7473
J. Phys. Chem. 1995, 99, 7473-7481
Kinetics and Product Branching Ratios for the Reaction HCO
+ NO2
Yili GuoJ Sean C. Smith,* and C. Bradley Moore* Department of Chemistry, University of California, Berkeley, and Chemical Sciences Division, Lawrence Berkeley Laboratory, Berkeley, California 94720
Carl F. Melius Center for Computational Engineering, Sandia National Laboratories, Livermore, California 94551 -0969 Received: December 21, 1993; In Final Form: June 27, 1994@
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The kinetics and product branching of the HCO NO2 reaction have been studied at room temperature using laser flash kinetic spectroscopy and modeled with microcanonical variational RRKM theory. The rate constant for the disappearance of HCO radical at 296 K is (5.7 f 0.9) x lo-" cm3 molecule-' s-', and it is independent of the pressure of SF6 buffer gas up to 700 Torr. The COZyield is 52 f 14%. Less than 10% of the reaction goes through the most exothermic product channel, HNO COz. The least exothermic product channel, H COz NO, is responsible for the remaining COZ. HONO has been observed, though not quantitatively, as a reaction product corresponding to the HONO CO product channel. It must account for the 48 f 14% of reaction which does not yield COz. The rates of formation of the collision complexes HCO(ON0) and HCO(N02) are calculated by variational RRKM theory to be comparable and only weakly temperature dependent. Branching ratios for the decomposition of these complexes are also calculated. They are found to match most experimental observations from 300 to 1600 K.
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Introduction
20,
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The radical-radical reaction HCO NO2 is important in atmospheric and combustion chemistry. It is involved in nitration and oxidation rea~tionsl-~ and pyrolysis of organic nitrates and nitrite^.^ Knowledge of the reaction rate, products, and reaction mechanism is essential for quantitative modeling of those processes. There are four possible product channels in the reaction between HCO and N02, as shown in Figure 1. Gutman's group5has measured the rate constant for this reaction using laser photolysis with photoionization mass spectrometry detection. Morrison and Heicklen studied the products of the HCO NO2 reaction by photolyzing a gaseous mixture of HzCO and NO2 at room They used gas chromatography to analyze the stable molecules after the reaction proceeded. The results were interpreted as indicating that H COz NO is the only product channel and that perhaps some adducts were formed. In extensive studies of the reaction H2CO NOz, Lin and co-workers derived rate constants in the temperature ranges 393-476 K7 and 1140- 1650 Ks for HCO NO2 to yield CO and COz. They fit these data using standard RRKM theory and gave rate constant expressions for CO and COz production covering the entire temperature range. Ab initio computation^^.^ (Figure 1) show that two possible collision complexes can be formed in the HCO NO2 reaction. The calculated heat of formation of HCO (ONO) at 298 K is lower than that of HCO (N02) by 10 kcallmol. Whether these collision complexes can be stabilized in this system is an interesting and important issue. Morrison and Heicklen have tentatively assigned an IR absorption band between 700 and 800 cm-' to these complexes.6 HCO (ONO) has been postu-
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t Present address: Research Laboratories, Rohm and Haas Company, 727 Norristown Road, Spring House, PA 19477. Present address: Department of Chemistry, University of Queensland Brisbane, QLD 4072 Australia. @Abstractpublished in Advance ACS Absrrucfs, August 15, 1994.
5
0-
E
L
-::
-20-
%
p
-40-
W
-6080
'
Figure 1. Potential energy diagram of the reaction between HCO and N01. Solid lines are the ab initio calculated heats of formation at 285 K by using the BAC-MP4 method. The connecting dashed lines are merely estimated.
lated as a reaction intermediate in organic synthesis,1° although it has not been independently isolated. In this work, the rate constant and product branching ratios for the HCO NO2 reaction have been determined using laser flash kinetic spectroscopy and modeled using microcanonical variational RRKM theory and ab initio potential surface parameters. The reaction products, HNO, CO2, and HONO, corresponding to three different product channels, were directly monitored as the reaction proceeded. The results provide valuable information not only for quantitative modeling but also for understanding the reaction mechanism.
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Experimental Section The detailed experimental setup has been described prev i ~ u s l y . ~ ' -Briefly, '~ HCO radical was produced by photolysis of acetaldehyde with a 308 nm excimer laser (Lambda PhysiW 103E equipped with unstable resonator optics). The pulse duration of the excimer laser was 8 ns, and the energy was 6080 mJ per pulse. The probe laser was either an Ar+ laser (Spectra Physics12030- 18) pumped ring dye laser (Spectra Physics1380A) or a difference frequency laser system. The single-frequency Ar+ laser (LexeV94-5) used in the difference-
0022-365419512099-7473$09.00/0 0 1995 American Chemical Society
7474 J. Phys. Chem., Vol. 99, No. 19, 1995
-0
0 C ._
*
.o
a a .*_
-0
d
0
2
4
6
a
Time (~1s)
Figure 2. HCO absorption at 16262.25 cm-' following the photolysis pulse with (b) and without (a) NO*. The signals were averaged over five photolysis shots. P(CH3CHO) = 6.00 Torr and P(N02) = 0.33 Torr in part b. W laser power 15 mJ-cm-2per pulse. The solid line in the bottom trace is the single-exponential fit with a decay rate of6.55 x 105 s-1.
frequency laser system was operated either at 514.5 or 488 nm, depending on the absorption frequency of the species being studied. The photolysis laser, apertured to 7 mm diameter, and the probe laser were linearly directed through the photolysis cell (156 cm long and 2.5 cm diameter) by using a UV reflecting and visible or IR transmitting mirror to combine the beams. Special attention was paid to ensure that the probe laser (less than 0.5 cm in diameter) overlapped the center part of the photolysis beam in order to minimize inhomogeneities in concentration and avoid shock wave and diffusion problems. The photolysis beam intensity was sampled through a 1 mm diameter aperture and found to be uniform to within a 10% range over the 7 mm diameter beam passed through the photolysis cell. The low divergence of the unfocused 308 nm laser beam resulted in less than a 5% intensity decrease over the length of the photolysis cell. Both UV and IR powers were monitored during the experiments for quantitative determination of the reaction products. The power of a small portion of each beam was split off prior to entering the photolysis cell and measured for power normalization. The fluctuation of each laser was less than 15% of its total power during the course of an experiment. The absolute UV laser power was measured with a calibrated UV power meter (ScientecW38-0 105). A photodiode detector (RCNC308 10) with large sensitive area (100 mm2) and fast response (12 ns) was used for monitoring HCO absorption in the visible. An InSb detector (Santa BarbaraE292; 0.25 mm in diameter; 50 ns response time) or Infrared Associates/H-313-IS (2 mm diameter; about 1 p s response time) was used when the IR laser was the probe beam for product detection. The signal was amplified and then either digitized (Tektronid7912AD) with a 20 MHz low pass filter at the input or integrated using a boxcar integrator (PAR/164/162). The data were stored and analyzed with a personal computer (FountaiflT). CH3CHO and NO2 were continuously flowed through the photolysis cell. Acetaldehyde (Mallinckrodt, min 99%) was purified by several freeze-pump-thaw cycles and kept at 0 "C. NO2 (Matheson, min 99.5%) was fractionally distilled and
Guo et al. stored at liquid nitrogen temperature. NO (Matheson, 99%) was distilled through a dry ice/ethanol trap. sF6 (Matheson, 99.99%) and COz (Matheson, 99.998%) were used without further purification. The flow rates were monitored with calibrated flow meters (HastinglALL-5 and LF-100). The total pressure of CH3CHO and NO2 in the photolysis cell was around 6 Torr with the ratio of CH3CHO to NO2 typically greater than 10 to 1. The pressure was measured with Baratron pressure gauges (MKS/310B). The absorption coefficients of CH3CHO and NO2 at 308 nm are (1.07 f 0.02) x and (5.41 f 0.14) x Torr-' cm-', as determined in this experiment, where the quoted errors are la values. The power of the UV photolysis laser was about 15 mJ/cm2 per pulse. Each data trace was an average of 5 or 10 photolysis shots. The total quantum yields for photodissociation of NO2 and CH3CHO at 308 nm are 0.99514 and O.925,l5J6respectively. Less than 0.1% of total CH3CHO and 1% of total NO2 molecules were photolyzed during the signal averaging. The branching ratio for photodissociation of CH3CHO has been studied by Horowitz and Calvert.15J6 The quantum yields for the three channels, CH3CH0
-
CH,
+ HCO
(1)
are pressure dependent. The quantum yields at 300 and 313 nm for P(CH3CHO) = 6.00 Torr and P(N02) = 0.300 Torr are 0.903, 0.010, 0.053 and 0.837, 0.016, 0.046 for (I), (IT), and (III), respectively (assuming the efficiency of collisional quenching of the vibrationally excited precursors by NO2 is the same as that by C02). The interpolated yields at 308 nm are 0.862, 0.014, and 0.049. The density of HCO produced by the photolysis thus can be calculated using Beer's law from the measured 308 nm fluence, the partial pressure of CH3CH0, the absorption cross sections reported above, and the yield of 0.862. It is typically on the order of 1014 ~ m - ~ .
Results and Discussion
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A. Reaction Rate Constant. HCO was monitored through the PQ1( 10) line of the (0,9,0) (O,O,O) vibronic transition at 16 262.25 cm-l.ll With more than 5 Torr of CH3CH0, the time for the relaxation of vibrationally hot HCO radicals after the 308 nm UV laser pulse should be shorter than 0.35 HCO absorption vs time without and with NO2 is shown in Figure 2. The loss of HCO due to diffusion or to reactions other than HCO NO2 is less than 2% during 5 ps, as seen in Figure 2a. The decay of HCO was least-squares fitted with a single exponential. Since the absorption of HCO produced by a single photolysis shot was less than 5 % , the absorption signal was considered to be the same as the absorbance. The resulting pseudo-first-order reaction rates vs NO2 pressure are plotted in Figure 3. The slope resulting from the linear least-squares fit (the solid line in Figure 3) gives (5.7 f 0.3) x lo-" cm3 molecule-' s-l for the rate constant of the HCO NO2 reaction at 296 K. The quoted error is 20. Considering the possible systematic errors resulting from gas pressure and flow rate measurements and the UV photolysis of NO2, the overall estimated error for the reaction rate determination is f15%, which means that the rate constant is (5.7 f 0.9) x lo-" cm3 molecule-' s-l. This measured value of the rate constant is in excellent agreement with the results of Timonen et al.,5 5.6 x IO-" cm3 molecule-' s-'.
+
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Kinetics and Product Branching for HCO
+ NO2
J. Phys. Chem., Vol. 99, No. 19, 1995 7475
Ij
.-E C
e
c
0)
.-
il 2665.90
NO2 Pressure (torr)
Figure 3. Pseudo-first-order reaction rate vs NO2 pressure. The slope of the linear least-squares fit gives the rate constant of 5.7 x lo-'' mol-' s-' for the reaction of HCO NO2 at 296 K.
+
I
0
100
1
200
1
1
I
300 400 5M)
2665.88
2665.86
Wovelength (cm-')
I
1
600
+
Figure 5. HNO monitored for both HCO NO (P(CH3CHO) = 5.49 Torr, P(N0) = 0.271 Torr) and HCO NO2 (P(CHXH0) = 6.43 Torr, P(N02) = 0.271 Torr) reactions. HNO CO is the only product channel for the HCO NO reaction (trace a).17 No HNO signal was observed for the reaction between HCO and NO2 (trace b). The boxcar had a 50 ,us gate and opened 5 ,us after the photolysis pulse.
+
+
I
I
1 0
+
I
I
I
I
I
50
IO0 Time ( p s )
I
I
700 800
Total Pressure (torr)
Figure 4. Reaction rate constant vs SF6 buffer gas pressure. The HCO HCO reaction contributes about 5 % in these measurements.
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B. Pressure Dependence of the Rate Constant. SF6 was used as a buffer gas in an attempt to collisionally stabilize HCO(N02) or HCO(ON0) intermediates. SF6 does not absorb 308 nm UV or 16 265.25 cm-I visible light. CH3CH0, NO2, and SF6 gases were premixed in a large bulb and then transferred through the vacuum line to the photolysis cell. The vacuum line and the photolysis cell were passivated with the gas mixture in order to avoid loss of NO;! from the static gas mixture due to wall absorption. A fresh gas mixture was used for each data trace. The decay rate of HCO was measured at various SF6 pressures at 296 K as shown in Figure 4. Since the HCO resonance absorption decreased due to pressure broadening when SF6 was added, up to 18 Torr of CH3CHO was used in order to increase the HCO absorption signal. The measured rate constant is about 5% larger than reported above, since at this high concentration some of the HCO was destroyed by the recombination reaction of HCO HCO HzCO CO. The observed rate constant is independent of total pressure from 6 Torr up to 700 Torr. The estimated error for each measurement is less than f20%. C. Reaction Products. 1. HNO Detection. The most exothermic product channel possible for the HCO NO;! reaction is HNO -I- CO2 (AH = -87.2 kcal/mo1).l8 The highresolution spectrum of the N-H stretch in HNO has been well studiedlg and provides accurate line positions for monitoring HNO concentration. Observation of HNO as a reaction product was attempted using the IR flash kinetic spectrometer. No HNO absorption was observed when the probe laser was scanned through the frequency region where HNO absorbs. In order to determine the system sensitivity for detecting HNO, the HNO absorption signal for the HCO NO reaction was used as a reference. It is presumed that the only product
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J
I50
200
I
I
_--
+
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4
O
1E-5
3E-5
2E-5
4E-5
5E-5
S
-
Figure 6. (a) CO2 appearance monitored on the R( 18) line in the (1,0,1) (O,O,O)vibrational band at 3728.41 cm-'. The signal was averaged over five photolysis shots with P(CH3CHO) = 6.05 Torr and P(N02) = 0.281 Torr for a UV laser power of 14.6 mJ*cm-2 per pulse. For times longer than 50 ,us, the observed absorption is 8.9%. (b) HONO monitored at 3590.41 cm-'. The signal was averaged over 80 photolysis shots with P(CH3CHO) = 6.20 Torr and P(N02) = 0.285 Torr.
+
+
channel for HCO NO is HNO C0.20 Traces a and b in Figure 5 depict the absorption signals of HNO obtained under exactly the same experimental conditions for the HCO NO and HCO NO;! reactions, respectively. The frequency at the line center is 2665.88 cm-' corresponding to the PQ1(6) line in the (1,0,0) (O,O,O)HNO absorption band. HNO was observed as a product in the reaction of HCO NO but not in that of
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7476 J. Phys. Chem., Vol. 99, No. 19, 1995
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HCO NO;?. A conservative estimate based on the signal-tonoise ratio of the measurements shows that the yield of HNO COZfrom the HCO NO, reaction is less than 10%. There is a possibility that HNO produced from the HCO NO2 reaction was removed by the secondary reaction of HNO NOz. However, the reaction between HNO and NO, is expected to be slower than the HCO NO2 reaction. No HNO was observed in this experiment with the delay of the boxcar gate (width 50 p s ) relative to the photolysis pulse varied between 2 and 20 p s . 2. C02 Detection. As shown in the calculated potential energy diagram (Figure l), COZcan be a product of the reaction between HCO and NO2 via the unstable intermediate HCO,. The barrier preventing the dissociation of HC02 to form H and COZis calculated to be only about 1.5 kcal/moLg HC02 should not be stable under the current experimental conditions. The top trace in Figure 6 shows the appearance of C02 as a reaction product monitored through the R( 18) line of the (l,O,l) (O,O,O) combination band at 3728.41 cm-1.21 The absolute yield of C02 was obtained by calibrating the transient signal against CW absorption of C02. The absorption coefficient of COz was measured in the presence of NO, and CH3CHO to take into account the effect of pressure broadening on the peak absorption intensity. With 0.350 Torr of NO2 and 6.000 Torr of CH3CHO and CO:! pressures to 50 mTorr, the measured C02 absorption coefficient is 0.187 f 0.005 Torr-' cm-'. The absorption coefficient of pure C02 with no pressure broadening was measured to be 0.241 torr-' cm-'. There are reactions in addition to HCO NO2 which can lead to the production of COz. The complete reaction scheme
+
+
+
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+
CH,CO
+ NO, -CO, + CH, + NO
k, = 2.5 x lo-" [25] (4)
+ hv -NO + 0 0 + CH,CHO - OH + CH,CO NO,
k, = 4.8 x lo-', [26] (5)
+ NO, - NO + 0, CH, + NO, - CH,O + NO 0
CH,
HCO
+ NO,-H + CO, + NO
-
H
+ NO,
+ CH,CHO
-
k, = 5.7 x lo-'' (la)
(1b)
+ CO + NO
(Id)
OH
H,
(1)
+ CO
HONO
-.OH
H
+ hv - HCO + CH,
+ NO
k, = 1.3 x lo-'' [22] (2)
+ CH,CO k,. = 9.8 x lo-', [23] (2')
OH
+ CH,CHO - CH,CO + H,O k3 = 1.6 x lo-'' [24] (3)
[27] ( 6 )
+ NO, + M - CH,NO, + M
k,, = 9.9 x lo-,* [29] (7b)
CH,
+ CH,CHO
-
CH,ONO
CH,
+M k7Jk7b = 2.2 [30] (7c)
+ CH,CO k8
CH,O
= 5.6 x
[31] (8)
+ NO, + M - CH30N0, + M
k9 = 3.2 x lo-', OH
321 (9)
+ NO, + M -HNO, + M k,, = 1.0 x lo-,' [
is22-32
CH,CHO
k6 = 9.5 x
k7a = 2.5 x lo-" [28] (7a)
-
+
Guo et al.
The rate constant units are cm3 molecule-' s-l, except k7b, kg, and k10, which are cm6 molecule-, s-'; the references are given in brackets. In reactions 7 and 10 the rate constants are for M = Ar, and in reaction 9 for M = He. According to the mechanism described above, CH3CO and H radicals formed from the photodissociation of CH3CHO and the 0 atom formed from the dissociation of NO, eventually lead to C02 production via reaction 4. Since the concentration ratio of CH3CHO and NO, is greater than l O : l , reaction 2 is about 100 times faster than reaction 2'. Thus, only reaction 2 is considered to be responsible for the removal of H atoms. On the other hand the removal rates of 0 atoms by reactions 5 and 6 are comparable under the experimental conditions. The fraction of 0 atoms removed through reaction 5 was calculated as ks[CH3CHOI/(k5[CH3CHOl k6[N021). When the UV photon flux was 2.26 x 10l6 cm-2 (14.6 mT cm-, per pulse) and CH3CHO and NO2 pressures were 6.049 and 0.281 Torr, the column densities of HCO, H, and 0 radicals formed from UV photolysis were 1.13 x 10l6, 0.064 x 10l6, and 0.32 x 10l6 cm-,, respectively. The column density of CH3CO generated by photolysis was the same as that of H atom (reaction 111). While each H and CH3CO radical produced one C02, each HCO reacted with NO2 via reaction l a followed by reactions 2-4 produced two CO2. The column density of C02 produced due to reaction 5 was 0.31 x 10l6 cm-2. The total column density of C02 measured in the experiment was 1.62 x 10l6 cm-*. After correcting for the C02 production from reactions initiated by CH3C0, H, and 0 radicals (0.44 x 10l6 cm-,), it is concluded that 52% of HCO which reacts with NO, formed COz in the first step. Five sets of experimental data using various CH3CHO and NO2 pressures ([CH3CHO]/[N02] % 17:l) give the result that 52% of the HCO NO2 reaction goes to the H C02 NO product channel. Considering the upper limit of 10% for C02
+
+
+
+
Kinetics and Product Branching for HCO
+ NO2
+
-
production from the HNO -t COz channel, the HCO NO;! H CO;! NO channel is responsible for 42-52% of the reaction. Errors arising from the following measurements contribute uncertainty in the determination of COz yields: (1) gas pressure and flow, (2) UV absorption coefficients of CH3CHO and NOz, (3) laser power normalization, (4) absolute UV energy and W beam cross section, (5) CW absorption coefficient of CO;!, (6) transient COz absorption intensity. The estimated error considering contributions from the uncertainties listed above is less than 15% of the C02 yield obtained. Since the calculation for the final yield of C02 is also directly related to the quantum yields of the photodissociation processes of CH3CHO at 308 nm and rate constants of reactions 5 and 6, the overall uncertainty in determining the COz yield is larger than 15%. The quoted errors of the rate constant measurements for reactions 5 and 6 are ~ t 1 4 % and~ ~ Assuming the error of the quantum yield measurements for the photodissociation of CH3CHO at 308 nm is &IO%, then the total COz yield from the reaction of HCO NO2 is 0.52 f 0.14. 3. CO and HONO Detection. An attempt was made to measure transient CO as a reaction product by monitoring its absorption at 4285.009 cm-', R(6) in the 2-0 overtone band.33 This experiment failed due to the limited detection sensitivity. The absorption strength of CO at 4285.009 cm-', which corresponds to one of the strongest absorption lines of CO in the 2-0 band, is about 17 times weaker than that of C02 at 3728.41 cm-1.21,34 HONO is relatively unstable at room temperature, but it can be generated in a mixture of NO NO;! H20 2HONO with the equilibrium constant K = 0.735 HONO has two isomeric forms, cis and trans. At 296 K, 33.5% of HONO exists in the cis form and 66.5% in the trans form.36The highresolution absorption spectrum of the 0-H stretching jn HONO has been recorded.37 Small portions of the Q-branch absorption spectrum of the 0-H stretching in trans-HONO were measured in this experiment. HONO was produced in the photolysis cell through the equilibrium reaction of 10 torr of NO, 0.5 Torr of NO;!, and 0.3 Torr of H20. Transient HONO following the reaction of HCO NO2 was then measured by averaging 80 photolysis shots with the probe laser tuned to the HONO resonance absorption frequency of 3590.41 cm-' and then subtracting 80 shots with the probe laser tuned off the resonance so as to eliminate the shock wave effect. The resulting signal is shown in the bottom trace of Figure 6. Since the observed signal-to-noise ratio for identical conditions is approximately 5 times less for the HONO product than for CO;! (Figure 6) and since absolute HONO absorption cross sections are at best difficult to measure, no attempt has been made to measure the absolute HONO yield from the reaction Of HCO NO;!.
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J. Phys. Chem., Vol. 99, No. 19, 1995 7477 imaginary vibrational frequency.) The geometries were optimized using the Hartree-Fock method (restricted Hartree-Fock (RHF) for closed shell species and unrestricted Hartree-Fock (UHF)for open shell species. A split-valence basis set with polarization functions on the heavy atoms (6-31G*) was used to describe the electronic wave function. Harmonic vibrational frequencies for each of the structures were then calculated at the same level of theory (RHF or UHF)with the same basis set (6-3 1G*). The vibrational frequencies were scaled downward by 12%. All calculations were carried out using the Gaussian 90 package of codes.39 The resulting vibrational frequencies are used subsequently to determine the zero-point energy of the molecule and other statistical mechanical properties of the molecule. The electronic energy of the molecule is obtained using MPler-Plesset perturbation theory through fourth order, MP4. The MP4 method includes electron correlation involving single, double, triple, and quadruple excitations. The calculation uses a split-valence basis set with polarization functions on all atoms (6-31G**). Empirical bond-additivity corrections (BAC) are used to account for systematic errors in the ab initio calculations, resulting primarily from basis-set truncation. The details of the BAC method are given in refs 9 and 38. The resulting BACMP4 procedure provides excellent thermochemical properties for stable species (see ref 9). For transition state structures, the geometries are expected to be quite reasonable, but the errors in the activation energies can be much larger, depending upon the particular system. In general, for transition state structures, the BAC-MP4 energies tend to be too high and should be shifted downward somewhere between 0 and 5 kcal mol-'. We believe this general case to be applicable for the HCO NO2 system. The reaction rate for HCO with NO2 has been modeled using a recently developed method for fast implementation of microcanonical variational RRKM theory.40 The reaction scheme is summarized as follows:
+
HCO
+ NO,
+
+
Calculations The theoretical determination of the branching ratios and rate constants for the HCO NO2 system requires molecular geometries, vibrational frequencies, and bond dissociation energies for the various possible stable intermediates and transition state structures. The BAC-MP4 (bond-additivity corrections to Moller-Plesset fourth order perturbation theory) m e t h ~ d was ~ . ~used ~ to provide this molecular data. In this method, the geometries are optimized for each stationary point on the potential energy surface (PES) of the reaction system. (Stable structures correspond to minima on the PES, Le., structures which have no imaginary frequencies, while transition state structures correspond to structures which have one
+
-
HCO(ON0)
f
HCO(N0,)
HCO,
+ NO - H +
CO,+NO ( l l a )
+ HNO HONO + CO C02
HONO
+ CO
(1IC)
(12)
Since both of the reactants are open shell species, in neither reaction is any significant barrier to association to be expected. This expectation is bome out by the fact that the measured rate coefficient for loss of the HCO radical, 5.7 x lo-" cm3 s-l, is essentially gas kinetic. Within the context of a statistical model, there is no rigorous way to exactly separate the capture flux into that which forms HCO(ON0) and that which forms HCO(NOz). A simple approach was therefore adopted for estimating the rates of formation of collision complexes: the association rate for reaction 11 was calculated as if the entrance channel for reaction 12 did not exist, and vice versa. The parameters for the RRKM calculations are summarized in the Appendix. Relatively simple model interaction potentials for the entrance channels were deemed appropriate for the present purposes. The reaction coordinate was chosen to be the separation between the atoms which are forming a bond. For the entrance channel of reaction 11, this is the separation between C and 0 atoms. For the entrance channel of reaction 12, this is the separation between C and N atoms. In both cases, the radial dependence of the bonding interaction was assumed to be given by a Morse potential, with a typical /3 value of 1.9. Nonbonded interactions
7478 J. Phys. Chem., Vol. 99, No. 19, 1995
Guo et al.
absence of collisions at 300 K. A further refinement of the capture rate calculation could be carried out by using a more complex model for the anisotropy which would include both of the association arrangement channels (i.e., reactions 11 and 12) at once and give a better estimate of the total rate of loss vanis = V,(1 - cos2 6 , cos2 f 1 2 ) 0 5 6 , 5 nl2,O Io2In of HCO (but not the branching ratio for formation of each complex). The rate of interconversion between HCO(ON0) and = v, n12 I6 , In (13) HCO(N02) should also be considered. However, given the lack where 61 is the angle of rotation of an axis embedded in the of quantitative knowledge of the potential surface in the entrance HCO fragment away from a reference value defined by its channels for formation of the two complexes and the transition orientation with respect to the axis of the bond being formed state for their interconversion, such further refinements were or broken in the equilibrium structure of the combined unimoconsidered inappropriate and so the original potential parameters lecular species. The embedded axis is the line joining the center were retained. of mass of the HCO fragment and the carbon atom. The Results concerning the predicted fate of the capture flux after reference angle is therefore the angle between this embedded collision complex formation are now summarized. In general, axis and the bond being formed or broken, measured at the an excited collision complex may rearrange or dissociate via equilibrium geometry of the unimolecular species. Ijf2 is likewise energetically allowed product arrangement channels, revert to defined for the ON0 fragment. For the entrance channel of reactants, or be collisionally stabilized to form the stable adduct. reaction 11, the axis embedded in ON0 is the line joining the With the potential surface parameters summarized in the bonding 0 atom and the center of mass of the fragment. For appendix, the lifetime of the HCO(ON0) collision complex at the entrance channel of reaction 12, the embedded axis in ON0 300 K is calculated to be 4.7 x s, and that of the HCOis the line joining the N atom and the center of mass of the (NOz) to be 3.8 x lo-" s. These lifetimes are calculated by fragment. The barrier to rotation VOis given by the absolute magnitude of the Morse interaction at the given s e p a r a t i ~ n ? ~ . ~ averaging the energy- and angular-momentum-resolvedlifetimes over the steady-state collision complex population distribution The anisotropy in eq 13 is symmetric with respect to rotation (see, e.g., ref 45). The calculated lifetimes of both collision of the ON0 fragment, since this fragment has the potential to complexes are much less than lo-', s, which puts the reactions form the complex with either oxygen. For reaction 12, firmly in the collisionless regime at the experimental pressures. This is in accord with the experimental observation of a lack = V,(1 - cos2 6, cos2 02) 0 I6,, o2I n12 of pressure dependence with SF6 as a buffer gas in the range = nl2 I O , In o r n l 2 5 6 , In 6-700 Torr. Branching ratios describing the probabilistic fate of the collision complex are calculated in a similar manner (e.g., (14) refs 45-47). The predicted branching ratios for dissociation The high-pressure-limiting recombination or capture rate of the HCO(ON0) collision complex are 0.18 for the C02 coefficients for reactions 11 and 12 were calculated using HNO channel (reaction llb), 0.82 for the H CO2 NO microcanonical variational transition state theory with angular channel (reaction lla), 0.00 for the HONO CO channel momentum resolution [cl VTST(E,J)] by variationally determin(reaction 1IC), and 0.00 for reversion to the reactants HCO ing the minimum sum of states WAE) over an energy and NO2. The predicted branching ratios for the dissociation of the angular momentum grid and subsequently thermally averaging HCO(N02) collision complex are 0.76 for HONO CO to obtain the capture rate coefficient, e.g.,"3 (reaction 12) and 0.24 for reversion to the reactants. These branching ratios are calculated at 300 K and pertain to the lowdensity (i.e., collisionless) limit of the reactions. The temperature dependence of the capture rates and branching ratios is an important factor in extrapolating the data to In eq 15, g, is the electronic degeneracy factor which is under higher temperatures relevant in combustion processes. In Table the usual assumption that collision complex formation occurs 1A and B the capture rate coefficients and branching ratios for only on the singlet electronic surface, Qr(Q is the partition the HCO(ON0) and HCO(N02) complexes are presented for a function for the reactants (center of mass motion excluded), a d range of temperatures between 200 and 2000 K. The capture k~ is Boltzmann's constant. The grid of values for the separation rate coefficients in both cases display only a very weak R between the bonding atoms (C and 0 for reaction 11 and C temperature dependence. The branching ratios for the HCOand N for reaction 12) used in the variational procedure included (ONO) complex (Table 1A) are essentially independent of 20 values ranging from 2.4 to 9.0 A. The capture rate temperature in the range 200-800 K. The branching ratio for coefficients so calculated for T = 300 K are, for reaction 11, formation of HONO CO products from the HCO(N02) 5.4 x lo-" cm3 s-l and, for reaction 12, 5.1 x lo-" cm3 s-l. collision complex is, on the other hand, sensitive to temperature. There is no simple way to combine these two rate coefficients The percentage of HCO(N02) complexes formed which disto obtain the total rate coefficient for loss of HCO radical. In sociate to HONO CO decreases from 87% at 200 K to just particular, one may not simply add the two, since the calculation 6% at 2000 K. This can be readily understood in terms of the of each capture rate coefficient includes a certain amount of sum-of-states profiles for the tight transition state in the product capture flux which rightly belongs to the other. The measured channel and the loose transition state in the reactant channel. rate coefficient for loss of HCO radical, 5.7 x lo-" cm3 s-l, Since the calculations indicate that HONO is formed from HCO is similar to the predicted capture rate coefficients. Some and NO2 reactants only via the HCO(N02) complex, these caution must be exercised in making a direct comparison of results suggest that the HONO product channel is of only minor the measured rate coefficient for loss of HCO with the capture importance under combustion conditions, whereas it is important rate coefficients, however, since the calculations indicate that about 25% of HCO(N02) redissociates to form reactants in the at room temperatures and below. were treated as Lennard-Jones pairwise interactions, with standard parameters?l The anisotropy of the bonding potential was modeled in a crude fashion by introducing a simple coupled cos2 6 ~ o t e n t i a l ? ~For . ~ ~reaction 11,
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Kinetics and Product Branching for HCO
+ NO2
J. Phys. Chem., Vol. 99, No. 19, 1995 7479
TABLE 1: Temperature-Dependent Capture Rate Coefficients and Branching Ratios for Reaction of HCO with NO*, Calculated by RRKM Theory with ab Initio Parameters and Methodology Described in the Text A. Via the HCO(ON0) Collision Complex
-
branching ratios
TIK 200 250 300 350 400 600 800 1000 1200 1400 1600 1800 2000
kcad IO-" cm3 s-1 4.1 4.9 5.4 5.8
6.0 6.1 5.9 5.7 5.5 5.3 5.1 4.9 4.7
COz+ HNO 0.18 0.18 0.18 0.18 0.17 0.16 0.15 0.14 0.14 0.13 0.12 0.12 0.11
HONO+
H+C02+ NO 0.82 0.82 0.82 0.82 0.83 0.84 0.85 0.86 0.86 0.87 0.87 0.87 0.87
HCO+ NOz 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.02
co
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
B. Via the HCO(N02) Collision Complex branching ratios
T/K
kc,dlO-ll cm3 s-]
200 250 300 350 400 600 800 1000 1200 1400 1600 1800 2000
4.7
HONO
+ CO
HCO
0.87 0.82 0.76 0.71 0.65 0.45 0.30 0.20 0.14 0.11 0.09 0.07 0.06
5.0 5.1 5.3 5.4 5.8 6.0 6.1 6.0 5.9 5.6 5.2 4.9
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Timonen et al.5 have shown that the reaction rate constant for the HCO NO2 reaction decreases with increasing temperature. The negative apparent activation energy indicates reaction via a collision complex with no barrier in the entrance channel. However, the pressure independence of the rate constant up to 700 Torr suggests a lifetime much less than 1 ns. Complex formation rather than direct hydrogen abstraction has been established as the mechanism of the reaction between HCO and NO.2o As indicated in Figure 1, the H C02 NO product channel is accessible only via the HCO(ON0) complex. On the other hand, HONO can be formed via both HCO(ON0) and HCO(N02) complexes. The barriers shown in Figure 1 are calculated assuming a five-center transition state (-6.6 kcaV mol) from HCO(ON0) to HONO CO and a four-center transition state (-1.3 kcdmol) from HCO(N02) to HONO CO. If HCO(ON0) is formed, one expects that it will dissociate into H CO2 NO rather than HONO CO, since the former dissociation process has a much higher A factor. This expectation is verified quantitatively by the RRKM calculations, which indicate that a negligible fraction of HCO(ON0) collision complexes will form HONO CO products. One therefore concludes that only reaction via the HCO(N02) collision complex leads to HONO formation and that it is the ratio of the amount of HCO(N02) to HCO(ON0) initially formed that determines the product branching ratios. The comparable yields of CO2 and HONO are in line with the theoretical estimate of comparable rates for formation of HCO(ON0) and HCO(N02). The exothermicities for the H CO2 NO and the HONO CO channels are 37.3 and 62.4 kcal mol-', respectively.'8
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0.70 0.80 0.86 0.89 0.91 0.93 0.94
Discussion
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0.55
+
+
+
+ NO2
0.13 0.18 0.24 0.29 0.35
+
These excess energies must be partitioned among the vibrational, rotational, and translational degrees of freedom of the products in each channel. It was noticed that the rise of the C02(0,0,0) state is much slower than the reaction. Some vibrationally excited CO2 has been observed by monitoring the R( 16) line in the ( l , l , l ) (0,1,0) hot band at 3735.63 cm-'. C02 IR emission was also observed. However, it is possible that some of these excited CO2 molecules are from the reaction between CH3CO and N02.25 The rise of the ground state HONO is not single-exponential, indicating that the initial HONO produced in the reaction is at least partly vibrationally excited. The dissociation energy of HONO to form OH NO is 49.9 kcal/mol.'* There is a 12.5 kcaYmol exothermicity for the OH NO CO product channel, which corresponds to 20% of the total amount of energy released when HONO and CO are formed. However, the amount of energy distributed among the CO vibrational and rotational degrees of freedom and the translational degree of freedom is expected to be larger than 12.5 kcal/mol, preventing significant dissociation of HONO to form OH NO. This is supported by the direct observation of HONO as a reaction product. Moreover, the rise of the ground-state HONO is comparable to the reaction rate, indicating that HONO produced by the HCO NO2 reaction is not highly vibrationally excited. Thus, it is unlikely that the internal energy of HONO is often sufficient for dissociation to OH NO. This eliminates the sequential mechanism of OH NO CO formation at room temperature. No transition state for simultaneous production of the three fragments has been found or suggested. If some OH NO CO were formed, this in turn would produce COz. The reported H C02 NO yield of 0.52 would have to be decreased by 0.5&)H+NO+CO. The OH NO CO product channel may be more significant at combustion temperatures. At room temperature it is clear that HONO CO is the product channel which does not yield CO2 and that its yield is 0.48 f 0.14. Several groups have studied the thermal reaction of H2CO and NO2 in the temperature range 391-476 K.7,48-50 Since the primary products of this reaction were proposed to be HCO and HON0,51the reaction between HCO and NO2 was indirectly investigated as a secondary reaction in some of those studies.7,50,52Most recently M. C. Lin and c o - ~ o r k e r smodeled ~~ the concentration-time profiles of HzCO, CO, C02, and NO, recorded using FTIR, and derived temperature independent rate constants for forming HONO CO and H CO2 NO products from the HCO NO2 reaction of 10-10,55*0.25 and 10-10,83*0.15 cm3 molecule-' s-', respectively. Their result in terms of C02 quantum yield, 34 k 20%, is significantly smaller than the 52% & 14 reported for 296 K in this work. The difference is even larger if the temperature dependence predicted theoretically is accounted for. It has been reported by Calvert et al. that at least some of the HCO produced by the reaction of H2CO OH is vibrationally excited and dissociates spontaneously to form H C0.53 This reaction (HCO H CO) was not considered in the reaction mechanism used by Lin and co-workers in their kinetic modeling, and it may change their rate constants for HONO CO and H C02 NO. Their shock tube data are consistent with the decrease in yield of HONO CO predicted theoretically.8 For modeling purposes, we suggest that the Timonen et aL5 data for the total rate for HCO NO2 be used in the range 294-713 K. Their room temperature rate is confirmed here, and their temperature dependence is in good accord with the theoretical results of Table 1. The room temperature branching ratio reported here, 52 & 14% l a and 48% lb, should be more reliable than that of He et al.,' 34% and 66%, because of the much less direct method
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7480 J. Phys. Chem., Vol. 99, No. 19, 1995
used by the latter. The experimental results may tentatively be extrapolated using the temperature dependences indicated in Table 1.
Summary For the first time the products formed in the reaction of HCO
+ NO2 have been directly detected as the reaction proceeds. The COz yield is 0.52 & 0.14, of which less than 0.1 corresponds to the COz + HNO product channel and the remainder is H + COz + NO. The remaining yield of 0.48 f 0.14 arises from the HONO + CO channel, which has been observed in this work, although some other channel(s), for example, OH + NO + CO or HNOz + CO, might contribute. The stability of HNO2 is not known, although there is indirect evidence of it being formed.54 The product formation can be explained by assuming that the reaction proceeds via formation of metastable collision complexes, the existence of which are verified by ab initio calculations. RRKM calculations based on ab initio parameters for the collision complexes and transition states provide good quantitative accord with the experimental observations. The rate coefficient of the HCO NO2 reaction at 296 K has been determined to be (5.7 iz 0.9) x lo-" cm3 molecule-' s-l. Both RRKM calculations and the observed yields suggest that the HCO(ON0) and HCO(N0z) complexes are formed with comparable probability. HCO(ON0) dissociates predominantly to H COz NO with some HNO COz possible. HCO(N02) dissociates predominantly to HONO CO at room temperature and back to HCO NO2 at high temperatures.
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+
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Acknowledgment. This research was supported by the Director, Office of Energy Research, Office of Basic Energy Science, Chemical Sciences Division of the U.S.Department of Energy, under Contract No. DE-AC03-76SF00098. We thank Dr. A. G. Maki and Dr. T. R. Todd for providing their unpublished 0-H stretching absorption spectrum of HONO. Appendix. Parameters for RRKM Calculations
this rearrangement channel by a microcanonical variational calculation for the simple fission dissociation of HCO(ON0) into HC02 and NO fragments. A Morse-type potential is assumed for the bonding interaction between the two fragments @ = 1.9), with anisotropy of the type indicated in eq 14 and standard Lennard-Jones potentials for nonbonded interactions between the fragments in the transition state. Exploratory calculations revealed that, at the high excess energies involved in this dissociation (the threshold for the entrance channel being 23 kcal higher than the barrier height for this exit channel), the transition state lay typically at a separation between the separating N and 0 atoms of 2.7 8,. In the final calculations, five transition states at separations in the range 2.3-3.1 8, (0.2 8, intervals) were included. Vibrational and rotational parameters for the HCOz and NO fragments follow. HC02. Frequencies: 507 (l), 821 (l), 1058 (l), 1218 (l), 1689 (l), 2010 (1). Rotational constants: 5.77, 0.401, 0.374. Symmetry number: 1. NO. Frequency? 1877 (1). Rotational constant: 1.74. 4. HCO(ON0) COz HNO Transition State. Ab initio parameters at the saddle point for this channel are as follows. Frequencies: 145 (l), 235 (l), 361 (l), 476 (l), 597 (l), 831 (l), 1030 (l), 1371 (l), 1504 (l), 1884 (l), 2194 (1). Rotational constants: 0.574, 0.096, 0.087. Symmetry number: 1. 5. HCO(ON0) HONO CO Transition State. Ab initio parameters at the saddle point for this channel are as follows. Frequencies: 101 (l), 208 (l), 288 (l), 409 (l), 721 (l), 859 (l), 1159 (l), 1363 (l), 1427 (l), 1817 (l), 2309 (1). Rotational constants: 0.536, 0.1 15, 0.095. Symmetry number: 1. 6. HCO(N02) Collision Complex. Reaction thresholds: reactant channel, 48.47 kcal; HONO CO channel (reaction 12), 30.06 kcal. Frequencies:' 310 (l), 514 (l), 598 (l), 755 (l), 926 (l), 1007 (l), 1287 (l), 1480 (l), 1720 (l), 1892 (l), 2985 (1). Rotational constants: external, 0.423, 0.164, 0.1 18, symmetry number 1; intemal (torsional),2.28, symmetry number 2. 7. HCO(N0z) HONO CO Transition State. Ab initio parameters at the saddle point for this channel are as follows. Frequencies: 207 (l), 254 (l), 374 (l), 475 (l), 633 (l), 845 (l), 1066 (l), 1346 (l), 1701 (l), 1905 (l), 2318 (1). Rotational constants: 0.443, 0.108, 0.087. Symmetry number: 1.
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The parameters for the reactants, collision complexes, and transition states in reactions 11 and 12 are summarized below. The numbers in parentheses following the frequencies are degeneracies. All frequencies and rotational constants are given in cm-'. Unless otherwise specified, rotational constants are calculated using standard bond angles and bond lengths. Note that details concerning the variational transition states for formation of the HCO(ON0) and HCO(N02) collision complexes are provided in the text. 1. Reactants. HCO. Frequencies: 1117 (l), 1914 (l), 2605 (1). Rotational constants: 26.10, 1.531, 1.446. Symmetry number: 1. NO2. Frequencies? 750 (l), 1320 (l), 1617 (1). Rotational constants: 9.13, 0.452, 0.431. Symmetry number: 2. 2. HCO(ON0) Collision Complex. Reaction thresholds: reactant channel, 58.9 kcal; H COZ NO channel (reaction lla), 34.37 kcal; COz HNO channel (reaction llb), 13.17 kcal; HONO CO channel (reaction llc), 35.26 kcal. Frequencies: 244 (l),565 (l), 688 (l),935 (l), 1047 (l), 1126 (l), 1380 (l), 1826 (l), 1856 (l), 2962 (1). Rotational constants: extemal, 0.80,0.116,0.101; internal (torsional), 5.5, 2.36. Symmetry number: 1. 3. HCO(ON0) H COz. NO Transition State. The dissociation of HCO(ON0) to produce H C02 NO proceeds in two stages, as indicated in eq 1la. As noted in the text, HC02 has only a very small barrier for dissociation to H and C02, and so this second stage is not expected to be rate determining. We, therefore, determine the transition state for
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References and Notes (1) Momson, B. M., Jr.; Heicklen, J. J . Photochem. 1981, 15, 131. (2) Topchiev, A. V.; Ballod, A. P.; Fedorova, T. V.; Shtem, V. Y. Pet. Chem. USSR 1962, 2, 150. (3) Barton, D. J . Phys. Chem. 1961, 65, 1831. (4) Fifer, R. A. 17th International Symposium on Combustion; The Combustion Institute: Pittsburg, PA, 1979; p 587. ( 5 ) Timonen, R. S.; Ratajczak, E.; Gutman, D. J . Phys. Chem. 1988, 92, 651. (6) Momson, B. M., Jr.; Heicklen, J. J . Photochem. 1980, 13, 189. (7) He, Y.; Kolby, E.; Shumaker, P.; Lin, M. C. Int. J . Chem. Kinet. 1989, 21, 1015. (8) Lin, C.-Y.; Wang, H. T.; Lin, M. C.; Melius, C. F. Int. J . Chem. Kinet. 1990, 22, 455. (9) Melius, C. F. Chemistry and Physics of Energetic Materials; Bulusu, S.N., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1990 p 21. (10) Hamann, H. C.; Swem, D. Terrahedron Lett. 1966, 28, 3303. (1 1) Langford, A. 0.;Moore, C. B. J . Chem. Phys. 1984, 80, 4204. (12) Petek, H.; Nesbitt, D. J.; Ogilby, P. R.; Moore, C. B. J. Phys. Chem. 1983, 87, 5367. (13) Dane, C. B.; Lander, D. R.; Curl, R. F.; Tittel, F. K.; Guo, Y.; Ochsner, M. I. F.; Moore, C. B. J . Chem. Phys. 1988, 88, 2121. (14) Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling; JF'L Publication 87-41, September 15, 1987, p 105. (15) Horowitz, A.; Kershner, A. C. J.; Calvert, J. G. J . Phys. Chem. 1982, 86, 3094. (16) Horowitz, A.; Calvert, J. G. J . Phys. Chem. 1982, 86, 3105.
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