, and the theoretical log k . The major question raised is that a t 295 K (the temperature at which the total rate constant for the 0 FCO reaction was measured) what is the contribution to the total measured rate constant by the 0 + FCO CO + FO reaction. The calculated rate for this reaction at 295 K is found to be negligible. In fact, it appears that the onset of significant contributions by this reaction does not arise until 1000 K. As aforementioned, the calculated rate constant could be as much as a factor of 4 to the experimental rate with this procedure. However, if we assume that the calculated rate constant is in error by a factor of 100, the resulting rate at 295 K is still negligibly small, and therefore has no contribution to the total rate constant reported by Ryan and Plumb for the loss of FCO radical via reaction with O(3P) atoms. Consequently, the reported rate constant of (9.3 f 2.1) X lo-" cm-3 molecule-' s-I due to reaction 8.
-
+
Summary Reaction pathways were calculated for the O(3P)and FC0(2A') system. A transition-state structure was found for the abstraction of fluorine from FCO by O(3P), while no transition state was located for the addition of O(3P) to FCO to form an FC(0)O radicals which is suggested to exist only transiently as it proceeds to dissociate to form F(*P) and C02('Z8+).The rate constant for the abstraction reaction at 295 K was determined to be negligible. This establishes that the previously measured rate coefficient of (9.3 f 2.1) X lo-" cm3 molecule-' s-' for the reaction of O(jP) with FC0(2A') radical is due solely to the following reaction:
O(jP)
+ FCO('A')
-
[FC(O)O]* -.+ F(2P) + C02('Z8+)
Supplementary Material Available: Table 1 listing total energies of species involved in the O('P) + FC0(2A') systems (1 page). Ordering information is given on any current masthead page.
Collisional Deactivation of Highly Vibrationally Excited Benzene Pumped at 248 nm Murthy L. Yerram; Jerrell D. Brenner, Keith D. King,*.' and John R. Barker*,$ The Department of Atmospheric, Oceanic, and Space Sciences, Space Physics Research Laboratory, The University of Michigan. Ann Arbor, Michigan 48109-2143 (Received: December 5, 1989; In Final Form: April 3, 1990)
Highly vibrationallyexcited gas-phase benzene was prepared with a pulsed excimer laser operating at 248 nm, and the subsequent collisional deactivation was monitored with time-resolved infrared fluorescence (IRF) from the C-H stretch modes near 3050 cm-I. Very low pressure photolysis mass spectrometric experiments were carried out to determine whether reaction of the benzene occurs at this laser wavelength and can complicate the energy-transfer investigation. The quantum yield for benzene loss was determined to be 3 f I%, consistent with previous experiments (Nakashima, N.; Yoshihara, K. J . Chem. Phys. 1982, 77,6040) that found photoproducts as the result of multiphoton excitation. The only detectable gas-phase product was H,,which had a quantum yield of 0.8 f 0.3%. Energy-transfer data were obtained for 19 collider gases, including unexcited , bulk average energy, was examined benzene. The inversion technique for converting the observed IRF decay to ( ( E ( t ) ) ) the the bulk average energy transferred in detail, and a careful propagation of errors analysis performed. For most colliders, ((AE)), per collision, exhibited an approximately linear dependence on vibrational energy for energies
