J. Phys. Chem. 1902, 86, 4257-4260
4257
Electron Spin Resonance Study of Nitric Oxide Adsorbed in Na-A Zeolite Paul
H. Kasal'
and R. M. Gaura
IBM Instruments, Incorporated, Danbury, Connecticut 06810 (Received:April 19, 1982; In Final Form: Ju/y 13, 1982)
The ESR spectrum of NO adsorbed in Na-A zeolite was reexamined. Unlike ESR spectra of NO observed earlier with other zeolites, the ESR spectrum of NO monomer in Na-A zeolite was found to be well-defined and resolved, reflecting the compositional uniformity (Si/Al ratio of 1)of the material. The study also revealed the formation of an unusual NO-NO triplet species within the cavities.
Introduction Electron spin resonance (ESR) examination of NO adsorbed in zeolites has been reported by many authors.'+ Although it is a stable radical, NO in its ground state (2111,z)does not exhibit an ESR signal unless the degeneracy of its K orbitals is removed. Lunsford studied ESR spectra of NO adsorbed in various cation-exchanged Y zeolite^.^^^ Gardner and Weinberger reported on ESR spectra of NO adsorbed in A and X zeolites.2 The spectra reported by these authors were broad but the observed patterns were consistent with the g tensor expected from NO interacting with the intracrystalline field of zeolitic cations. In our independent study of NO adsorbed in Y zeolite, we discovered that the ESR signal was initially broad but became sharp on standing at room temperature for several daysS4 The sharpening of the signal must result from improved uniformity of the intracrystalline field responsible for.the lifting of the degeneracy of K orbitals. NO is thermodynamically unstable toward the disproportionation reaction 3N0 NzO NO2. It was proposed and substantiated that zeolite not only catalyzed this reaction but induced a further electron transfer reaction between the resulting NOz and additional NO creating NO2- and NO+ ions. It was asserted that the generation and a proper arrangement of these additional ions improved the uniformity of the crystal field.4 The building block of X or Y faujasite-type zeolite is a sodalite cage, a truncated octahedron, the chemical composition of which may be given as (Si24-nAln04s)n-.The cages are connected tetrahedrally through six-member rings, and the exchangeable cations (n Na+ ions per cage in Na-ion zeolite) are held electrostaticallyto the negatively charged network of cages.' The &/A1 atomic ratio in general is not a simple number. I t is 1.23 for X and 2.43 for Y. Thus, it is not unexpected that the intracrystalline field centered about zeolitic cations would vary significantly from one cation site to another. The A zeolite is also built of sodalite cages. I t differs from faujasite-type zeolite in two significant aspects; the cages are connected octahedrally through four-member
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+
(1)J. H.Lunsford, J. Phys. Chem., 72, 4163 (1968). (2)C. L. Gardner and M. A. Weinberger, Can. J. Chem., 48, 1317 (1970). \ - - . -,-
(3)J. H.Lunsford, J. Phys. Chem., 74, 1518 (1970). (4)P.H.Kasai and R. J. Bishop, Jr., J. Am. Chem. SOC.,94, 5560 (1972). (5)C. Naccache, M. Che, and Y. Ben Tarrit, Chem. Phys. Lett., 13, 109 (1972). (6) C. C. Chao and J. H. Lunsford, J . Phys. Chem., 76, 1546 (1972); 78, 1174 (1974). (7)D.W. Breck, "Zeolite Molecularseives",Wiley, New York, 1974, Chapter 4. 0022-365418212086-4257$01.25/0
rings, and the Si/Al ratio is exactly unity. It follows that there should be little site-to-site variation in the intracrystalline field of A zeolite. A recent CP/MAS (magic angle spinning, cross polarization) 29Si NMR study of zeolitess showed the presence of several chemically nonequivalent Si peaks in X and Y, but only one peak in A. The previously reported ESR spectrum of NO adsorbed in Na-A, however, is broad and shows no feature attributable to this compositional uniformity.2 The present study was undertaken in an effort to elucidate this problem. The study not only confirmed that the NO/Na-A system could exhibit a well-resolved ESR spectrum of NO reflecting the compositional uniformity of the material but also revealed the formation of an unusual NO-NO triplet species within the cavities.
Material and Instrumentation Na-A (Linde 4A) obtained from Sigma Chemical Co. and Na-Y obtained from Ventron Corp. (Alfa Division) were used. Zn-A was prepared from Na-A by a standard exchange process. Samples were activated under vacuum torr) at 500 "C for 3 h, cooled to room temperature or lower, and then exposed to NO introduced through the vacuum manifold. A notation such as NO/Na-Y will be used to indicate a Na-Y sample exposed to NO. No ESR signal was observed from any of the NO/zeolite systems at room temperature. All the spectra reported here are those observed at liquid-nitrogen temperature. The ESR spectrometer used was an IBM ER-100-E, a X-band system. A typical spectrometer frequency locked to the loaded sample cavity was 9.236 GHz. Results As stated earlier the ESR spectra of NO adsorbed in X or Y zeolites were initially broad but then became sharper on standing at room temperature for several days. Such spectral changes observed with NO/Na-Y and NO/Ba-Y systems are shown in Figures 1and 2. The analysis of the sharper spectra (Figures l b and 2b) in terms of an orthorhombic g tensor and an axially symmetric 14N hyperfine coupling tensor has already been delineated.4 In the case of NO/Na-Y, improvement of the crystal field uniformity is most clearly manifested in the emergence of a well-definedg, peak. In the case of NO/Ba-Y, not only the definition of the g, peak improved with time, but also the resolution of the 14Nhyperfine splittings along the gr direction improved, reflecting a stronger interaction between adsorbed NO molecules and divalent Ba2+ ions. An ESR spectrum observed immediately after activated Na-A had been exposed to NO (200 torr) is shown in (8) E. Lippmaa, M. Magi, A. Samoson, M. Tarmak, and G . Englehardt, J . Am. Chem. Soc., 103, 4992 (1981).
0 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86, No. 21, 1982
4258
'-p"
t
t Y
I
X I
I
I
I
I
I
I
I
3200
G.
3600
Figure 1. ESR spectra of NO/Na-Y observed: (a) immediately after preparation and (b) 5 days later.
n
X 1
3200
I
Kasai and Gaura
1
I
I
3600
I
G.
Flgure 2. ESR spectra of NO/Ba-Y observed: (a) Immediately after preparation and (b) 5 days later.
Figure 3a. The spectrum of the same sample observed 5 days later is shown in Figure 3b. The spectrum of "fresh" NO (200 torr)/Na-A is much more complex than those encountered earlier; the spectrum of "aged" NO (200
I
3200
I
I
1
I
3600
I
G.
I
Flgure 3. ESR spectra of NO (200 torr)/Na-A observed: (a) immediately after preparation and (b) 5 days later.
torr)/Na-A is exactly like that reported by Gardner and Weinberger.2 A thorough examination of the ESR spectra of several fresh NO/Na-A samples prepared under a variety of conditions (e.g., different NO pressure and/or temperature. of Na-A at the time of exposure to NO) revealed the following: (1) An ESR spectrum of fresh NO/Na-A is generally a superposition of two signals I and 11. Signal I resembles the spectrum of aged NO/Na-Y or NO/Ba-Y; it is characterized by a well-definedg, peak and a partially resolved 14Nhyperfine structure along the gr direction. Signal I1 is of hitherto unknown origin; it has an appearance of doublet with the spacing of -300 G and flanks the signal I, thus giving an overall appearance of a triplet to the total spectrum. (2) Signal I prevails when the NO pressure is low ( < l o torr), while signal I1 becomes dominant with higher NO pressure (>200 torr). (3) When a fresh NO/Na-A sample is evacuated at room temperature, both signals I and I1 are removed completely. If evacuated at lower temperature (e.g., -78 "C), signal I1 is removed preferentially. We concluded that signal I arose from NO monomers interacting with Na cations, and surmized that a triplet species (S = 1)involving two NO molecules might be responsible for signal 11. The half-field signal due to AM, = f2 transition was therefore sought and was indeed detected. The half-field signal wm not observed when signal I1 was weak or absent, as it should be. Figure 4 shows the ESR spectrum of fresh NO (400 torr) /Na-A observed immediately after preparation, and the spectrum of the same sample observed after subsequent evacuation at -78 "C for 10 min. Both the normal region and the half-field region are shown. In Figure 4a a weak negative dip is indicated by an arrow. This feature also belongs to the triplet species and is discussed later in the analysis section.
ESR Study of NO Adsorbed in Na-A Zeolite
The Journal of Physical Chemistry, Vol. 86, No. 27, 7982
4259
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A 0
1.00
,
,
'
1110
,
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I
1 1I 0 0
I
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,
Flgure 4. ESR spectra of fresh NO (400 torr)/Na-A observed: (a) immediately after preparation and (b) after subsequent evacuation at -78 OC for 10 min. Both the half-field region and the normal region are shown. The signal at 1500 G is due to Fe impurity in the glass sample tube.
\
/
I
3200
1
3600
I
C
Flgure 6. (a) Computer-simulated spectrum of NO monomers in NO/Na-A. (b) Computer-simulated spectrum of NO-NO triplet in NOINa-A. (c) Superposition of a and b with the weighted ratio of 1:l.
of the ESR spectra of NO in fresh Na-A and Zn-A were determined as follows:
a
AYa
gx gY gz 1.987 1.905 Na-A 1.980 Zn-A 1.999 1.999 1.918 In gauss. In both cases Ax 2 A z E 0 G.
30.0 30.0
The most intriguing revelation of this study is the formation of NO-NO triplet species in Na-A. The ESR spectrum of a triplet state is generally characterized by the following spin Hamiltonian: I
3iOO
I
I
I
I
3600
I
G.
I
Flgure 5. ESR spectrum of fresh NO/Zn-A.
Figure 5 shows the ESR spectrum of fresh NO/Zn-A. It is a spectrum of NO with a well-defined g, peak and a well-resolved hyperfine structure along the gy direction; its similarity to the spectrum of aged NO/Ba-Y (Figure 2b) is conspicuous. With the NO/Zn-A system this was the only signal observed, though the spectrum observed 5 days later was slightly broader.
Analysis and Discussion The most gratifying result obtained in the present study is that the ESR spectra of NO in fresh Na-A (Figure 4b) and fresh Zn-A (Figure 5) are both well-defined and resolved attesting to the predicted uniformity of the crystal field in the starting zeolite cages. In sharp contrast to the result obtained with other zeolites, signal I of NO/Na-A becomes broader with time, while signal I1 concurrently loses its intensity (Figure 3b). The uniformity of the field within A zeolite must be such that it is adversely affected by the products of the disproportionation reaction 4N0 N20 + NOz- + NO+. Also it appears that the disproportionation reaction proceeds consuming the NO-NO triplet species and/or the space required for ita formation. The g tensors and the 14Nhyperfine coupling tensors
-
7f = P S p H
+ D [ S Z 2- 1/3S(S + l)]+ E(SX2- SY2)(1)
where S represents the electron spin operator (S = l),g the g tensor, and H the magnetic field. The D and E terms represent the electron-electron dipolar interaction and are responsible for the fine structure splitting. A relatively small splitting observed here (-300 G) suggests that the triplet is of a radical-pair type, where E would be negligibly small and D is dependent on R, the average separation between the two unpaired electrons, according to eq 2.
D = 3g2P2/(2R3) (2) For an axially symmetric case ( E = 0) the spin Hamiltonian in eq 1predicts the resonance positions of the fine structure components ( M , = 1 0, M , = 0 -1) as follows:
-
-
(3)
Here 0 is the angle between the magnetic field and the symmetry axis. The powder pattern or the line shape of a randomly oriented triplet system was first analyzed by Wasserman et aL9 For an axially symmetric system two strong signals occur at the positions corresponding to 0 = (9) E. Wasserman, L. C. Snyder, and W. A. Yager, J. Chem. Phys., 41, 1763 (1964).
J. Phys. Chem. 1982, 86,4260-4265
4260
r / 2 , and two weak signals at the positions corresponding to t9 = 0. The former pair is separated by D/(g,P), and the latter by 20/(g,@). Thus,in the spectra seen in Figures 3a and 4a, the doublet flanking the normal NO signal was recognized as the perpendicular (6 = 7r/2) components of the triplet species. In Figure 4a a weak negative dip is indicated by an arrow at 3750 G. This was recognized as the high-field member of the parallel (6 = 0) components. This assignment then places the low-field parallel component at 3200 G where it would be masked by the lowfield perpendicular signal. The g tensor and the crystal field parameter of the triplet species were thus determined as follows: g, = 1.976 g,, = 1.912
The crystal field parameter determined above and eq 2 yield R of 4.6 A. It has been shown that the largest deviation of the g tensor of NO from the spin only value (2.0023) occurs along the N-0 bond dire~tion.',~ The assignment given above then states that the symmetry axis for the dipole-dipole interaction must closely parallel the N-0 bond direction. A plausible structure of the NO-NO triplet formed in Na-A is thus envisaged as follows:
D/(g,P) = 288 G (or D = 0.0266 cm-')
Zeolite A has a cubic structure. Each unit cell contains a roughly spherical cavity having a diameter of 11A,defined by eight sodalite units placed at each corner of the unit cell. Each cavity thus has 8 walls of six-member ring, 12 walls of four-member ring, and 6 windows of eightmember ring shared with the adjacent cavities. In Na-A Na+ ions are located near the centers of all the six-member rings, and about half of the eight-member rings.1° Although the exact location and the orientation of the triplet species cannot be ascertained from the present study, there is certainly an ample space and there are enough cations within each cavity so that the formation of such a NO-NO triplet as depicted above would be possible.
Figure 6a, and b show respectively the computer-simulated spectra based upon the parameters determined above for the NO monomer, and the NO-NO triplet in the NO/Na-A system. A Lorentzian line shape with the line width of 20 G was assumed for the computation. Superposition of the spectra with the weighted ratio of 1:l is shown in Figure 6c. It is in an excellent agreement with the observed spectrum (Figure 4a). The powder pattern of the triplet species seen here is unusual in that it is extremely askewed by the anisotropy of the g tensor (gL> g,,). Interestingly, though, the g tensor of the triplet given above is very similar to that of NO monomer detected in the same NO/Na-A system. It strongly substantiates the notion that the triplet state in question is a radical pair comprising two NO molecules.
I
I
(10)R. Y. Yanagida, A. A. Amaro, and K. Seff, J. Phys. Chem., 77,805 (1973).
Solid Phase Transition Kinetics. The Role of Intermolecular Forces In the Condensed-Phase Decomposition of Octahydro- 113,5,7-tetranitro- 1,3,5,7-t etrazocine T. B. Brill' and R. J. Karpowlcz Department of Chemistty, University of Delaware, Newark, Delaware 19711 (Received: May 73, 1982; I n Final Form: July 13, 1982)
The fundamental basis of the interconversions between the polymorphs of the monopropellant octahydro1,3,5,7-tetranitro-l,3,5,7-tetrazocine (HMX) has been established. A Fourier transform infrared method was developed to study the rates of solid-solid phase transitions on the molecular level. Approximate first-order rates were observed. The activation energy (kJ mol-') and frequency factor (log A ) for the transitions are /3 -,6 (204 f 14; 19.9 f l),a -,6 (208 f 18; 19.9 f 2), and y 6 (219 f 20; 21.8 f 2). These Arrhenius data explain why the 6 transformation is the predominant thermally induced phase transition of P-HMX. Of much greater importance is the close resemblance of the Arrhenius data for the phase transitions to those for the condensed-phase event to the Arrhenius data. It is proposed that rupture of the intermolecular forces rather than cleavage of covalent bonds within the molecule is what largely controls the rate of thermal decomposition of HMX. With this new interpretation a number of heretofore unclear facets of HMX decomposition come into sharper focus. The implications for altering the rate of decomposition are noted.
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Introduction Octahydro-1,3,5,7-tetranitro1,3,5,7-tetrazocine, shown in Figure 1, is known as HMX and is one of the most important energetic nitramines. In spite of extensive work on HMX, the initial step in the thermal decomposition of the condensed phase remains uncertain. Differential scanning calorimetry,l differential thermal analysis,2mass (1) Rogers, R. N. Thermochim. Acta 1972, 3, 437. 0022-3654/82/2086-4260~01.25/0
spectrometry: and gas evolution4measurements yield rates from which frequency factors, A , and activation energies, E,, can be calculated from the Arrhenius equation: log k (s-l) = log A - E,/RT. log A = 18-20 and E , = 209-220 (2) Sinclair, J. E.; Hoondee, W . Proc. Symp. Explos. Pyrotech. 1971, 7, 1; Chem. Abstr. 1972, 76, 1 4 3 0 5 0 ~ .
(3) Goshgarian,B. B. AFRPL-TR-78-76;Air Force Rocket Propulsion Laboratory: Edwards, AFB, CA, 1976. (4) Robertson, A. J. B. Trans. Faraday SOC.1949, 45, 85.
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