The rates of the elementary reactions of NH(a1 Δ) with NH3(X) and HN3(X). The temperature dependences

Article abstract of DOI:10.1524/zpch.219.2.197.57303

The reactions NH(a) + HN₃(X) → prod. (1) and NH(a) + NH₃(X) → prod. (2) were studied in a quasi-static reaction cell in the temperature range 293 ≤ T/K ≤ 501 at a pressure of 10 mbar and 20 mbar, respectively, with He as the main carrier gas. The electronically excited reactant NH(a) was generated by laser-flash photolysis of HN ₃, at λ = 308 nm and detected by laser-induced fluorescence (LIF). Also the ground state species NH(X) was detected by LIF. From the measured concentration-time profiles of NH(a) under pseudo-first order conditions, the rate coefficients k₁(T) and k₂(T) were obtained. For the rate coefficient k₁ a positive temperature dependence was observed: k₁(T) = (8.1 ± 0.5) × 10 ¹³ exp[(-0.76 ± 0.05)/RT] cm³/mol s with a small activation energy of E_A = 0.76 kJ/mol. For reaction (2) the rate coefficient k₂(T) = (8.6 ± 0.6) × 10¹³ (T/298)^-(0.6±0.1) cm³/mol s with a negative temperature dependence was measured indicating that the intermediate N ₂H₄, which decomposes to 2NH₂, is formed without a barrier. The rate of the quenching channel NH(a) + NH₃ → NH(X) + NH₃ (2q) has the same temperature dependence as the rate of the overall reaction (2). The contribution k_2q/k₂ = 0.008 over the temperature range 293 ≤ T/K ≤ 501.

Full text of DOI:10.1524/zpch.219.2.197.57303

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Z. Phys. Chem. 219 (2005) 197–211
by Oldenbourg Wissenschaftsverlag, München
The Rates of the Elementary Reactions of
1
˜
˜
NH(a ) with NH3(X) and HN3(X).
The Temperature Dependences
2
By L. Adam1, W. Hack1, , and M. Olzmann
∗
1
Max-Planck-Institut für biophysikalische Chemie, Am Fassberg 11, 37077 Göttingen,
Germany
2
Institut für Physikalische Chemie, Universität Karlsruhe, Kaiserstr. 12,
76128 Karlsruhe, Germany
(Received September 9, 2004; accepted in revised form November 8, 2004)
Kinetics / Laser-Photolysis / Laser-Induced Fluorescence /
Complex-Forming Bimolecular Reaction
˜
The reactions
and
NH(a)+HN3(X) → prod.
(1)
(2)
˜
NH(a)+NH3(X) → prod.
were studied in a quasi-static reaction cell in the temperature range 293 ≤ T/K ≤ 501
at a pressure of 10 mbar and 20 mbar, respectively, with He as the main carrier gas.
The electronically excited reactant NH(a) was generated by laser-flash photolysis of HN3,
at λ = 308 nm and detected by laser-induced fluorescence (LIF). Also the ground state
species NH(X) was detected by LIF.
From the measured concentration-time profiles of NH(a) under pseudo-first order
conditions, the rate coefficients k1(T) and k2(T) were obtained.
For the rate coefficient k1 a positive temperature dependence was observed:
k1(T) = (8.1 0.5)×1013 exp[(−0.76 0.05)/RT]cm3/mol s
with a small activation energy of EA = 0.76 kJ/mol.
For reaction (2) the rate coefficient
k2(T) = (8.6 0.6)×1013 (T/298)−(0.6 0.1) cm3/mol s
with a negative temperature dependence was measured indicating that the intermediate
N2H4, which decomposes to 2 NH2, is formed without a barrier.
*
Corresponding author. E-mail: whack@gwdg.de
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L. Adam et al.
The rate of the quenching channel
NH(a)+NH3 → NH(X)+NH3
(2q)
has the same temperature dependence as the rate of the overall reaction (2). The
contribution k2q /k2 = 0.008 over the temperature range 293 ≤ T/K ≤ 501.
1. Introduction
The gaseous detonation of hydrazoic acid HN3 and the decomposition mech-
anism of HN3 have been studied experimentally for many years [1–3]. For the
detonation propagation models the knowledge of the chemical kinetics is of
vital importance. One of the key reaction in HN3 decomposition mechanism
is the reaction of NH(a1∆) with HN3 [3]. The rate coefficient and in particu-
lar its temperature dependence is thus essential for the understanding of the
detonation structure. Moreover, HN3 is often used as a precursor for the pho-
tochemical production of NH(a) and NH(X). Also in the description of these
systems the rate coefficient of the reaction
NH(a)+HN3 → prod.
(1)
should be known. Reaction (1) has been intensively studied both experimen-
tally [4, 5] and theoretically [6] and is assumed to essentially produce vibron-
2
˜
ically excited NH2(A A1) [7, 8], the chemiluminescence of which was used
to study the kinetics [5, 9, 10]. To our knowledge the temperature dependence
of the rate coefficient, however, has not yet been determined experimentally.
In a laser-photolysis experiment of HN3 at low pressure [5], by detecting the
2
˜
visible emission of NH2(A A1), the rate coefficient of reaction (1) was ob-
served to depend on the photolysis wavelength. The rate was found to be lower
at λph = 193 nm than at λph = 266 nm. An ab initio MO investigation on the re-
activity of the NH(a) radical in the bimolecular abstraction gas-phase reaction
with the HN3 molecule predicts a barrier height of 12.1 kJ/mol [6].
The reaction of NH with NH3 is of importance in ammonia and hydrazine
pyrolysis and also in the combustion kinetics of N-containing fuels. The re-
action of NH(X), i.e., NH in the electronic ground state, has a high activation
energy of EA = 112 kJ/mol [2], whereas the reaction of the electronically ex-
cited NH(a1∆),
1
˜
NH(a ∆)+NH3(X) → prod.
(2)
has a rate coefficient near the collision limit already at room temperature. The
rate coefficient for the reaction of the next higher singlet state, NH(b1Σ+),
however, is again two orders of magnitude smaller than k2 at room temperature.
Its temperature dependence is complicated probably due to the competition
between a direct and a complex-forming mechanism [11].
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˜
˜
The Rates of the Elementary Reactions of NH(a ∆) with NH3(X) and HN3(X) . . .
199
Reaction (2) is assumed to proceed via highly vibrationally excited
N2H4 in the electronic ground state [12, 13]. The rate coefficient was de-
termined at room temperature [14–16], but its temperature dependence has
not yet been investigated. We note that the isoelectronic radical CH2(a˜) re-
acts with NH3 in an insertion reaction with a rate coefficient k = 1.7×1014
(T/295 K)−1.2 cm3/mol s in the temperature range 210 ≤ T/K ≤ 475 [17, 18].
2. Experimental
The experiments were performed in a quasistatic laser-flash photolysis/laser-
induced fluorescence (LIF) system, where “quasistatic” means that the flow
through the reaction cell is negligible between the pump and the probe pulse
but sufficient to exchange the gas volume between two subsequent pump
pulses. The carrier gas was He at a total pressure of 10 mbar and 20 mbar,
respectively.
The temperature was varied in the cell via a copper inset with four perpen-
dicular openings for the laser beams and the observation of the laser-induced
fluorescence. The temperature was varied in the range 293 ≤ T/K ≤ 501 and
measured by a resistance thermometer with an accuracy 0.1 K.
The experimental set up is described in detail elsewhere [19], and only
the essentials are repeated here. For the photolysis, a XeCl-exciplex laser
(Lambda Physik LPX 205) with pulse energies in the range 200 ≤ E/mJ ≤
400 and a beam area of about 1.1 cm2 was used. The probe laser was
a dye laser (Lambda Physik FL 3002) with a beam area of 7 mm2. It
was pumped by an exciplex laser (Lambda Physik LPX205, XeCl; 230 ≤
E/mJ ≤ 290).
˜
˜
The NH(a) radicals were produced by HN3 photolysis in the A−X band
at λ = 308 nm with a rotational excitation significantly above room tempera-
ture. The rotationally hot population, however, relaxes to a thermal distribution
mainly by collisions with He on a µs time scale.
NH(a, v = 0) was detected by exciting the P3 line at λ = 326.22 nm of the
transition c1Π, vꢀ = 0 ← a1∆, vꢀꢀ = 0. The undispersed fluorescence from the
excited state was observed in the wavelength range 325 ≤ λ/nm ≤ 328 per-
pendicular to the laser beam using filters (long pass filter KV370 (Schott) or
interference filter 326.3 nm (Schott)) to suppress scattered radiation from the
excitation beam.
Gases with the highest commercially available purity were used: He,
99.9999%, Praxair; NH3, 99.998%, UCAR; and N2, 99.995%, UCAR. HN3
was synthesized by melting stearic acid, CH3(CH2)16 COOH, with NaN3. It
was dried with CaCl2 and stored in a bulb at partial pressures ≤ 200 mbar
diluted with He (overall pressure ca. 1 bar). For safety reasons, the HN3 con-
taining devices were covered with a wooden box since HN3 is highly explosive
even at low pressures.
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L. Adam et al.
3. Results and discussion
The reactions (1) and (2) were studied under pseudo-first order conditions with
the precursor hydrazoic acid and ammonia in large excess over the imino radi-
cals. In this system the NH(a) radicals also react with the precursor molecules,
NH(a)+HN3 → products
(1)
and, hence, the depletion of NH(a) has to be described by:
−d[NH(a)]/dt = k1[NH(a)][HN3]+k2[NH(a)][NH3]+kq[NH(a)][He] .
(I)
Since all terms in Eq. (I) are first-order with respect to the imino radical, they
can be combined to give:
−d[NH(a)]/dt = keff[NH(a)] .
(Iꢀ)
In general, the physical quenching of NH(a) by He can be neglected, be-
cause the rate kq < 6×108 cm3 mol−1 s−1 [21] is too small. To determine the
rate of the NH(a) + HN3 reaction, experiments without NH3 were performed
and Eq. (I) reduces to:
−d[NH(a)]/dt = k1[NH(a)][HN3] = keff[NH(a)] .
(II)
The rate coefficient keff is determined from plots of ln(I(t)/Io) versus reac-
tion time, where I(t) is the fluorescence intensity due to NH(a) at a given
time t, and Io the fluorescence intensity at t = 0. The experimental details
are summarized in Table 1. The HN3 concentration was varied in the range
1.2 ≤ [HN3]/10−10 mol/cm3 ≤ 10. In all cases straight lines keff vs. [HN3] were
obtained, and from the slopes of these lines k1 was determined; the results are
also contained in Table 1. The room temperature rate coefficient of reaction (1)
has been previously determined by several groups [8, 9, 19–22, 28]. The values
vary in the range from 5.6×1013 up to 11×1013 cm3/mol s. The value we ob-
tained, k1(293 K) = (5.9 0.5)×1013 cm3/mol s, is in good agreement with
the results given in some of the more recent publications. In the present work,
however, we emphasize the temperature dependence of k1. The rate coeffi-
cient slightly increases with increasing temperature, and from the plot shown
in Fig. 1, the following Arrhenius equation is obtained:
k1(T ) = (8.1 0.5)×1013 exp[(−0.76 0.05)kJ mol−1/RT] cm3/mol s .
This is the first direct determination of the temperature dependence of the
rate coefficient k1.
In a laser photolysis experiment by Watanabe et al. [5], HN3 was decom-
posed at very low pressures (p = 40 µbar with no buffer gas) and two different
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