9865
J. Phys. Chem. 1992,96,9865-9870 1450 (2), 1340, 1320, 1280, 1220, 1160 (2), 1050 (2), 1020, lo00 (3). 960,950 (2), 900,840,830,800 (3), 750,600,500,410,300 (2). Transition-state frequencies [cm-'I: 3 100, 3050 (S), 3020, 1620, 1600 (2), 1500 (l), 1450 (2), 1340, 1320, 1280, 1160 (2), 1020, lo00 (3). 900,840 (2), 800 (3), 780 (2), 750,744,743,700 (3), 500 (21, 260 (2). Ratio of overall moments of inertia (I+/I) = 1. Critical energy = 75.00 kcal/mol; collision frequencies calculated as ( A w ) (see text). Appendix II Thermochemical Data are given in Table I. Thermochemical Estimates (298 K).23 Barrelene (bicyclo[2.2.2]octa-2,5,7-triene). Entropy: Open-chain starting com= 9 1.O cal/mol K; pound: 3 vinyl- 1,Ccyclohexadiene. Soint entropy decrement in closing a C6 ring ASoint = 4 . 7 cal/mol K;symmetry corrections ( u = 6) ASo, = -3.6 cal/mol K, thus So = 82.7 cal/mol K. [4.1.O]-7-MHD (7-methylenebicyclo[4.1.O]hepta-2,4-diene). Entropy: Open-chain starting compound 5 vinyl 1,3 cyclohexadiene. Soht= 87.4 cal/mol K. Entropy decrement in closing the C3 ring ASoht mt = -4.9 cal/mol K. Thus,So = 82.5 cal/mol K. Heat of formation: AHOf = AHOr[Cd-(H)z] AHof[Cd-(C),] + 2 AHof[C-(Cd),] + AHOf[Cd-(H)(C)] + 2AHOf[Cd-(H)(Cd)] + AHoRc(methylenecyclopropane) + HR,(~,~-CHD) AHOf = 6.76 10.34 2(-1.48) 2(6.78) 2(8.59) 41 4.8 = 90.7 kcal/mol
References and Notes (1) Hassenruck, K.; Martin, H.-D.; Walsh, R. Chem. Rev. 1989,89,1125. (2) Martin, H.-D.; Urbanek, T.; Pfohler, P.;Walsh, R. J. Chem. Soc., Chem. Commun. 1985,964. (3) Yu, C.F.;Young, F.; Bersohn, R.; Turro, N. J. J. Phys. Chem. 1985, 89,4409. (4) Durln, R. P.;Amorebieta, V. T.; Colussi, A. J. J . Am. Chem. Soc. 1987,109, 3154. ( 5 ) Durln, R. P.;Amorebieta, V. T.; Colussi, A. J. J. Phys. Chem. 1988, 92,636. (6)Ghibaudi, E.;Colussi, A. J. J. Phys. Chem. 1988, 92, 5839. (7) Duran. R. P.; Amorebieta, V. T.; Colussi, A. J. 6 1 . J . Chem. Kiner. 1989, 21, 847. (8) Kiefer, J. H.; von Drasek, W. A. Inr. J . Chem. Kiner. 1990,22,747. (9)NIST Standard Reference Database 25,version 1.0, National Bureau of Standards and Technology, Gaithersburg, MD, 1991. (10) Muller-Markgraf. W.:Troe. J. J. Phvs. Chem. 1988. 92.4914. (11) Martin, H . - 6 ; Urbanek, T:;Braun,-R.; Walsh, R. X r . ' J. Chem. Kiner. 1984, 16,117. (12)Dudek, D.; Glanzer, K.; Troe, J. Ber. Bunsen-Ges.Phys. Chem. 1979, 83. 776. (13) Amorebieta, V. T.; Colussi, A. J. J . Phys. Chem. 1982,86,2870. (14)Robaugh, D. A.; Barton, B. D.; Stein, S.E. J. Phys. Chem. 1981,85, 2378. (15) Grela, M. A,; Colussi, A. J. J . Phys. Chem. 1986,90,434. (16)Kiefer, J. H.; Mitchell, K. I.; Kern, R. D.; Yong, J. N. J. Phys. Chem. 1988,92,677. (17) Chanmugathas, C.; Heicklen, J. Inr. J. Chem. Kiner. 1986,18,701. (18) Chanmugathas, C.; Heicklen, J. Inr. J . Chem. Kiner. 1987,19,659. (19)Dick, P. G.; Gilbert, R. G.; King, K.D. Inr. J. Chem. Kiner. 1984, 16, 1129. (20)Gilbert, R. G.; Smith, S. C.; Jordan, M. J. T. UNIMOL A program for calculation of rate coefficients for unimolecular and recombination reactions. Department of Theoretical Chemistry, University of Sidney, Australia, 1990. (21) NIST Standard Reference Database 17 (Chemical Kinetics Database, version 2.01) National Bureau of Standards and Technology, Gaithersburg, MD, 1991. (22)Gallo, M. M.; Hamilton, T. P.; Schaeffer, H.F. I11 J. Am. Chem. SOC.i990,112,a714. (23) Benson, S.W. Thermochemical Kinetics, 2nd ed.;Wiley: New York, 1976. (24) Ervin, K. M.; Gronert, S.;Barlow, S.E.; Gilles, M. K.; Harrison, A. G.; Bierbaum, V. M.; DePuy, C. H.; Lineberger, W. C.; Ellison, B. G. J. Am. Chem. Soc. 1990,112,5750. (25)Tsang, W. J. Phys. Chem. 1986, 90, 1152.
+
+
+
+
+
+
+
[2.2.1] -7-MHD (7-methylenebicyclo[2.2.I ]hepta-2J-diene). Entropy: Open-chain starting compound 3 vinyl 1,4 cyclohexadiene. Soint = 9 1.Ocal/mol K. Entropy decrement in closing a tight C5 ring Soht mt = -4.7 cal/mol K. Symmetry corrections ( u = 2) ASo, = -1.4 cal/mol K. Thus So = 84.9 cal/mol K. Biradical BR (5-vinyl (2'-yl) 1,3-c-hexadien-6-y1): Mor(BR) = BDE[CZH3 - HI + BDE[(1,3-CHD-S-y1) HI - BDE[H - HI + AHO~[5-vinyl-l,3-cyclohexadiene] = 111.2 76.2 (ref 24) - 104 (ref 25) 44.2 = 127.4 kcal/mol
+
+
Dynamics of the O('D)
+ CINCO Reaction
Steven M. Singleton and Robert D. Coombe* Department of Chemistry, University of Denver, Denver, Colorado 80208 (Received: June 19, 1992)
The reaction of excited O('D) atoms with ClNCO was studied by pulsed photolysis of 03/ClNC0 mixtures with the 249-nm output of a KrF laser. The rate constant for O(lD) quenching by ClNCO was determined to be k = (1.3 f 0.3) X cm3s-I. Investigation of the possible products of this reaction indicated that production of C10 + NCO is a major channel, with a branching fraction greater than 0.20. No evidence was found for the production of NCI in its X'Z-, alA, or blZ+ states. An upper limit for the branching fraction to NCI + C02 is 0.005.
Inboduction In previous issues of this journal, we have presented results from studies of the dynamics of reactions of excited O(lD) atoms with HN3 and HNC0.1-3 These processes are of interest in view of the similarity of O(lD) to isoelectronic NH(alA), a species whose reactions with HN3 and HNCO are known from studies of the photochemistry of these m~lecules.~*~ O(lD) appears to behave much like NH(a) in its reaction with HN3; chemiluminescence from the system is indicative of formation of OH and N3 in the 0022-3654/92/2096-9865$03.00/0
+
initial step. The O(lD) HNCO reaction is very different, however, in that the dominant path is electrophilic attack by the excited oxygen atoms on the NCO chain, generating NH and C02. Although much of the energy released by this reaction comes from formation of the new C-O bond (a major difference from the O(lD) HN3 reaction), angular momentum constraints force the system to produce NH in its excited alA state. Indeed, production of NH(a) + COz is the dominant path for the reaction, a remarkable result given the complexity (and presumed lack of
+
0 1992 American Chemical Society
9866
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992
Singleton and Coombe
SYCHRONOUS
u GENERATOR
Figure 1. Diagram of the experimental apparatus.
symmetry) in the intermediate. In this paper, we present the results of a similar study of the O(ID) + ClNCO reaction. This work was undertaken for a number of reasons, one of which was to test the strength of the angular momentum constraints under conditions where spin-orbit coupling is increased by the presence of the C1 atom. If the mechanism and its constraints were to remain intact in the ClNCO system, the reaction might offer an excellent source of NCl(a'A), an important metastable energy storage agent in a number of potential laser systems. On the other hand, the mechanism in O(lD) HNCO is thought to involve electrophilic attack on the NCO chain. In the ClNCO reaction, this attack might well be diverted to the electron-rich chlorine atom, leading to production of CIO and NCO. In the work described below, we present measurements of the overall rate of O('D) quenching by ClNCO and a determination of the dominant reaction path.
+
Experimental Details The O(ID) + ClNCO reaction was studied by photolysis of C1NC0/O3 mixtures with a pulsed laser at 249 nm. ClNCO was prepared as described previously6 by passage of ClZ(a 10%mixture in He) over solid AgNCO suspended on glass wool at 160 K. The ClNCO effluent of the generator was monitored continuously by its IR absorption near 2200 cm-I. The ClNCO/He was collected and stored in 5-L Pyrex bulbs. The purity and composition of the ClNCO/He were verified prior to each experiment by measurement of IR and UV absorption spectra.6 Typically, the only impurities evident were C 0 2and C12,both in only trace quantities. HNCO was prepared3 by the reaction of KOCN with excess stearic acid at 363 K. Since this process also generates considerable amounts of COz, the HNCO was purified with a freeze/thaw cycle at 196 K. This procedure served to reduce the C02 levels to trace or unobservable amounts (by IR absorption). The purified HNCO was collected in a Pyrex bulb and diluted with He to a 5% mixture. Ozone was prepared by passing Oz though a commercial ozonizer and trapping the product on silica gel at 196 K. Prior to experiments, the ozone was released by warming the silica gel trap and collected in a thoroughly passivated Pyrex bulb. It was diluted with He to a 5% mixture. A diagram of the photolysis apparatus is shown in Figure 1. The aluminum photolysis cell used in the experiments was of standard design, except for an extended side arm used for direct measurements of the O3density in the cell. The O3 density was determined by monitoring its absorbance at 253.7 nm over the 8oCm combined length of the side arm and width of the photolysis
cell. The flow rates of the ClNCO/He mixture and the Ar diluent were determined with calibrated mass flow meters (Tylan FM360). The total pressure in the cell was measured with a capacitance manometer (MKS). O('D) was generated by photolysis of the ozone present in the cell with the 249-nm output of a KrF excimer laser. The laser fluence was typically 2Ck30 mJ/cm2. As is described below, the rate of O(lD) quenching by ClNCO was measured by using the chemiluminescent O('D) + HNCO reaction2as a tracer. Emission from excited NH(A311) produced by this reaction was detected by a cooled GaAs photomultiplier tube (RCA C31034) in combination with a narrow band-pass filter. The time decay of the emission was amplified, digitized, and averaged over successive pulses with a Nicolet 1270 data acquisition system. The averaged decays were analyzed with a personal computer using RS/1 statistical analysis software. NCO produced by the O('D) + ClNCO reaction was detected by laser-induced fluorescence (LIF) on the X211 Az2+transition of this radicalz*'in the 420-440-nm range. The LIF probe beam was generated by an excimer-pumped dye laser operating with Coumarin 440 dye. The LIF signal was detected by the GaAs photomultiplier tube noted above in combination with a band-pass filter which passed the spectral region in question. The delay time between the photolysis and probe pulses was adjusted by using an SRS DG535 delay generator which was triggered by the photolysis pulse. With this configuration trigger jitter was less than 10 ns. The LIF signals were amplified, digitized, and stored with an SRS 250/245/240 gated integration system triggered by the probe laser pulse.
-
Results Rate Constant for O(lD) Quenching by CINCO. Since the O('D) ClNCO reaction produces no chemiluminescence (see below) which might be used to monitor its rate, an external tracer of the relative O(lD) density in the system was needed for this measurement. A convenient tracer for this purpose is the O(ID) + HNCO reaction, which produces2.excitedNH. A small proportion (0.02%) of the NH is produced in the excited A% state, which has a strong characteristic emission (A311 X3Z-) near 336 nm. Since the radiative lifetimeEof the NH(A) state is relatively short at 455 ns, the time decay of the A X emission tracks the decay of the O('D) density in the system. The rate constant for O(lD) quenching by ClNCO was determined by measuring the decay rate of the NH(A) emission in the presence of fixed densities of parent O3and HNCO and variable densities
+
-
+
Dynamics of the O(lD)
+ ClNCO Reaction
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 9867
is expected that the two most probable channels are as follows: NCI
O('D)
+
CINCO
i
NCO
+
+
CO2
AH=-65kcaVmd
(2a)
CIO
AH=-96kcaVmol
(2b)
Reaction 2a is the analog of the dominant path observed in the HNCO reaction. Much of its exothermicity,6 65 O('D) kcal/mol, arises from the formation of the new C-0 bond in the COz product. This product would be expected to carry signifcant vibrational excitation. The energy released is also sufficient for excitation of the lowest excited singlet states of NC1, alA and b'Z+. As noted above, the alA state of N H was the dominant product in the analogous reaction with HNCO. In that case? no NH(b'Z+) was observed and the production of ground-state NH(X3Z) was less than onethird that of NH(a). In the prcaent work, numerous experiments were conducted in an attempt to observe the X3Z-, a'h, and b'Z+ states of NC1 from reaction 2a. The alA and b'Z+ states were sought via their characteristic emissions9 X emission was at 1076 and 665 nm, respectively. NC1 b sought using the cooled GaAs photomultiplier tube described above in combination with either a monochromator (0.25 m) or narrow band interference filter centered at 665 nm (Tmx = 54%). NC1 a X emission was sought using an intrinsic germanium detector (cooled to 77 K) in combination with an interference filter centered at 1080 nm (T,,, = 40%). No NC1 emission was observed in either case. The sensitivity of these measurements can be ascertained by comparison with observations of excited NCl produced from photolysis" of ClN3 at 249 nm, using an identical apparatus to detect the emission. For this purpose, dilute flows of ClN3 in Ar were produced by passage of C12/Ar mixover H,O-moistened NaN3 suspended on glass wool at 273 K, in a synthesis described previously.I0 The ClN3 density in the effluent of the generator was monitored by the IR absorption of the molecule near 2050 cm-I. The IR absorbance of this feature in our spectrometer was independently calibrated against known densities of CINl as determinedloby UV absorption. Densities of NCl(a) produced by the photolysis pulse were calculated from the known densities of ClN3 in the photolysis cell, the absorption cross sectiont0of this molecule at 249 nm, and the measured fluence of the incident laser radiation. The quantum yield for production of NCl(a) was assumed to be unity for these calculations. Using this method, NCl(a) densities greater than 5 X 10" were readily observed, with a signal-*noise ratio of at least 5:l. Consequently, if NCl(a) densities greater than this value were produced by the O(lD) ClNCO reaction, NCl a X emission would have been detected. The detection limit for NCl(b) is far lower since the b X radiation ratel0 is more that lo3 times greater than the a X and the b X radiation (at 665 nm) is detected radiation with a more sensitive photomultiplier tube. The detection limit for NCl(a),
