Symmetric Atom
The Journal of Physical Chemistry, Vol. 83, No. 1, 1979 69
Exchange Reactions
Symmetric Atom Exchange Reaction CI' 4- HCI. An Arduous Task for Theory and Experimentt M. Kneba" and J. Wolfrum Max-PLenck-lrtstitut fur Stromungsforschung, Abteilung Reaktionskinetik, D 3400 Gatti'ngen, Federal Republic of Germany (Received June 28, 1978) Publication costs assisted by Max-Planck-Institut fur Stromungsforschung
Reaction rate constants for the hydrogen atom exchange reaction C1 + HCl(u = 0.1) have been determined using a pulsed HC1 chemical laser for isotopically selective vibrational excitation of HCl and time-resolved nozzle-beam mass spectrometry, infrared fluorescence, and atomic line resonance absorption for quantitative detection of the reactants and products. The thermal rate for 37Cl+ H3Tl H37Cl+ 35Cl(1)was measured to be k l = (2.5 Z!C 1.5) X lo9 cm3/mol.s at 358 K. The reaction rate increases by more than a factor of lo3 after vibrational excitation. For 37Cl+ H3%l(u = 1) H37Cl(u= 0,l) t 35Cl(8) a rate of ha = (6.6 k 4.0) X 1OI2 cm3/ moles at 298 K and an Arrhenius activation energy of (l0.8?$ ltJ/mol was obtained over the temperature range 250-358 K. The rate of vibrational relaxation of HCl(u = 1) by C1 atoms was measured as hz = (3.7 f 0.8) :< 10l2cm3/mol-sat 298 K. For the V-E energy transfer process HCl(u = 1) + C1(2P312) HCl(u = 0) + C1(2P112)(3) the rate was found to be k 3 = (5.0 f 2.0) X 1O1O cni3/mol.s at 298 K.
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I. Introduction Numereous attempts have been made in recent years to measure and to describe quantitatively the dynamics of simple exchange reactions in the hydrogen-halogen system, e.g.
+ H3jC1
37Cl
-
H37Cl+ 35Cl
(1)
These reactions provide an important testing ground for experiments and theoretical calculations on thermal reaction rate constants. They are also interesting model systems for investigations on the competition between energy transfer processes and chemical reactions under nonequilibrium conditions. In the present and a following paper new experimental investigations on the kinetics of the reactions C1' HCl(u) and H' HCl(U)l using selective vibrational excitation of halogen halides by chemical lasers and time resolved I13 fluorescence, resonance absorption in the vacuum 'IJV, and nozzle beam mass spectrometry will be described. In 1964 Klein, I'ersky, and Weston2 measured the thermal rate of the reaction C1' HC1 by using the radioactive 36CIisotope and the C1 + Dz system as a reference reaction in a competitive method. With more direct measurements for the reference reaction3 one derives from their data an Arrhenius activation energy of 25 kJ mol-'. However, as shown in Figure 1, quite contradictory evidence on the potential energy surface was obtained from the assignment of infrared spectra observed in matrix isolation studies by Noble and Pimente14 to the HC1, molecule. Similar spectra were assigned to matrix isolated HBr2 and These conclusions have been supported by the results of BE(B0calculations6on the properties of X-H-X (X .- C1, Br, or I) species. In the case of C1-H-C1 a well depth of 6.5 kJ mol-' and a bond strength of 12.6 k J mol-l were predicted. On the other hand, calculations using the LEPS method' and more recent ab initio calculations do not agree with these results. A barrier height of 70 kJ mol-l has been calculated for the collinear C1' HCl Cl'H + C1 exchange reaction by means of the coupled electron pair approximation CEPA. Taking into account the remaining correlation errors in this calculation
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Dedicated to Professor Dr. Dres. 1i.c. W. Jost on his 75th birthday. 0022-3654/79/2083-0069$01 .OO/O
a reduction of the barrier height down to 8-30 k J mol-' is predictedas Clearly, the calculations are still not accurate enough to derive a precise value for the Arrhenius activation energy of 1;he C1' HC1 reaction. However, it seems unlikely that the symmetric C1-H-Cl complex is stable with respect to dissociation into HC1 and C1. Also the original interpretation of the matrix isolation spectra of HClz has been questioned by Milligan and Jacoxg and more recently by Ault and Andrews.'O They conclude that the species observed in the earlier studies was the isolated HC1; anion. Further information on details of the potential energy surface of the C1-H-Cl system can be obtained by observing the behavior of vibrationally excited HC1 molecules in the presence of C1 atoms. The open electronic shell as well as the spin-orbit components of the ground state chlorine atom provide several effective pathways for vibrational energy transfer in electronically adiabatic and nonadiabatic col1isions.l' Conflicting values of 10l2 and lo1' cm3 mol-l 3-l were first reported for the vibrational deactivation rates h2 of HCl(u = 1)by C1 atoms a t room temperature12J3
+
HCl(u = 1) + C1---* HCl(U = 0)
+ C1
(2)
More recent measurements support the larger value.I4 The experimental data can be compared with the results of classical trajectory studies on various LEPS potential surfaces. Calculations using reaction barrier heights from 40 to 25 kJ mol-' predict vibrational relaxation rate coefficients much smaller than the experimental data. In contrast, calculations employing surfaces with a small potential energ:y well are in better accord with the experiment~.'~The best agreement with the experimentally observed rates was obtained with a barrier height around 4 kJ More direct experimental information is seriously needed. Beside this aspect such experimental investigations are of basic interest for a number of practical applications. The effective vibrational deactivation by atoms represents a large loss of potential power in chemical laser systems and an important elementary step in the quantitative description of nonequilibrium situations in flames, electrical discharges, and laser induced chemical reactions. 0 1979 American Chemical Society
70
The Journal of Physical Chemistry7 Vol. 83,
No. I , 1979
M. Kneba and J. Wolfrum ne c12
THEORY I__
ff\ ,/ \
Meyer et ai CEPA ob initio c a l c u l o l i ~ n
f2
EXPERIMENT
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IR D E T E C T O R U
THERMOSTATE 4 -
f;:o
CI
pL)MpS
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C I ' *(v-0,
CL'H [ v z O ) + CI
-___ Pimentei et ai tic12 r n O l l l X 1501at110"
Trublar et ai .BE BO calculation
CI'
H
CI
Figure 1. Experimental and theoretical results for the potential energy profile of the symmetric CI f HCI exchange reaction. He C12
He CI 2
'
HCL
I
u
l
a
I
b 4 l"1 :',IR-DETECTOR A
MWD ,
1
PUMP
Figure 2. Schematic of the discharge-flow system with laser-induced infrared fluorescence detection
11. Experimental Section The experiments were performed using the arrangements shown in Figures 2 and 3. They consist of an HCl chemical laser and two different flow systems. System A is a laser excited infrared fluorescence cell similar to the one described in ref 17. System B is a thermostated discharge flow reactor (10 mm i.d.1 coupled by a molecular beam sampling system to a quadrupole mass spectrometer for the direct detection of laser-induced reaction products.ls A s pulse of laser radiation from an HC1 chemical laser is used for the production of HCl(u = 1) in a flowing mixture of He, HC1, Cl,, and C1 atoms. The laser consists of a plexiglass tube of 2 m active length with CaF, Brewster's angle windows. Laser action is initiated by a transverse electrical discharge (20 J) switched by a thyratron over 1600 tungsten pin electrodes (four rows equally spaced) electrolytically coupled by a CuSO, solution in a flowing Hz-C12 m i x t ~ r e The . ~ ~optical ~ ~ ~ cavity is formed by a 3-m radius gold-coated mirror and a Ge flat (50% transmission) 2.5 m apart. With a 1 O : l H,-Cl, mixture (total pressure 70 torr) the laser delivers 10 mJ/cm2 at a repetition rate of 1Hz. At the entrance window to the flow tube (distance to the Ge flat 3 m) the laser beam has a nearly rectangular profile of 25 X 25 mm power half-width. The HC1 infrared fluorescence was monitored as a function of time using a liquid nitrogen cooled InSb photovoltaic detector (Texas). The detector signal is amplified and averaged over typically 100 laser shots on a Datalab DL 102 signal averager. The signal-to-noise of all fluorescence signals was > 10 for PHCl > 10 mtorr. The exponential decay times of the infrared fluorescence were determined by comparison with a calibrated RC unit. The u =2 1 fluorescence is separated by a cold gas HC1 filter in front of the InSb detector. The fraction of laser excited HC1 molecules, determined by comparing the intensities of the u = 1 -* 0 and u = 2 1 fluorescence,17 varied
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PUKP
H C l - LASER
H2IC12
Figure 3. Schematic of the experimental arrangement for time-resolved mass spectrometric detection of laser-induced reaction products
between 5 X 10 and 10 depending on the laser intensity. For direct observation of the laser-induced reaction products the flow tube (Figure 2 ) is coupled to a quadrupole mass spectrometer by a molecular beam sampling system. During the experiment the intensity of a certain mle peak is followed as function of time starting with the onset of the laser pulse. A signal is obtained during the transit time of the volume excited by the laser pulse (10 mm diameter, 150 mm long) passing the sampling nozzle. With a time resolution of 10 bs the detection limit forH35 C1 ( m / e 36) is 5 X 10" molecules ~ m - ~ . C1 atoms are produced in the flow tubes by dissociating (up to 80%) Clz diluted in He in an electrodeless microwave discharge (2.45 GHz, 200 W). The chlorine atom concentration in the flow tube was determined by titration with NOC121,22 using the C1 atom recombination emission as end point indicator or by monitoring the Clz concentration at the mass spectrometer with the microwave discharge on and off. The discharge flow systems were coated with metaphosphoric acid to minimize surface catalyzed atom recombination. The He flow was measured with a calibrated capillary flow meter. All other gas flows were determined by measuring the pressure increase in a calibrated volume. The total pressure in the flow tube (typically around 3 torr) was measured with a calibrated Wallace and Tiernan vacuum gauge. The flow velocity in the flow tubes varied between 5 and 20 m/s. Gases of the best commercial purity were used. He (99.9996%) was obtained from Messer Griesheim. The other gases were supplied by Matheson, HCl (99.99%), C12 (99.965%), and NOCl (97%). All gases were taken from cylinders without further purification. Further experimental details can be found in ref 17 and 23.
111. Deactivation of HCl(v = 1) by C1 Atoms The rate constant k 2 for the vibrational deactivation of HCl(u = 1)by C1 atoms was determined by subtracting the exponential decay constants of the HCl u = 1 0 fluorescence with and without atomic chlorine present in the system. Semilog plots of the fluorescence intensity against time were linear over almost two decades. Experimental conditions and results are shown in Figure 4. The subtracted decay constants vary linearly with the C1 atom concentration and give a value of kz = (3.7 f 0.8) X 10l2 c1n3 mol-l s-l a t 298 K. The vibrational deactivation constant obtained here is in good agreement with measurements of Arnoldi and Wolfrum17 but about 30% lower than values obtained in Moore7sl4and Smith's', laboratories. It is unlikely that this small discrepancy is caused by calibration errors of the discharge flow method. All authors have very carefully +
The Journal of Physical Chemislry, Vol. 83, No. 1, 1979 71
Symmetric Atom Exchange Reactions
t AB
t
I
0 LASER
10 [ms]
[mol cm-31
Figure 5. Mass-spectrometric detection of H3'CI formed in reaction 8: P, = 5.6 torr, [HCI], = 8.1 X lo-' (mol/cm3), [HCI(v= l)lo= 6.3 X (mol/cm3),[CI], = 2.4 X to9 mo1/cm3. T = 358 K.
I 0
0
0.5
1.0
1.5
25
2.0
3.0
mol cm"i
CI-At~m-Concentration
Figure 4. Observed HCI(v = 1)decay rates as a function Of W chlorine
atom concentration: P, = 2.9-3.4 torr, [HCI], = 1.04
