J. Phys. Chem. 1993, 97, 10028-10034
10028
The F
+ HNCO Reaction System:
A Flow Reactor Source for NCO(W211) and NF(X3E-)
S. Wategaonkart and D. W, Setser' Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 Received: February 18, 1993; In Final Form: June 8, 1993'
The reaction of H N C O with F atoms has been investigated as a source for NCO(R211) and NF(X3Z-) radicals in a room temperature flow reactor by monitoring the N C O concentration using laser-induced fluorescence. The first step gives HF N C O with a rate constant of (3.4 f 0.7) X lo-" cm3 s-l; the second step gives NF(X3Z-) COwitharateconstantof (9.2f 1.8) X 10-12cm3s-'. Theoverallreactionseemstobestoichiometric for NF(X3Z-) formation and for properly adjusted reaction conditions the F / H N C O system can be used to provide a selected [NCO] or [NF(X)]. The bimolecular, self-removal rate constant for N C O was measured as (5.0 f 2.0) X cm3s-1. The dynamics of the F H N C O and N C O reactions are discussed and compared to the analogous F HN3 and N3 reactions. The utility of the F H N C O reaction system for study of N C O chemistry in a flow reactor was demonstrated by measuring the N C O removal rate constants by C2H2, CzH4, C3H6, C4H6, NO, NO2, HzS, and HI. The reaction of 0 2 with N C O was confirmed to be very slow at 300
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K. Introduction The feaction of F atoms with HNCO has been used to generate NCO(X2II) radicals' and the reaction of F with NCO has been suggested as a source for ground-stateN F radicals.2 In the present study the reaction of excess F atoms with HNCO has been investigated as a clean source for NF(X32) radicals in a flow reactor. The first step, abstraction of a H atom, and the second step, the reaction of F with NCO, have been characterized by using laser-induced fluorescence (LIF) to monitor the NCO formation and removal kinetics.
F
F
+ HNCO
+ NCO(R)
-
-
HF(UI2) + NCO(R%)
(la)
other products
(1b)
NF(X32-)
+ CO
(2)
Since NF(a'A) can be formed directly by several chemical reactions,3 the NF(b'Z+, alA, X3Z-) system is of interest for possible energy storage applications. The NF(a and b) states have been extensively studied,qd but less is known about the chemistry of NF(X'Z-) radicals because of the lack of a suitable source for their generation and the difficulty of monitoring their concentration. Heidner and co-workers'have demonstrated that the UV photolysis of NF2 can be used to generate NF(X) in a static cell, and they have begun to characterize some of the reactions. W,e will show that reactions (1) and (2) can provide either NCO(X) or NF(X) in suitable concentrations for kinetic studies in a flow reactor by selecting the [F]o/[HNCO]o ratio. Based on the inability to observe NF(alA) formation from F + NC02, the AHP(NF) was assigned 1 4 9 kcal mol-I (or 44 kcal mol-' from the more recent AHP(NC0)). In fact, the dissociation of FNCO to NF(a) CO probably has a activation barrier5 that is in excess of the thermochemical limit, and the limit to AHP(NF) from this experiment is not very useful. Recent ab initio calculations favorsa AZffoo(NF)= 55 kcal mol-', which can be combined with AHfo0(NCO)*f= 31.4 and AHfoo(HNC0)8f= -27.5 kcal mol-' to obtain A H 0 0 = -24.6 and -22 kcal mol-' for (la) and (2), respectively. The formation of NF(a) from F + NCO requires, at least, 10kcal mol-' more energy and this channel
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Present address: Tata Instituteof Fundamental Research,ColabaBombay 4oooO5, India.
Abstract published in Advance ACS Absrracts, September 1, 1993.
0022-3654/93/2097- 10028$04.00/0
is firmly precluded at 300 K. Recent interpretationsa of the predissociation of NCO(&Il) give AHfOo(NCO)= 30.5 f 1 kcal mol-' and very extensive ab initio calculationssegf report AHfoO(HNCO)= -27.5, AZffoo(NCO)= 3 1.4, and Do(H-NCO) = 1 10.5 kcal mol-'. We have adopted this set of thermochemical values. Since HF(u = 2) is formed from (l), the AHo0 must be more negative than 22 kcal mol-', which is in accord with this thermochemistry. Although our main interest was in developing a flow reactor source for NF(X), we studied the reactions of NCO with some unsaturated molecules (C2H2, C2H4, C3H6, C4H6),with the open-shell NO and NO2 molecules, and with HI and H2S to demonstrate the utility of reaction l a as a flow reactor source for NCO. We also characterized the bimolecular, selfdestruction rate constant of NCO in order to determine the concentration limit where second-order loss of [NCO] must be considered. The rateconstant of reaction l a has been determined previously by comparing the HF(u = 1,2) infrared chemiluminescence9*to that from the F CH4 reaction. The P I / Pratio9* ~ was reported as 0.67:0.33, which was confirmed in a more recent independent experiment? In order to obtain the total HF(u = 0,1,2) formation rateconstant, therelativeHF(u = 0) populationmust beestimated. If the POcontribution is assigned by extrapolation, POwould be -2.5 times larger than for PI, and the rate constant for reaction l a is -4.5 X 10-l' cm3 s-I. If reaction l a proceeds by a direct abstraction mechanism, PO is expected to be less than PIgband the rate constant would be less than 4.5 X 10-I' cm3 s-I. Sloan and co-workersI0proposed another channel in addition to (la), because they observed HF(u 1 3) levels in their cold-wall reactor and (la) can give only HF(u I 2). They suggested that HF(u L 3) was formed from the secondary reaction of F atoms with H N F radical formed in (1b). As already noted, two independent studies of the HF infrared emission from a flow reactor in our l a b o r a t ~ r y *gave ~ ~ no HF(u = 3) emission. The AHP(HNF) recently has been set'' at 35.6 kcal mol-' and this thermochemistry makes H N F + CO formation from F HNCO about 14 kcal mol-' endoergic. Based upon monitoring the [NCO] vs time, the present work supports a total rate constant for reaction 1 that is comparable to the rate constant reported for the formation of HF, and (1b) must be a minor competing process. Distinguishing whether (la) proceeds by direct abstraction or by additionelimination remains as a challenge. The rate constant for reaction 2 was estimated2 as >(5 f 2) X 10-12 cm3 s-l from observation of the relative [NCO] using the NCO(A X) emission excited
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0 1993 American Chemical Society
F
+ HNCO Reaction System
The Journal of Physical Chemistry, Vol. 97, No. 39, 1993 10029
by theN2(A3Z,+)excitation-transferreaction.2 The present study using direct monitoring of [NCO] for various [F]o and [HNCOIo gave a 2-fold larger value for k2. The NCO radical is an important intermediate in the combustion of nitrogen-containing fuels and it plays a role in proposed schemes to reduce NO, emission from combustion products.12 Most measurements of NCO rate constants were based upon indirect monitoring of the [NCO], until PerryI3studied the reactions with H2 and NO using LIF to monitor the [NCO]. The 300 K rate constant with H2 is very small, 4.4 X lO-I* cm3 s-l, but the reaction with NO has a rate constant of 3.3 X 1 O - I l cm3 s-l. Black and co-workers12b reported a few reactions of NCO in their study of the chemiluminescent from the reaction of CN with 0 2 . Hancock and McKendrickI4 studied the vibrational relaxation of NCO(3) by rare gases, as well as the reaction with NO. Atakan and Wolfrum15 measured the temperature dependence of the 0 2 and N O reactions; NCO was generated from the 0 2 CN reaction and monitored by LIF. There are long-standing questions about the presence of N2(A3Z,,+) in reaction systems containing NC0.2J2b For example, Du and Setser2 inferred thc ptesence of N2(A) in the F HNCO system from the NCO(A-X) chemiluminescnece. Black and coworkers12b suggested that the N(2D) NCO reaction could explain N2(A) formation in the CN 0 2 system. Du and Setser showed that N(2D) was not present in the F/HNCO reaction system at room temperature. FormationofN2(A) via bimolecular self-destruction of NCO is endoergic by 25 kcal mol-’, based on AHP(NC0) = 31.4 kcal mol-’. Unless NCO retains 150% of the available energy from reaction 1, this pathway for Nz(A) formation is not viable. Another source of N2(A) could be the reaction betweenN(4S) atomsandNCO(AP= -171 kcalmol-I) from the “minor”concentrationof N atoms that are nearly always present in discharge-flow reactors. During- the course of the present work, we observed that the NCO(A-X) emission, as well as other chemiluminescence, was very sensitive to the presence of small (impurity) concentrations of reactive atoms. A few experimentswere done in which N atoms were deliberately added to the flow_reacto_rdownstream from the NCO generator, and the NCO(A X) emission was enhanced. The N + NCO reaction may- be indirectly responsible for the excitation of the NCO(A X) emission via the generation of N2(A).
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Experimental Methods Experiments were done in a 5-cm-diameter, 80-cm-long, halocarbon wax coated Pyrex glass reactor, pumped by a blower backed by a mechanical pump; the maximum flow speed was 110 m s-l. Due to the modest rates of (1) and (2), most of the experiments were done under throttled conditions with a flow velocity of 15 m s-1, The entrance of the reactor was fitted with a microwave discharge tube for producing F atoms from CF4/Ar mixtures. The reactor was fitted with laser baffle arms and quartz windows for the LIF measurements. A movable inlet permitted the addition of HNCO at various positions in front of the LIF observation region. The reaction time between F and HNCO was varied by adjusting the distance between the inlet and the LIF zone. Another reagent inlet located immediately before the observation zone was used either to add the C2H6 (5% mixture in Ar) as the monitor for [F] in the F atom titration38~5aor to add HNCO for experiments to measure the NCO vibrational distribution from (1) at short reaction time. The large flow rates were measured with floating ball type flow meters that were calibrated with a wet test meter. The small flow rates were measured by the rate of pressure rise in a calibrated volume. The absolute [F] was determined by titration with CF31 for each e~periment.’~The HF(3 0) emission at 890 nm from the F + CzHs reaction was monitored with a 0.3-m monochromator and RCA C-3 1034 photomultiplier tube. The end point of the titration was determined by extrapolating the I(HF) vs [CF3I]
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plot to zero H F intensity. Alternatively, the [F] was determined by fitting the observed I(HF) vs [CF3I] profile by kinetic simulation, using the known rate constant for F+ CF3I. Good agreement was found between both methods. The fractional dissociation of CF4 depends upon the total flow speed,the CF4/ Ar ratio, and the absolute [CFd]; a constant fractional dissociation of CFd cannot be assumed. The HNCO was prepared by using the same method as in ref 2, Le., the reaction of KOCN with stearic acid. After collection at 77 K, the products were purified by dynamic pumping through a 175 K (methanol/liquid N2 slush bath) trap until the IR absorption peak due to COZ(2350 cm-I) disappeared and only the HNCO band (2280 cm-I) remained. The HNCO was stored as a 5% mixture in Ar in a 10-L Pyrex reservoir. The CF4, C2H6r C2H4r C2H2, C3H6, C4H6, NO, N02, HI, and H2S gases were purchased from Matheson and were used after purification by distillation. The CF4 and Ar gases were used after passing the flows through commercial traps that adsorb hydrocarbons and 0 2 . The Ar carrier gas was further purified by passing the gas flow through several molecular sieve traps cooled to 77 K to remove H20 and C02. Extensive purification of the Ar and CF4 was required in order to reduce the background chemiluminescence. The NCO(AW-%II) transition is ideal for LIF because frequency doubling is not required, the radiative lifetime is 360 f 10 ns and predissociation is not important. We used a Lambda Physik (FL3002) dye laser (Coumarin 440) pumped by the third harmonic of a Nd3+:YAG(Quantel International,Model YG66 ls30) laser for these experiments. The dye laser was used with the grating in the seventh order, which gave a !and pass of 0.2 cm-I. The dye laser was tuned to the A(000) X(2113/2,000)band at 438.5 nm and the resulting fluorescence was observed with Hamamatsu R212 photomultiplier tube and two cut-off filters (Schott CG-475 Corning 5-61), which passed the 460-510nm region. The 000 100 and 200 plus the weaker 000-001 transitionswere observed. The waveformswere recorded by using a digitizer (LeCroy) interfaced to a laboratory computer. The time-integrated fluoJescence intensity was used as a measure of the relative NCO(X) concentration. The laser intensity was sufficiently low that the fluorescence signal was linearly proportional to the laser pulse energy. For a given series of kinetic measurements, the wavelength of the dye-laser was initially set to obtain maximum LIF intensity and then not changed. The rate constants for reactions 1 and 2 were determined by monitoring the relative [NCO] as a function of reaction time for various initial [F]o and [HNCOIo. The differential rate law for [NCO] can bewrittenas (3),providing thattherearenocompeting reactions.
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d[NCO]/dt = k,,[F] [HNCO] - k,[F] [NCO]
(3)
The relative [NCO] was measured as a function of reaction time for which the integrated rate law must be used. The NCO formation and removal steps were modeled by using numerical integration of the rate law; calculated profiles were then compared ot the experimental [NCO] profiles. In practice klb was ignored, so the measurements actually are for k l ,and hereafter the notation will be just kl. After values for kl and k2 were established from experiments with low [HNCOIo, a few experiments were done under conditions such that [HNCOIo >> [F]o to assign the importance of the bimolecular, self-destruction reaction. 2NC0
-
N2
+ 2CO
(4)
The reactivity of NCO with several unsaturated hydrocarbons was studied by extending the reactor by 50 cm to provide a prereactor section to generate NCO from reaction 1 with [F]o < [HNCO],; the remainder of the reactor was used to study the decay of NCO in the presence of the added reagent. The distance between the HNCO inlet and the reagent inlet was 75 cm, which corresponded to a reaction time between F and HNCO of 47 ms.
Wategaonkar and Setser
10030 The Journal of Physical Chemistry, Vol. 97, No. 39, 1993
TABLE I: Summary of the Measurements. for kl and 4 [F],, loL3 [HNCO],, 10” kz, 10-l2 kl,b 1 V 1 no. molecule cm-1 molecule cm-1 cm3 s-l cm3 s-1
N C O ( X 1 excitation spectrum
1.o 1.2 1.1
0.10
0.90
0.14
0.90
426
430
434
438
Lt42
Wavelength9 n m
Figure 1. NCO(A g)excitation spectrum (uncorrected for the laser power) taken at 0.6 Torr of Ar for a reaction time of 0.5 ms with [F]o and [HNCOIo = 4 X 10l2and 6.4 X 10” ~ m -respectively. ~, The curve showsthe variationof the laser power with wavelength. The strongbands at 440 and 438 nm are the Au = 0 transitions from zII1p(OOO) and 2II3/2(000). Thebandsinthe434-436-nmregionarethe Au = 0 transitions z The transitions from the from the 010 levels of the zE+and 2 A ~ / states. 2A3/2 and 2E-states are overlapped with the ZII3/2 band. The two bands between 430 and 434 nm are the sum of the Au = 0 transition from 001 and 020 levels; the latter is probably the more important. The two bands in the 426428-nm region are the 010 OOO transition. +
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The reagents were added 14-cm upstream (-9 ms) of the LIF monitoring window. For a fixed [F]o and [HNCOIo, the reagent concentration was varied and the relative [NCO] was determined by LIF. Since [Q]o>> [NCOIo, the kinetics follow the pseudofirst-order rate law SinceI(NC0) is proportional to [NCO] for constant laser power and fixed detection geometry, the log of the LIF intensity was plotted against the reagent concentrations for fixed reaction time to determine the rate constant for each reagent. Although the LIF probed the NCO(X,2113/~) concentration, the populations of the two spin-orbit states are in equilibrium, and the k~ values pertain to the total NCO concentration.
ReSults A. The Excitation Spectrum of NCO(X). The HNCO was added immediately in front of the LIF observation window with the optimum pumping speed, which corresponded to a reaction time of 0.5 ms with [F]o and [HNCOIo equal to 4 X 1Ol2and 6.4 X 1011 molecule cm-3, respectively. The waveforms from NCO(A X) fluorescence were single exponential with a decay time of 360 ns. The excitation spectrum of NCO from 425 to 442 nm taken at 0.6 Torr is shown in Figure 1. The strongest features of the s p t r u m are the two spin-orbit bands of the NCO(~2~+,000+X~~1~2,3~z,000) transition. The ratio of the areas of these two bands was 0.64, indicating that the population in the X(zII3/2) and X(ZII1/2) levels was 300K boltzmann. The Au = 0 transition fromthe (010) statesofNCO(2) wereobserved in the 435-436-nm region. The weak bands between 430 and 434 nm could be either (or both) the 020 020 or 001 001 transition.16 Since the (020) level is lower in energy, it probably makes the major contribution. The only other features that we observed were the 010 000 transitions a t 426-428 nm. The 020 100 band at 436.4 nml& was not observed, nor was the 100- 100band observed; the latter seems to be rarely recorded.16 The filtep pass the same emission bands from the (A,OlO) level as the (A.000) level, namely, 010 110, 210, and 01 1. We assumed that the Franck-Condon factors for the three observed band systems were the same as for (A,000) to estimate the relative
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0.12
9.06 9.12
0.05
8.64 av 9.2 & 0.8
0.08
0.88 0.88
0.14
0.64 0.45
0.03
0.09
3.0 3.7 3.2 3.1
3.5 3.8 av 3.4 i 0.3 a Experiments 1-3 were used to determine kz and experiments 4-9 were used to determine kl. In selecting the best-fit values for kl, the cm3 s-l. value for k2 was set as 9.2 X 0.08
populations. From the integrated areas of the ba_nds, corrected for variation of the laser energy, the ratios of the (X;OOO:O10:001 + 020) populations were calculated as 100:20:6. The 010 level is split into four vibronic states, W , 2A5/2,zA3/~,and Q-. The intensity from the two lowest energy levels (2Z+and 2 A ~ /could ~) be measured, see Figure 1, and the intensity from the two higher levels was estimated by assuming a boltzmann distribution for the four states. The zA3/z and 2Z- intensities were subtracted from the 2113/z(OOO)band to obtain the population ratio just mentioned. The observed populations are approximately a thermal NCO vibrational population. According to Hancock and co-workers,14 the rate constant for the relaxation of NCO(X,OlO) in Ar is 4.1 X lO-I3 cm3 s-I. This value suggests that considerable relaxation could have occurred at 0.6 Torr and 0.5 ms, our best condition. Our excitation spectrum cannot provide information about the nascent NCO(X) vibrational distribution from (1); however, the excitation spectrum does demonstrate that the vibrational energy released to NCO by (1) is not very large, and the steady-state NCO(%) distribution is not vibrationally excited. Since both direct H-atom abstraction9J7J8and addition4minationl9 would tend to give low cf,(NCO)), the vibrational energy disposal to NCO is not a useful diagnostic for the dynamics of (1). B. The Rate Constants for Reactions 1 and 2. On the basis of the estimates from previous investigations, the rate constant for (1) was expected to be larger than that for reaction 2. Experiments first were done with excess [F]o so that NCO formation was complete within the first few milliseconds and the removal of NCO by reaction 2 was the important process during most of the observed reaction time. Since [ F]o had been carefully determined, k2 was the only parameter needed to fit the data. The [NCO] was sufficiently low that reaction 4 need not be included in the analysis, vide infra. Several experiments were done with [F]o >> [HNCOIo, and the reaction time was varied from 5 to 25 ms by moving the HNCO inlet. The results from three of these experiments are listed in Table I, and Figure 2 shows a plot of the LIF intensity vs reaction time for [F]o = 1.2 X l O ” ~ m - ~ a n[HNCOIo= d 1.2X 1012~m-3,whichisexperiment 2 of Table I. The profiles from several experiments were fitted by kinetic simulation in which kl, (=kl) was fixed at 3.0 X cm3 s-I and kz was varied. In fact, the decay of [NCO] was virtually independent of k l and thevalue chosen for the simulations is not important. Table I lists the best-fit rate constants for the three experiments. The average value for kz was (9.2 f 0.8) X 10-12 cm3 molecule-’ s-1. The important variables for the determination of k2 are the reaction time and [Fo]. As previously noted, the [F]o was measured for each experiment. Since the flow rates were calibrated carefully, Ar should be known to f 10% and kz should be reliable to f20%. In order todetermine k l ,the [F]o and [HNCOIo wereadjusted so that the formation and removal of NCO both were taking place during the observation time. This was done by reducing
F
+ HNCO Reaction System
The Journal of Physical Chemistry, Vol. 97, No. 39, 1993 10031
8,
I
-6 --
4.OE1 I
._
5
--
3.OE1 I
.-
4
--
2.OEll
.-
3
--
7
t
0-0
Simulotion
'P
l'OEl1
05
Y "
I
10
0
30
20
Time in ms
Figure 2. Plot of the LIF intensity from [NCO] vs time for [F]o = 1.15 X 10l3and [HNCO]o= 1.2X 1 0 1 2 m ~ l ~ l e ~ m Thebest-fitcalculated -3. [NCO] profile with kl = 3.0X 10-" and k2 = 9.7 X cm3s-I is also shown. Thecalculated result from the averagevalue of kl and k2 (overall fit) also is shown.
Simulation
A-A
0 Data
Z
O e 5 V
Y
"._"a
0
10
20
30
Time in ms
Figure 3. Plot of the LIF intensity from [NCO] vs time for [F]o = 6.4 X 1012and[HNCOIo = 3.0X 10" m~leculecm-~. Thesimulated [NCO] profile vs time for kl = 3.5 X 10-11 cm3 s-l and kz = 9.2 X 10-12 cm3 s-1 is also shown. 5.OE1 1
-_
4.OE1 1
-_
3.OE1 1
.-
2.OE11
--
1 .OEl 1
40
I-7 I
0 3 -
-.-8 C
z
Y
I 5
10
15
20
25
Reoction time (ms)
Figure 4. [NCO] vs reaction time for [HNCOIo = 8.0 X 10" and [F]o of 9.0 X 1012 molecule ~ m - ~The . calculated results (-) with k2 = 9.2 X 10-12 cm3 s-1 and kl 5 3.7 X om3 s-I provided the best fit to the data (0).Two more curves with kl = 3.0 X 10-I1 cm3s-I (- -) and 4.5 X 10-11 cm3 s-1 (- -) are also shown to demonstrate the sensitivity of the simulated results to the value of kl;see text for discussion.
the [FIo and data for two experiments, as well as the calculated [NCO], are shown in Figures 3 and 4. These [NCO] profiles were fitted by simulations for which k2 = 9.2 X 10-12 cm3 s-1 was fixed, and thevalue for kl was varied until the best fit was obtained. Table I lists [F]o, [HNCOIo, and assigned rate constants for several experiments; the average of the six experiments gave kl as (3.4 0.3)X 10-" cm3s-l. Thecrucial part for determination of kl is fitting the position of the maximum of the [NCO] profile, which is sensitive to the product of [Flo, kl, and time. Figure 4 shows a family of curves with kl = 3.0X 10-11, 3.7 x 10-ll, and 4.5 X 10-11 cm3s-l to show the sensitivityof the calculated profiles
*
2.OE11
4.OE11
6.OEll
8.OEll
1.OE12
1.2E12
14E12
[F] molecule
Figure 5. Plot of [NCO] vs [F]o for [HNCOIo = 4.3 X 10l2molecule cm-3under theconditionof [HNCO]o>> [F]oandareactiontimeof0.17 s. A computer simulation using k4 = 5.0 X kl = 3.4X 10-l1, and kz = 9.2 X 10--'2cm3s-l is also shown. If k4 was insignificant,the [NCO] would be equal to [F]o for the whole [F]o range of the plot.
with respect to kl. We could not observe the rising part of the [NCO] profiles for [Flo, [HNCOIo, and At that could be used. The uncertainty in kl is larger than implied by the standard deviations in Table I because of the uncertainty in [Flo, and we quote (3.4 f 0.7) X lo-" cm3 s-l. The decay of [NCO] in experiments 4-9 also provides a check upon k2. After kl was fixed, we examined the data from longer reaction times; however, there was no need to change k2. C. Rate Constant for the Bimolecular Reaction of NCO. The bimolecular, self-removal rate constant of NCO radicals is important, because it sets the concentration regime where secondorder kineticsbecome important. The experimentstocharacterize reaction 4 consisted of observing [NCO] for a long reaction time asafunctionof [Floforafixedlarge [HNCOIo. Since [HNCOIo was larger than [Flo, reaction 2 would not compete with reaction 4 and changing [F]o controls the [NCO]. Figure 5 shows the experimental data for a reaction time of 174 ms and [HNCOIo = 4.3 X 1012 molecule ~ m - The ~ , calculated [NCO] profileusing k4 = 5.5 X 10-12 cm3 s-l matches the experiment result. Two other independent experiments were done and the average value for k4 is (5.0 f 1.0) X 10-12cm3 s-1. The uncertainty in k4 is larger than for kl and k2, because [F]o could not be determined for each data point in Figure 5. Since [CF4] was go%) NCO F rather than NF(X) CO5b is supporting evidence for the redissociation argument. Regardless of the specific mechanism, NF(X) + CO is the only possible exit channel and reaction 2 is a stoichiometricsource for NF(X3Z-) from HNCO, providing that excess [F] and sufficient reaction time are used. SincetheNF(a) COandF+NCOreactionsareboththought to proceed via the bound FNCO (R,lA’) potential, RRKM calculations were done to obtain an order of magnitude estimate for the dissociation lifetimes of FNCO* formed by reaction 2 and by NF(a) CO. For these calculations, &(F-NCO) = 83 kcal mol-’ was assumed by analogy to the bond energies of the NF3 (NH3) and FNCO (HNCO) pairs. The bending frequencies of the F-NCO transition state were set so as to give a preexponential factor (partition function form) of -5 x 1014 s-I for the thermal (1000 K) unimolecular dissociation of FNCO. The calculated RRKM unimolecular lifetimes of FNCO with 5 and 15 kcal mol-’ of energy above the dissociation threshold are 14 and 0.5 ps, respectively. These values are only order of magnitude estimates, because of the uncertainty in D(F-NCO). However, the time needed for successful competition between dissociation and crossiFg to the triplet potential seems to be 110 ps, since the FNCO (XIA’) formed from NF(a) CO with an excess energy of -5-10 kcal mol-’ gives only F + NCO. B. Reactions 1 and 2 as Sources of NCO and NF. Reaction 1 can be used to generate NCO for systematic studies of the reactions of NCO in a flow reactor. Since the reaction rate is only moderately rapid at 300 K, the [HNCOIo should be larger than [F]o for delivery of a known [NCO] in the absence of [F]. We checked the reactivity of NCO with HNCO and found that there was no measurable loss of NCO for [HNCO] I 1 X 1015
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molecule cm3 at 300 K and At = 9 ms. Since LIF is a sensitive detection method ( C 1 X 1011molecule cm3can be easily observed), low NCO concentrations can be used and reaction 4 need not complicate the kinetics for removal of NCO. Reaction 1 can serve to calibrate the LIF signal for absolute [NCO], as we demonstrated in our study of the NF(a) + CO reaction.5’~ Reaction 1 should be suitable for calibrating LIF signals for in situ monitoring of the absolute [NCO] in complicated environments. Reactions 1 and 2 can serve as a source of NF(X) radicals, if excess [F]o is used. The 300 K rates are slower than desirable and using an independently heated prereactor to deliver [NF(X)] in the absence of [NCO] may be advantageous for some applications. The bimolecular, self-destruction reaction7c.k between 2NF(X) has a rate constant in the (3.5 f 2.0) X 10-12cm3 s-l range and second-order removal of NF(X) will not be serious in a flow reactor using (1) (2) as the NF(X) source, providing [NF(X)] C 2 X 10l2molecule ~ m (see - ~Figure 1 of ref 5c). We already have used reactions 1 and 2 to study the interaction between NF(X) and NF(a).24 Our attempts to use LIF on the NF(b X) transition to monitor [NF(X)] in the flow reactor were unsuccessful, because of background chemiluminescence in the 520-nm region for both the 2F + HNCO and 2F HN3 reaction systems. For that reason we developed the Nz(A) excitation-transfer reaction as the monitor for NF(X).SC With suppression of the background chemiluminescence, the relative concentration of NF(X) probably could be measured by LIFa7 However, for many applications the addition of N2(A) to the reactor is a satisfactory monitor of [NF(X)]. The 2F HNCO reaction system offers the same possibility for systematic study of the chemistryof NF(X) as the 2F + HN3 reaction has provided for study of the reactions of NF(a).S As a practical matter, we do not recommend the stearic acid KOCN synthesis for HNCO. The HNCO yield is low because of the need to remove the C02 impurity. Knowing the exact HNCO flow rate is especially critical, if absolute [NCO] or [NF] are needed. Another method for the preparation of HNCO would be preferable, if large quantities of HNCO are needed. Most of our experiments were done with CF4 as the F atom source. However, trial experiments using a 10%F2 in He mixture seemed to offer the advantageof reduced background chemiluminescence. The residual F2 from the F2/He discharge probably would be converted to FNCO and would cause no difficulty. C. Comparison octhe Properties of NCO vs N3. The enthalpies of formation of N3(X2n) and NCO(R211)are 11325Jaand 3 1-48‘ kcal mol-1, which correspond to bond dissociation energies of -0 and 53 kcal mol-’, respectively. Thus, the N atom abstraction reactions by atoms and radicals with NCO are less exoergic and the rates are probably slower. However, for selected reactants, such as N atoms, the NCO reactions can still provide enough energy to be a source of electronically excited products. Our preliminary work indicates that the N atom reaction gives N2(A?&+), whereas the N N3 reaction givesN2(B311,) and possibly also N2(A3Z,+ and 3AJ.26 Investigation of some of the reactions of NCO with atoms that gave chemiluminescence with N3 could be usefu1.22.26 Simultaneous studies of the NCO and N3 reactions can be done in the same reactor by using LIF detection. In this context, the question22 of the actual N3 concentration in the extensive series of e~periments26.2~ reported from the Denver laboratory should be remembered. The C1+ N3 reaction certainly has a large rate constant [(2.8 f 0.4) X l0-lo cm3 and the yields of NCl(a1A and blZ+) are high.28 Formation of NCl(a) from the C1+ NCO reaction appears to be endoergic and the C1 NCO reaction could be a clean source for ground-state NC1 radicals. The H-atom reactions30-31with N3 and NCO can be contrasted with the reactions of F and C1 atoms. The cross section for H + NCO is reported to be 1.3 times larger than H + N3, which has a 300 K rate constant of (1.3 f 0.1) X 10-l0 cm3 s-l,
+
+-
+
+
+
+
+
Wategaonkar and Setser
10034 The Journal of Physical Chemistry, Vol. 97, No. 39, 1993
according to the most recent study.22 The product branching from H N3 seems to be abnormal30 with NH(X3Z) favored (by a ratio of 3.2 f 1.3) over NH(alA), even though the singlet channel is exoergic. This has been explained31 as a consequence of the H N3 reaction proceeding by the HN3(B3A")potential, but it is difficult to understand why there is a barrier on the singlet entrance channel in this atom + radical recombination reaction and not for others. Reinvestigation of the product branching ratio from the H + N3 reaction could be worthwhile. The NH(a1A) + CO reaction32mainly gives NCO + H rather than NH(X) CO, and the singlet channel does seem to be open from HNCO(%A'), if there is sufficient energy. Formation of NH(a1A) from H NCO is endoergic and only NH(X3Z-) is observed.30 Since D(HN-CO) < D(H-NCO), the thermal decomposition of HNCO gives NH(X'Z-) + CO rather than H
+
+
+
+
+~
~ 0 . 3 3
Another interesting point of comparison is the bimolecular, self-destruction reactions, which are exoergic by 226 and 117 kcal mol-' for N3 and NCO, respectively. The rate constant measured here, (5.0 f 2.0) X 10-12cm3 s-l, for NCO is in modest agreement with an estimate from Quinones et al.30 The N3 selfdestruction constant has not been carefully measured, but a consensus v a l ~ e ~ is ~ 9-23 ~X 10-12 cm3 s-I, and the bimolecular rate constants for N3 and NCO are approximately the same. Such small rate constants for radical recombination reactions are remarkable. The delocalized nature of the odd electron in the g2Ii states must lead to a significant activation barrier; the large number of electronic states in the entrance channel that do not correlate to the lowest energy transition state also reduces the rate constants. The NCO and N3 radicals have the 4 ~ , 2 3 ~ , , ~ configuration 1 ~ , ~ 1 ~ ~with spin-orbit separation of the 2 I i 3 / 2 (lower) and 2 I i 1 / 2 (upper) levels of 95.6 and 71.3 cm-1, respectively. Quite significant changes in electron configuration are required to form products by either addition to unsaturated molecules or abstraction of H atoms. Studies of the relative reactivity of the 'Ii3/2 and 2111/2 states and their X-doublet components, not to mention the four vibronic states associated with the 010 vibrationally excited state, provide an interesting challengefor state-testatekinetic studiesof NCO and N3 radicals.
Conclusions The reactions of F atoms with HNCO and NCO were characterized by monitoring the [NCO] via laser-induced fluorescence. The rate constants kl and k2 were determined as (3.4 f 0.7) X 10-11 and (9.2 & 1.8) X cm3 molecule-l s-l, respectively,at 300 K. Although themeasurements arenot totally conclusive, the data suggest that the formation of HF and NCO is the only pathway for reaction 1. By controlling the [F]o/ [HNCOIo,either NCO or NF(X) can be generated in concentrations up to (3-5) X 1012 in a flow reactor. The bimolecular, self-destruction rate constant for NCO was found to be (5.0 f 2.0) X 10-12 cm3 s-1 and the self-destruction reaction will not interfere with first-order decay rates for modest NCO concentrations. The bimolecular, self-destructionrate constant for NF(X)seems smaller than for NCO and higher NF(X) concentrations can be used and still maintain the first-order kinetic regime. The utility of the F + HNCO reaction for study of NCO kinetics was shown by measuring the reaction rate constants with NO and NO2, several unsaturated hydrocarbons, and HI and H2S. Our NO rate constant is in excellent agreement with independent measurements in the literature. The F + HNCO reaction can be used to calibrate the LIF intensity for the measurement of absolute NCO concentrations.
Acknowledgment. This work was supported by the U S . Air Force Office of Scientific Research and the Strategic Defense Initiative under Grants 88-0279 and 92-J-0275.
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Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. J . Phys. Chem. ReJ Datu 1985, 14, Suppl. 1. (c) Spiglanin, T. A.; Perry, R. A., Chandler, D. W. J . Phys. Chem. 1986, 90,6184. (d) Cyr, D. R.; Continetti, R. E.; Metz, R. B.; Osborn, D. L.; Neumark, D. M. J . Chem. Phys. 1992, 97, 4937. (e) East, A. L. L.; Johnson, C. S.;Allen, W. D. J . Chem. Phys. 1993,98,1299. (f) East, A. L. L.; Allen, W. D. J . Chem. Phys. 1993, In Press. (9) (a) Manocha, A. S.;Setser, D. W.; Wichramaaratchi, W. A. Chem. Phys. 1983, 76, 129. (b) Agrawalla, B. S.;Setser, D. W. In Gus Phase Chemiluminescence and Chemiionizution; Fontijn, A., Ed.; North Holland: Amsterdam, 1985. (10) Sloan, J. J.; Watson, D. G.; Wright, J.S. Chem.Phys. 1981,63,283. (11) Chen, J.; Dagdigian, P. J. Chem. Phys. 1992, 96, 7333. (12) (a) Louge, M. Y.; Hanson, R. K. Combust. Flame 1984,58,291. (b) Black, G., Jusinski, L. E., Taherian, M.-R., Slanger, T. G.; Huestis, D. L. J. Phys. Chem. 1986,90,6842. (c) Wong, K. N.; Anderson, W. R.; Kotlar, A. J.; Vanderhoff, J. A. J. Chem. Phys. 1984, 81, 2970. (d) Cooper, W. F.; Hershberger, J. F. J . Phys. Chem. 1992, 96, 771. (e) Becker, K. H.; Kurtenbach, R.; Wiesen, P. Chem. Phys. Lett. 1992, 198, 424. (13) Perry, R. A. J . Chem. Phys. 1985,82, 5485. (14) (a) Hancock, G.; McKendrick, K. G. Chem. Phys. Lett. 1986, 127, 125. (b) Astbury, C. J.; Hancock, G.; McKendrick, K. G. J. Chem. Soc., Faraday Trans. II 1993,89, 405. (15) Atakan, B.; Wolfrum, J. Chem. Phys. Lett. 1991, 178, 157. (16) (a) Patel-Misra, D.; Sauder, D. G.; Dagdigian, P. J. J. Chem. Phys. 1990, 93, 5448. (b) Wu, M.; Northrup, F. J.; Sears, T. J. J. Chem. Phys. 1992,97,4583. (c) Hemmerling, B.;Vervloet, M. Md. Phys. 1993,78,1423. (17) (a) Agrawalla, B. S.;Setser, D. W. J. Phys. Chem. 1986,90, 2450. (b) Wategaonkar, S.;Setser, D. W. J . Chem. Phys. 1987, 86, 4477. (18) Beaman. R. A,: Nelson. T.: Richards. D. S.: Setser. D. W. J . fhv.9. Chem. 1987, 91, (20) Tsai, C. P.; Belanger, S.M.; Kim, J. T.; Lord, J. R.; McFadden, D. L. J . Phys. Chem. 1989, 93, 1916. (21) (a) Yamada, K. J. Mol. Spectrosc. 1980, 79, 323. (b) Misra, P.; Mathews, C. W.; Ramsay, D. A. J. Mol. Specrrosc. 1988, 130, 419. (22) Liu, X.; MacDonald, M. A.; Coombe, R. D. J. Phys. Chem. 1992, 96,4907. (23) Du, K. Y.; Setser, D. W. Unpublished work. The evidence for the lack of vibrational relaxation of NF(X) for reaction 2 is somewhat indirect.
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