7282
J. Phys. Chem. 1991, 95,7282-7290
Very Low Pressure Reactor Chemiluminescence Studles on N Atom Reactions with CHCI, and CDCI, Sae Chae Jeoung, Kwang Yul Cboo, Department of Chemistry, Seoul National University, Seoul, Korea
and Sidney W. Benson* Loker Hydrocarbon Research Institute. University of Southern California, Los Angeles, California 90089-1661 (Received: January 17, 1990; In Final Form: April 30, 1991)
Ground-state (N(S4)) nitrogen atom reactions with chloroform4 and chloroform-d were studied by using the VLPR technique at room temperature. Relative N atom concentrations were monitored via mass spectrometry, and their absolute values were determined by the chemical titration reaction with nitric oxide. It was possible to obtain a more accurate rate constant for the bimolecular reaction: N + NO N2 + 0, kNO = (2.4 f 0.2) X IO-'' cm3 molecule-' s-I at 298 K. N atom decay in the presence of CHC1, and CDCl, was found to have an apparent induction period and to have a large isotope effect. Chemiluminescence signals emitted from the reactor in the range of 300-600 nm were also observed, and identified as coming from the excited CN radical. The detailed study of reaction products, intermediates, N atom decay kinetics, and chemiluminescence signals are interpreted by a slow reaction of CI atoms with CHCl, followed by fast branching chain reactions of N atoms with the intermediate radicals. A successful numerical simulation of the experimental results supports the suggested chain branching mechanism. The following rate constants were estimated from the experimental results: kl(N + CHC13 NCI + CHCI2), 1.00X 10-l6, k2(N + NCI N + Cl), 2.57 X lo-", k3(Cl + CHCI, HCI + CCI,), 3.70 X IO-", k3D(Cl+ CDCI, DCl + CCI,), 0.95 X lO-I4,k4(N + CHC12 HCN + 2C1), 1.98 X lo-", ks(N + CC13 ClCN + 2C1), 1.67 X IO-", k6(N + ClCN CCI + N2), 1.00 X and k,(N + CCI CN(B22:) + Cl),5.70 X IO-", all in the units of cm3 molecule-' s-l.
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Introduction
Many studies have been carried out on the reactiolis of active nitrogen since Strutt' first suggested that the high chemical reactivities of active nitrogen should be due to the presence of free nitrogen atoms. Because of the very large bond dissociation energy of nitrogen molecules, active nitrogens have always been produced by passing nitrogen gas through either a condensed electrode discharge or a radio frequency discharge. Under normal experimental conditions, pure nitrogen downstream from the discharge cavity emits the Lewis-Rayleigh afterglow. This afterglow has been the principal means used to investigate the nature of active nitro Metastable species, such as N(2D), N(2P), and N2(A3Z,), 8 that have relatively long radiative lifetimes, are important. If these species exist in a reaction zone they can initiate the reactions that lead to the apparent decay of ground state nitrogen atom, N(%).4 Therefore, it is of great importance to confirm the absence of energetic species other than N('S) atoms in the reaction zone. Our understanding of the reactions of N(4S) atoms with hydrocarbons is still very limited. It has been reported that reactions of the type N(%) R H N H R are always endothermic and are therefore slow at room temperature because the N H radical has a rather low bond energy (77.7 f 0.69kcal/mols) compared with C H bond energies of 94-105 kcal/mol in aliphatic compounds. In contrast, active nitrogen reacts with saturated hydrogen-containing compounds such as isobutane, SiH4, and trimethylsilane at room temperature.69 Up to the present time the
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(1) Strutt. R. J.; Fowler, A. Proc. Soc. 1911,A&, 106. (2) Brocklehurst, B.; Jennin 8, K. K. Pro .React. Kinet. 1967, 4, 1 . (3) Herron, J. T.; Huie, R. J . Phys. Cfem. 1968, 72, 2538. (4) Michael, J. V. Chem. Phys. Lett. 1980, 76, 129. (5) Bauschlicher Jr., C. W.; Langhoff, S.R. Chem. Phys. Leu. 1987, 135, 67. ( 6 ) Evans, H. G.; Freeman, G. R.; Winkler, C. A. Cun. J . Chem. 1956, 34, 1271. (7) Zabolotiny, E. R.; Cesser, H. J . Am. Chem. Soc. 1959, 81, 6091. (8) Takahashi, Saku. Mem. Jpn. Acad. 1972, 12, 149. (9) Horie, 0.; Potzinger, P.; Reiman, B. Chem. Phys. Lett. 1986. 129, 231.
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reactivity of N(4S) has been studied mostly by resonance fluorescence,4J0resonance absorption," and mass spectrometry3J2 at relatively high pressures of 1-3 Torr. Under these conditions, it is difficult to eliminate reactions by impurities unless low N atom concentrations were sed.^*'^ We have used the very low pressure reactor (VLPR) kinetic system to reduce the coinplications of higher pressures. This system consists of a Teflon-coated reactor operated a t low total pressure of 10-3-10-5Torr, a molecular beam mass spectrometer with which all the reactants and products can be monitored, and a chemiluminescent detector with which UV-visible luminescence in the reactor can be detected and analyzed. The reactions of N(4S) atoms with some chlorine-substituted olefins have already been studied in this laboratory with this experimental apparatus, and rate constants have been rep0rted.I.' In this work we show that N(4S) atoms react with CHC13 and CDC13significantly at room temperature. By carefully accounting for all the chlorine isotopic mass spectra and by resolving the emissions, the reaction products and chemiluminescent species have been identified. Detailed studies of reaction products, intermediates, N atom decay kinetics, and chemiluminescencesignals have led us to conclude that the mechanism of the N atom disappearance in the presence of CHC13 is a branching chain reaction initiated by CI atom reaction with CHCl, and of N atoms with intermediate radicals. The rate constants for each step in the suggested branching chain mechanism were deduced from the experimental results. A successful numerical simulation of the observed results from the deduced rate constants supports the validity of our suggested mechanism. Experimental Section 1. Materials. Nitrogen (Matheson, 99.999%) was further purified by passing through copper wire heated at about 300 OC.
Nitric oxide from Matheson was used after trapping the high(IO) Michael, J. V. Chem. Phys. Left. 1979,68, 561. (1 I) Sato, S.;Sugawara, K.; Ishikawa, Y . Chem. Phys. Lett. 1979,68,557. (12) Herron, J. T. J . Chem. Phys. 1966, 70, 2083. (13) Kim, Y.K.; Choo, K. Y . Chem. Phys. Lerr. 1983, 102, 281.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 7283
N Atom Reactions with CHC13 and CDC13
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E? analyzer
TURBO
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TURBO
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Osciloscope
Figure 1. Schematic diagram of the very low pressure reactor (VLPR) chemiluminescence detection system: (P)photomultiplier; (M) monochromator; (F) field lens; (S)skimmer: (C) chopper; (I) ionization chamber; (Q)quadrupole; (E) electron multiplier.
boiling impurities with liquid nitrogen. CHC13and CDCI, were both from Aldrich (spectroscopic grade). All the samples were used after several freeze-pumpthaw cycles before each experiment. No detectable impurities were found by mass spectrometry in any of the chemicals used in the experiment. 2. The Very Low Pressure Reactor.I4 A schematic representation of the stainless steel very low pressure reactor (VLPR) is shown in Figure 1. Under ordinary experimental conditions, the pressure in the reactor is in the range of 10-3-10-5 Torr. The collision number with walls in a Knudsen cell type reactor is given by the following equation
zw
y4&t,
= (Av/Ah)V
where A, is the surface area of the walls of the reactor and Ah is the area of the escape hole. The mean residence time, t,, of a molecule of mass M in the reactor is given by the Knudsen equation. t , = 4V/?Ah (s) where Vis the volume of the reactor and E = ( ~ R T / T M ) is ’/~ the mean molecular speed. Then, the first-order rate constant, kcM,which characterizes the escape rate of the species of mass M from the reactor, can be defined.
If FM is the flow rate of species of mass M into the reactor cell, the steady-state concentration of M in the reactor is given by the following equation:
[MI, = FM/VkeM (molecules/cm3) The reactor volume (V) and the surface area of the wall of our VLPR cell (A,) are 214 cm3 and 918 cm2, respectively. Two different escape hole areas (Ah) of 0.43 and 0.09 cm2 were used in this work. The experimentally measured keMvalues for the species used in this work were all in good agreement with the values calculated from the reactor geometry and the Knudsen equation above. keMis measured directly by flowing N or N2 or more generally species M into the spectrometer, shutting off the flow and measuring the first-order decay of the MS signal at the appropriate mass. 3. Kinetic System. The experimental system consists of three parts: the glass vacuum system, the metal vacuum system, and the chemiluminescence detection unit. The glass vacuum system has sample reservoirs and a flow measurement section for intro-
ducing sample molecules into the reactor. The metal vacuum system has the VLPR reactor, mass spectrometer detection system, and differential pumping system. Figure 1 shows the schematic drawing of the metal vacuum system and the chemiluminescence detection unit. Since the glass vacuum system and metal vacuum system have already been described elsewhere,Is only the chemiluminescence detection unit will be described in detail here. An Oriel Type 7070 photomultiplier detection system (P) was used for detecting the chemiluminescence signals that were emitted through the quartz window. The field lens (F) (Kratos, QPMA321, 50 mm field lens) was placed between the quartz window on the VLPR reactor and the photomultiplier. All the spectra were resolved by a double grating monochromator (M) (Kratos, GM200) with 240-nm blazed gratings. To eliminate signals from the afterglow that originated in the discharge, we used a Woods Horn with 90-deg bending of the flow tube and confirmed that the light interference from the discharge was negligible under usual experimental conditions. Under our experimental conditions, the mass spectral peak a t m / e = 14 with ca. 21 eV ionization voltage was found to be solely attributed to the ground-state nitrogen atoms. The emitted chemiluminescence originated from the reactions of ground-state nitrogen atoms (see Discussion section). All the reactants and products as well as the N atoms in the VLPR reactor were analyzed by the mass spectrometer with ca. 21 eV ionization voltage. Results and Discussion Since the N H radical has a rather low bond dissociation energy of 77.7 kcal/mols compared with the C-H bond dissociation energies of ca. 90-104 kcal/mol, it has been predictedI6 that hydrogen abstraction reactions from hydrocarbons by ground-state N(4S) atoms is expected to be slow a t room temperature. In contrast, it has been frequently reported that the characteristic afterglow was emitted during the reactions of actiue nitrogen with saturated hydrocarbons.”-20 It has been that the apparent reactivity of N atoms with hydrocarbons might be due to either excited species such as N2(A3Z:), N(2D), and N(2P) in active nitrogen or impurities such as 0, H, and other active species from the discharge. If any energetic species other than ground-state nitrogen atoms exist in the VLPR reactor, they should be capable of dissociating the C H bond in a saturated hydrocarbon, and a fast long-chain radical and atomic reaction involving N(4S) consumption could follow. These chain steps will give false information that ground-state N(4S) atoms cause the apparent reactivity with the saturated hydrocarbons. The N(‘S) atoms may not be solely responsible for the observed reactivity in this situation. Therefore, it is of great importance to confirm the absence of any energetic species other than N(4S) in our VLPR reactor. Many ~ t u d i e s ~ of ’ ~the ~ active nitrogen produced by microwave discharge have shown the presence of large concentrations of N(%) atoms. The vacuum UV absorption spectrum2 of the active nitrogen stream showed a strong absorption at 1200 A by N(4S) (15) Choo, K. Y.; Choe, M. H. Bull. Korean Chem. Soc. 1985, 6, 196. (16) Jones, W. E.; Winkler. C. A. Can. J . Chem. 1964,42, 1948. (17) Herzberg, G. Z . f h y s . 1928,49, 512. (18) Wager, A. T. fhys. Reu. 1943, 64, 18. (19) Radford, H. E.; Broida, H. P. fhys. Rev. 1962, 128, 231. (20) Radford, H. E.; Broida, H. P. J. Chcm. fhys. 1963, 38, 644. (21) Safrany, D. R. frog. React. Kfner. 1971, 6 . (22) Michael, J. V.; Lee, J. H. Chem. f h y s . Lorr. 1977.51, 303. (23) Westenberg, A. A.; detfaas, N. J . Chcm. fhys. 1964, 40, 3087. (24) Von Wessenhoff, H.; Patapoff, M. J . fhys. Chem. 1965,69, 1956. (25) Jackson, 0. S.; Schiff, H. 1. J . Chem. fhys. 1955, 23, 2333. (26) Berkowitz, J.; Chupka, W. A.; Kistiakowsky, G. B. J . Chem. fhys. 1955. 25. 451. (27) Tanaka, Y.; Innes, F. R.; Jursa, A. S.;Nakamura, M. J. Chem. fhys. 1965,42, 1183. (28) Young, R. A.; Sharpless, R. L.; Stringham, R. J . Chcm. fhys. 1964, 40, 256. (29) Foner, S.N.;Hudson, R. L. J . Chem. fhys. 1962, 37, 1662. ~
(14) Bagdal-Vayjoocc, M. H.; Colussi, A. J.; Benson, S.W. J . Am. Chcm. Soc. 1978, 100, 3214. Choe, M.H.; Choo, K. Y . Chem. fhys. Le#. 1982, 89, 281.
1284 The Journal of Physical Chemistry, Vol. 95, No. 19, 1991
atoms. However, the spectra also showed absorptions with very much weaker intensities at 1495 and 1745 A by N('D) and N(ZP), respectively. The relative concentrations of N(2D) and N(2P) atoms with respect to N(%) were found to be less than 1%. Negligible concentrations of excited N atoms downstream from the discharge were also shown by Foner et al.29in a mass spectrometric study of active nitrogen. A sample taken 2 ms downstream at a somewhat lower pressure (0.45 Torr) failed to show the presence of any excited atoms and suggested that they were rapidly deactivated at the walls of the flow tube. Using an optical absorption technique, Morse and Kaufman30reported that collisional deactivating efficiencies of excited N atoms with Pyrex wall were almost unity. The second-order rate constant of N(2D) with N2 at 300 K was found to be (1.6 f 0.7) X lo-'" cm3 molecule-' s-l. Besides excited-state nitrogen atoms, energetic species that are capable of initiating long-chain radical and atom reactions are metastable nitrogen molecules, vibrationally excited ground-state N2 molecules, and free char ed particles. The excited nitrogen molecule, N2(A3Z,), is known to be capable of dissociating the CH bond in hydrocarbons3' and reacting with several inorganic compounds32such as Cot, NH,, and H20. The energy of the N2(A31;:) molecule is 142 kcal/mol, and its radiative lifetime is very long. Therefore, this highly energetic metastable molecule has been regarded as a source of the reactivity for the reaction of the active nitrogen in many studies. It has been suggested that the metastable N2(A3Z:) molecule be produced either by a termolecular N atom recombination with the rate constant of about 10-3'-10-33 cm6 molecule-2 s-', or by the transition from highly excited N2(%:) produced in the dischargee2 The metastable N2 molecules presumably disappear through wall collisional deactivation (every collision with Pyrex Thrush" reported that the steady-state concentration of N2(A3Z:) molecules was about 6 X X [N][M] molecules/cm under the normal experimental conditions. Because the concentration of N atoms and total pressure in our VLPR reactor are about 10l2 molecules/cm3 and 10-3-10-s Torr, respectively, the steady-state concentration of N2(A3Z:) should be very small (
