THERMAL DECOMPOSITION OF NITRONIUM PERCHLORATE
drogen bonds between the solvent and the peptide carbonyl seem to be important for maintaining the balance of forces in favor of a stable solution form.
4077
Acknowledgment. The authors wish to thank Mrs. Mary Clemmer for her assistance with the experimental work and the preparation of the manuscript.
The Thermal Decomposition of Nitronium Perchlorate1
by J. N. Maycock and V. R. Pai Verneker Research Institute for Advanced Studies (RZAS), Martin Marietta Corporation, Baltimore, Maryland dld.97 (Received May 8, 1967)
The thermal decomposition of nitronium perchlorate has been studied in the temperature range 1O0-16O0 by isothermal constant-volume techniques and by mass spectrometry. Kinetic analyses have been performed for all the major decomposition species, e.g., Oz NO, and Clz. The activation energy is found to be 15 f 1 kcal mole-'. This is found to be in good agreement with the activation energy derived from E = hueo/€ where hv is the absorption edge and eo and E are the high- and low-frequency dielectric constants of the solid.
Introduction
ClO4'
The solid-state chemistry of the perchlorates is very important owing to their effectiveness as solid oxidizers. Considerable information is available relating to the metallic perchlorates,28but the only nonmetallic perchlorate which has received considerable attention is ammonium Another nonmetallic perchlorate of interest is nitronium perchlorate, whose decomposition between 70 and 112" has been studied by Cordes' and at 65" by Marshall and Lewis.6 The kinetic analysis performed by Cordes fitted the MampeP theory of solid-state decompositions remarkably well. As a result of this kinetic analysis he postulated that the rate-controlling step in the decomposition was the transference of the anionic electron to the nitronium ion with subsequent gas phase reactions to produce NOz, Clz, CIOz, NO&l, and Oz. This can be represented by ClOa- +c104' e-
+ NOZ+
+ eNOZ
with the subsequent gas phase reactions being
----)
CIOz
0 2
+ ClOz +NO3 + OC1 OC1 + NO2 +NO&l Clz + 20c1
NOz
--t
0 2
The isothermal decompositions of Marshall and Lewis have been interpreted such that nitronium perchlorate (NOzC104) decomposes into nitrosonium perchlorate (NOCI04) and oxygen with subsequent decomposition of the nitrosonium perchlorate. (1) Supported by the U. 8. Army Missile Command, Huntsville, Ala., Contract No. DA-01-021-AMC-l2596(Z). (2) (a) R. D. Stewart, "Perchlorates," J. C. Schumacer, Ed., American Chemical Society Monograph, Reinhold Publishing Corp., New York,N. Y.,1960,p46; (b) L. L. Bircumshaw and T. R. Phillips, J . C h a . Soc., 4741 (1957). (3) A. K. Galway and P. W. M. Jacobs, Proc. Roy. Soc. (London), A254, 455 (1960). (4) H. F. Cordes, J . Phys. Chem., 67, 1693 (1963). (5) M. D. Marshall and L. L. Lewis, Advan. Chem. Ser., 54, 82 (1966). (6) K.L. Mampel, Z . P h y d k . Chsm., A167, 235 (1940).
Volume 71, Number l d
November 1967
4078
J. N. MAYCOCK AND V. R. PAIVERNEKER
+ '/zOz NOz + ClOz (Clz + +
NOzC104 ----f NOClOI NOClOi
----+
02)
Both of these reaction schemes are derived from analysis of the condensed decomposition products and both investigators have observed remarkably large induction periods. I n this paper, results will be presented for the decomposition of NOzC104 whereby the experiments have been performed in a conventional closed volume system and also for time-of-flight mass spectrometric direct inlet decompositions such that the gaseous products formed are continuously being analyzed. The kinetic analyses have been performed using both the ProutTompkins equation' and the more general kinetic analysis technique of Jacobs and Kureishy.8
Experimental Section Commercial grade nitronium perchlorate (Callery Chemical Co.) was used initially in the as-received form for some exploratory runs. However, the irreproducibility of the isothermal data with this material necessitated the standardization of all future material. The problem was undoubtedly the extreme hygroscopicity of the nitronium perchlorate. To eliminate this problem, all transference operations of material from the stock batch to the sample containers were performed in a drybox having a stream of pure, dry nitrogen passing through it. Even with this precaution it was found by differential thermal analysis (D.T.A.1 that all samples invariably absorbed a certain amount of water. But by using D.T.A. and mass spectrometric analyses it was found that complete removal of water and products of hydrolysis could be accomplished by pumping on each individual sample for a period of 18 hr in a vacuum line maintained a t lo-* torr. Samples undergoing this treatment then gave reproducible data. The isothermal decompositions were studied by means of a conventional constant-volume high-vacuum apparatus previously des~ribed,~ the course of the reaction being followed by an ionization gauge and a Pirani gauge. Simultaneously, the course of the reaction was followed by bleeding the gaseous products through a Varian 951-5100 adjustable leak valve having a minimum leak rate of 1 X torr I./sec into a Consolidated Electrodynamics 21-613 residual gas analyzer. Using this technique we could either run complete mass sweeps or, as was more often the case, its growth as gate On the Oxygen) O2, peak and function of time. The use of the residual gas analyzer enabled US to determine the molecular species formed during decomposition' Owing to the of this decomposition we have also followed the The Journal of Physical Chemiatry
isothermal decompositions with a Bendix Model 14 time-of-flight mass spectrometer where we have been able to gate simultaneously on five different chemical species as a function of time. Again all samples underwent the pumping treatment prior to the isothermal decomposition data being obtained. Specifically we constructed a very simple, all-glass vacuum decomposition line which was attached to a variable leak on the inlet port of the Bendix mass spectrometer. The arrangement is shown diagrammatically in Figure 1. The actual decomposition line was maintained a t a pressure of approximately torr and the main flight tube a t lo-' torr. At each experimental constant temperature, e.g. loo", the preliminary procedure was to take a complete mass spectrum every minute for the complete duration of the run, the mass spectra being recorded by a Honeywell Visicorder. As a further check the spectrum was continuously displayed on an oscilloscope. All actual recorded runs were performed using an ionization voltage of 65 eV. Having established the fundamental mass spectrum, we then repeated the isothermal decompositions by gating simultaneously and continuously on five different mass species. This procedure allowed the establishment of the partial pressure vs. time curves for any spectral species. These isothermal decompositions have been studied in the temperature range 100-160'.
Results After the elimination of the instrument background species, a typica1 mass analysis of the total decomposition products is given in Table I.
Figure 1. A diagrammatic display of the constant-volume apparatus connected via a variable leak to the Bendix time-of-flight mass spectrometer.
(7) E. G . Prout and F. C. Tompkins, Trans. Faraday Soc , 40, 488 (1944). (8) P. W. M. Jacobs and A. R. T. Kureishy, J . Chem. SOC.,910, 4718 (1964). (9) J. N. Maycock, V. R. Pai Verneker, and L. Witten, J. Phys. Chem., 71, 2107 (1967).
THERMAL DECOMPOSITION OF NITRONIUM PERCHLORATE
4079
Table I : Mass Species from the Thermal Decomposition of Nitronium Perchlorate at 140" Peak height, current X 109 d e
amp
14 16 28 30 32 35 37 46 51 53 67 70 72 74
1.6 10.2 0.2 13.3 17.8 7.6 2.8 5.5 1.7 0.8 1.4 6.9 4.8 1.2
Identification
The identification of the species has been based on the precalibrated cracking patterns for Clz, NP,0 2 , NO, NzO, NOz, and the oxides of chlorine. An important feature of these products is that we did not find any evidence of NOaCl as suggested by the Cordes' mechanism and also no evidence of the parent or fragments of NOC104 as suggested by the low-tempera ture study of Marshall and Lewis.6 From Table I and the cracking patterns mentioned above, it is apparent that the only gaseous decomposition products are 0 2 , CIS, OC1, C102, NO, and NO2. All the pressure-time curves, Figure 2, both obtained in the conventional system and by the mass spectrometer gating method exhibited the usual sigmoid characteristics as observed by Cordes.' In the temperature range investigated the residue was either very small (-5%) or nonexistent above approximately 120". The kinetic data produced from the time-of-flight mass spectrographic study were initially in the pressure vs. time form since we gated on the Oz, Clz, and NO peaks simultaneously as a function of time. These data have been reduced into a fractional decomposition (a)as a function of time form. Kinetic analysis of the a-t data has been tried using the Pro~t-Tompkins~ equation. As will be seen from Figure 3 a plot of log a/(l - a) vs. time gives a reasonably good straight line. This type of analysis has been used for Oz, Clz, and NO over the temperature range 100-160". From these plots the rate constants k have been calculated and are presented for the three decomposition species as a function of temperature in Figure 4. By inspection of this figure it is apparent that the data for all three species agree reasonably well if possible experi-
"
o
i
2
3 4 5 TIME (minutes)
6
7
Figure 2. Pressure-time plot for the production of oxygen (o), chlorine (O), and nitric oxide ( 0 )from the thermal decomposition of nitronium perchlorate at 160".
2.t 0
,
,
4
8
l
12
,
16 20 TIME (MINS)
,
,
,
24
28
32
42.0
Figure 3. Log CY us. t analysis (0)and log a / ( l - CY) us. t analysis (0) for the production of Ozfrom the thermal decomposition of nitronium perchlorate at 120".
mental error is accounted for. A line drawn through these points gives a value of 14.3 kcal mole-' for the activation energy of thermal decomposition of nitronium perchlorate. Volume 71, Number 18 November 1067
4080
J. N. MAYCOCK AND V. R. PAIVERNEKER
+ e- (conduction band) C102 + O2 solid phase + NOz+ -+ NOz
c104- +C104' C104'
4
e-
The resultant gases, NOz, 0 2 , and CIOz then undergo possible gas phase reactions in the following manner ClOz (3
+ NO2
33.0
NO
20c1+
0 2
+ + C10 + Clz 0 2
whereby the following complete reaction scheme can be written as
+ Clz + 2N0
2NOzC104 = 502
I
I
2.20
230
1
2.40
I
I
I
250
2.60
2.70
1
1 0 3 1 ~( O K - ' )
Figure 4. Relationship between log k (rate constant) and 1 (A),C4 (O), and NO (0). 108/T (OK-') for 0
The major difficulty in using the above analytical method is the uncertainty in the applicability of the Prout-Tompkins equation? to the experimental data. We have therefore reanalyzed the oxygen data using the Jacobs-Kureishy technique8 whereby the rate equation can be expressed in the general form f(CY)
= kt
k being the rate constant and CY,, CY,+I values of the fractional decomposition (a)at times t, and tn+1. Thus, one sees that F(a,+d
- F(a,)
=
W,+l
- t,)
and for a different temperature, but the same values of a then
F(a,+d
- F(%)
= k'(t,+1'
- in')
and hence a plot of log (t,+' - t,) against 5"-l should be linear with a slope of 2.303R. The data for the production of oxygen have been analyzed in this manner and gives an activation energy of 16.12 kcal mole-'.
Discussion After careful consideration of various molecular cracking patterns to be expected from the species produced during a thermal decomposition of nitronium perchlorate, it is apparent that the main species are 02, NO, and Cl,. We would like to propose the following, possible decomposition reactions which take place in the thermal degradation of nitronium perchlorate. The Journal of Phusicd Chemistry
The traces of OC1 and ClOz as observed mass spectrometrically are undoubtedly intermediate products as shown in the proposed mechanism for the decomposition of nitronium perchlorate. Owing to the complexity of the decomposition it is felt that the Jacobs-Kureishy* treatment of the kinetics is more valid than the approach using the Prout-Tompkins' equation. A survey of the activation energies obtained by these two different approaches is given in Table 11. ~~
~
~
~~
Table II : Activation Energies (Least-Squares Calculation) for the Thermal Decomposition of Nitronium Perchlorate
Oadata
1. Jacobs-Kureishy method E = 16.12kcal mole-'
Oa data C1, data NO data
2. Prout-Tompkins method E = 16.11 kcal mole-' E = 14.43kcal mole-' E = 15.14kcal mole-'
The activation energy for all these products is the same, L e . , 15 h 1 kcal mole-'. Cordes14on the other hand, finds the activation energy to be 26 kcal mole-'. This difference is too great to be an experimental error. Upon replotting the Cordes data, using the JacobsKureishy type of analysis, we find that his log t us. 1/T data exhibit a break at about 100" such that the activation energy above 100" is of the order of 15 kcal mole-' and below 100" it is of the order of 25 kcal mole-'. Hence the observed divergence of data appears to be due to a possible wrong usage of the Mampel theory. An estimate within 10% of the activation energy can be made from the relationship E = hveo/e where hv is the absorption edge and E and EO are the static and high-frequency dielectric constants of the solid. We have measured the absorption edge of nitronium perchlorates (3400 A) and although the dielec-
THERMAL DECOMPOSITION OF NITRONIUM PERCHLORATE
tric constants are not known, these can be estimated from the ultraviolet and infrared absorption of the material. From the dispersion theory of optics it is possible to show that the refractive index, n, of a material can be expressed by
408 1
at 1100 cm-l. This band is the asymmetric C1-0 stretch of the clod- ion. Probably a more appropriate frequency would be the one corresponding to the motion of the NOz+ lattice against the c104- lattice. Acknowledging the shortcoming of this calculation we have calculated the static dielectric constant, E , to be 13.3. Hence
E where e and m are the charge and mass of the electron, q the number of active electrons per molecule (ion), N the atomic density, vo the characteristic frequency of oscillator, and v the variable frequency. As the expression stands it is only applicable to electronic transitions, i.e., vo will be an ultraviolet frequency. To include effects at longer wavelengths where nuclear motion will become important, the formula will be n2-
1 =
qe2N mr(vv2 - v2)
qezN +
CM7r(vr2 -
VZ)
vv and vr being the ultraviolet and infrared frequencies
and M the mass of the ion. The low-frequency dielectric constant E will therefore only involve the infrared frequency since the ultraviolet term will be negligible. Using the X-ray data of Truter, et ~ 1 1 . ~we ' ~ have calculated the atomic density of nitronium perchlorate to be 0.83 X Assuming a value of 1 for q eo=
.:
EO N
1
0.83 X
+9 X
X (4.8 X 10-10)2 X 3.142 X (0.88 X 1016)2
IJ
3.65 X 1.87 = 0.514 ev = 11.8 kcal mole-] 13.3
This value is in reasonably good agreement with our activation energy derived experimentally. Thus one can say that the activation energy for the thermal decomposition is 15 kcal mole-' and is the same for 02, NO, and C12. Although we did not observe NOC104 mass spectrometrically, Marshall and Lewis' work using Raman spectroscopy positively shows that it is formed at 65". However, Rosolovskii and Rumyantsev12report that the major product in the decomposition of NOC104 above 90" is N02C104. From our recalculation of Cordes4 data and the Marshall and Lewisb data concerning the formation of N0C1O4, it is a possibility that the decomposition of NOzC104goes via two different mechanisms, one above 100" and the other below 100". Further work on the low-temperature decomposition is in progress and will be reported later. Acknowledgments. We wish to thank C. 8. Goraynski, Jr., and D. E. Grabenstein for their valuable assistance in the experimental part of this program.
1.87
Our calculation of E, the static dielectric constant, is probably more approximate than that for BO since the most intense infrared absorption band known for nitronium perchlorate is found by Nebgen, et ~ l . to, be ~ ~
(10) M. R. Truter, D. W. J. Cruikahank, and G. A. Jeffrey, Acta Cryst., 13, 855 (1960). (11) J. W. Nebgen, A. D. McElroy, and H. F. Klodowski, Znorg. Chetn., 4, 1796 (1965). (12) V. Ya. Rosolovskii and E. S. Rumyantsev, Ruse. J . Znorg. Chem., 4, 1796 (1965).
Volume 71, Number 1% November 1967
