P. Van
210
Khe, J. C. Soullgnac, and R. Lesclaux
Pressure and Temperature Dependence of NH2 Recombination Rate Constant Pham Van Khe, J.
C. Soullgnac, and R. Lesclaux'
Laboratoire de Chimie Physique A, Universtti de Bordeaux I, 33405 Talence, France (Received April 22, 1976; Revised Manuscript Received November 2, 1976) Publication costs assisted by Laboratoire de Chimie Physique, Universiti de Bordeaux I
The recombination rate constant of the NH2 radical is measured by flash photolysis of ammonia as a function of total pressure from 0.3 to 1000 Torr of nitrogen. The recombination kinetics is third order between 0 and 20 Torr and the relative third body efficiencies of ammonia, nitrogen, and argon, determined in this pressure range, are 4.0, 1.0, and 0.4, respectively. The termolecular rate constant is, in the case of nitrogen, JZla,N2 = 2.5(*50%) X 10" M" S-'. The high pressure limiting value of k l a ( m ) = 1.5(*50%) x 10'O M-' s-l is reached at about lo00 Torr of nitrogen. A low pressure limit k1(0) = 8.5(*50%) X los M-' s-l is found when the pressure tends to zero. The disproportionationreaction of amino radicals seems to be the principal reaction at pressures lower than 1 Torr. No significant temperature effect is found either near the high pressure limit or at the low pressure limit. A small negative temperature coefficient (-1 < E < -0.5 kcal/mol) is observed at 20 Torr of nitrogen, when the reaction is essentially termolecular. These results are discussed in connection with previous data concerning this reaction.
Introduction Data concerning the rate constant of NHz radical reactions are relatively scarce to date. The radical is known to be weakly reactive'-3 and among the NHz reactions, the recombination process NH, + NH, products (11 appears to be one of the fastest. As a result it often competes with other NHz reactions and it follows that reaction mechanisms involving the NHz radical cannot be completely elucidated unless the recombination kinetics is well known over a wide range of conditions. This is particularly true for the pressure dependence of the process
-
NH, t NH,
+ M-
N,H,
+M
(la)
The first evaluation of the recombination rate constant was made by Hanes and Bair4 using a pulsed radio frequency discharge in ammonia and simultaneously monitoring NH2 concentrations by visible absorption spectroscopy. They measured a value of 2.3 X lo9 M-' s-l at room temperature under pressures of 0.4-0.8 Torr. Later, Diesen5 found that around 10 Torr the recombination rate was pressure dependent and suggested that the rate constant measured by Hanes and Bair corresponded in fact to the following disproportionation process rather than to the combination reaction la: NH,
+ NH,
-
NH
+ NH,
Ub)
In a new investigation, Salzmann and Bair6 again found a pressure dependence of the NHz disappearance rate between 0 and 10 Torr and determined a low pressure limiting rate of 0.46 x IO9 M-l s-l that they attributed to process lb. The occurrence of this reaction at low pressure was shown by Mantei and Bair in a flash photolysis study of ammonia' while Gehring et al.879measured kla/klb = 4.7 X lo3 M-' (M = Ar) using a fast flow reactor coupled to an ESR spectrometer and a mass spectrometer. The first high pressure determination of the rate constant Itl was performed by Gordon et al.1° by pulse radiolysis of ammonia. They found that kl reached the high pressure limiting value of 6.2 X 10'O M-' s-l for 250 Torr of ammonia. Back and Yokota" obtained about the same value (4.7 X 1O'O M-I 5-l) in the same range of pressure.
t Equipe de Recherche Associee au CNRS No. The Journal of Physical Chemistv, Vol. 81, No. 3, 1977
167.
Despite these attempts, not enough attention has been paid to NH2 recombination reactions in order to be able to relate all the available data obtained at different pressure. It thus appeared to us necessary to have a complete determination of the kinetics of reaction 1 in a range of pressure continuously covering the falloff region. This is important not only to obtain precise data needed for a complete understanding of reaction mechanisms involving NH2 but also for a better theoretical approach to the forward and reverse processes. This paper reports measurements of the rate constant of reaction 1by flash photolysis of ammonia in the pressure range 0.3-1000 Torr, using NH3, Nz, and Ar as third bodies, and the temperature dependence between 300 and 500 K. One of the features of this study is that all the measurements are made using a single technique under all conditions.
Experimental Section Materials and Apparatus. Ammonia (99.96%) was dried over sodium and distilled at low temperature. Nitrogen (99.99%) and argon (99.99%) were used without further purification. SF6 (99.5%) was distilled at low temperature. The flash photolysis apparatus and the method for transient absorption measurements were described in detail in a preceding paper.I2 The apparatus was essentially composed of the following: a pulsed 150-W xenon arc as the analyzing light source; two flashlamps of 400 J emitting pulses of light with a duration of about 20 p s ; a multipath cell set for path lengths from 3 to 15 m; a monochromator selecting the strong NH2 absorption lines around 5977 A; a differential absorption measurement system allowing determination of light absorptions smaller than 5%. For such low absorptions and except for the lowest pressures, the measured optical densities are a linear function of NHz concentration in spite of the fact that the monochromator band width is broader than the absorption line.'2 Therefore no correction were necessary for pressures higher than 20 Torr to take into account deviations from the Beer-Lambert law. Figure 1 shows typical plots of the measured absorbance vs. the number of passes in the cell. A t low pressure, the correction was applied directly from these calibration curves. Kinetic measurements were made with absorbances smaller than 0.05.
21 1
NH2 Recombination Rate Constant
i
Absorbance
/
0.07
/
/
-
Optical path length(a.u)
2
4
8
12
Flgure 1. Measured NH2 absorbance as a function of the optical path length: (0)total pressure = 20 Torr: (X) total pressure lower than 5 Torr.
Second-Orders Kinetics Measurements. In all measurements performed, the NH2 disappearance kinetics is essentially second order. It was thus necessary to determine the absolute radical concentration or the apparent extinction coefficient of the absorption line. Two methods were used for this determination. First, the transient absorption of NH2 was measured by flash photolysis of 5 Torr of NH3 in the presence of 600 Torr of isobutane. Ammonia is dissociated according to hu
+H
NH,-NH,
(21
while hydrogen atoms react very fast (during the flash duration) and quantitatively with the isobutane in excess H + I'C,H,, -,H, + C,H, (3) The amount of molecular hydrogen produced by 100 flashes was measured by mass spectrometry and was related to the NH2 initial concentration. The amino radicals disappeared by various radical recombination reactions with a half-life of about 150 ps. No change in the NH2 absorption was found from the first to the last flash and H atoms could not react with the products of the reaction, given the large excess of isobutane. In the second method 5 Torr of ammonia was flashed in the presence of 0.04 Torr of NO and 700 Torr of nitrogen. The reaction of NH2 + NO being very fast, the following reactions were assumed to take place12 NH, + N O N, t H,O H + NO + M - HNO + M 2 H N O - N,O + H,O -+
Two NO molecules were thus consumed for one NH2 radical produced. By measuring the increase of the NH2 lifetime (about 5%) as NO is consumed from one flash to the other it was possible to relate the initial NH2 concentration to the initial NH2 absorption. Both methods gave similar values, resulting in an apparent extinction coefficient t of 330 f 50 M-' cm-' for our particular experimental conditions. The question arises here as to the extinction coefficient variation with pressure due to the pressure broadening of NH2 absorption lines. In fact, the apparent extinction coefficient was found constant over the range 0.3-1000 Torr because the spectral width of the analyzing light (1.2 A) is much broader than the NH2 absorption lines (the NH2 absorption was actually due to a group of three rotational lines around 5977 AI3). Thus the absorption
Figwe 2. Recombination rate constant kl of NH2 radicals as a function of nitrogen pressure.
measured was not really proportional to the extinction coefficient value at a particular wavelength, but rather to the oscillator strength of the lines which is constant with pressure. It should be noted that the measured apparent extinction coefficient is independent of line broadening only for low absorption (i.e.,
