J. Phys. Chem. 1983,87,5065-5068
5085
Chemistry of N204at High Pressure: Observation of a Reversible Transformation between Molecular and Ionic Crystalline Forms S.
F. Agnew, B. I. Swanson,'
L. H. Jones, R. L. Mills, and D. Schiferl
Los Alamos National Laboratory, University of California, Los Alamos, New Mexico 87545 (Received: August 22, 1983)
Diamond-anvil cells have been used in Raman spectroscopy to characterize the behavior of N204at high pressure. Evidence is presented for three crystalline modifications, two of which have not previously been observed. The a-phase appears to be identical with the known low-temperature form, cubic Im3. Laser irradiation of a-N204 caused the formation of a second phase, 8-Nz04,which is also composed of planar nitro-N204molecules and is noncubic. While the crystal structure of (?-Nz04is as yet unknown, polarized Raman spectra give evidence for the alignment of the N-N bond axis as opposed to the cubic a-form which shows no molecular alignment. Both a-N204and P-Nz04are molecular crystals. A t pressures in the range 15-30 kbar, however, P-N,04 exhibits a reversible phase transition with a large hysteresis to a third structure, ionic NO+N03-. In contrast, a-N204 is stable to at least 76 kbar.
Introduction The chemistry and spectroscopy of the nitrogen oxides have been and continue to be topics of considerable interest. Virtually nothing is known, however, about the behavior of the nitrogen oxides a t high density. Nitric oxide, NO, is one of the simplest known high explosives' and is, therefore, an appealing system for detailed experimental and theoretical studies of detonation. Information concerning the chemistry of the nitrogen oxides at high pressure may help clarify the molecular level processes which accompany shock-initiated detonation of NO. However, in past attempts2 to observe nitric oxide a t high static pressure in diamond-anvil cells, only the products (N20, N203,and N204)of the rapid condensedphase disproportionation of N202were observed. In order to explain the very interesting chemical disproportionation of N202,it is necessary first to understand the behavior of all four oxides of nitrogen at high d e n ~ i t y . ~ The present study of N204at pressures up to 76 kbar (1 kbar = 0.1 GPa) was undertaken to accomplish this goal, and is in line with our general interest in the chemistry of small molecules at high density. Evidence is presented for at least two molecular forms, a- and P-N204,as well as a heretofore unobserved ionic crystal, NO+N03-. Facile conversion of crystalline P-N204to the ionic form is observed, while a-N204is apparently stable to quite high pressures. , Experimental Section Merrill-Bassett diamond-anvil cells4 with either hardened beryllium5 or beryllium-copper backings and type IIa diamonds were loaded by the indium dam technique previously described,6 with the modifications mentioned below. An NO impurity in our N204was eliminated by allowing O2 to stand over NzO, liquid in a closed cylinder overnight. The N204was then frozen, cooled to 195 K, and (1) Ribovich, J.; Murphy, J.; Watson, R. J . Hazard. Mater. 1975-77 I , 275-87. (2) Swanson, B. I.; Agnew, S. F.; Jones, L. H.; Mills, R. L.; Schiferl, D., to be submitted for publication. (3) Swanson, B. I.; Jones, L. H.; Babcock, L.; Schiferl, D.; Mills, R. L., spectroscopic studies of N20to be submitted for publication. (4) Merrill, L.; Bassett, W. A. Reu. Sci. Instrum. 1974, 45, 290-4. (5) Schiferl, D.High Temp. High Pressures 1977, 9, 71-5. (6)Mills, R. L.; Liebenberg, D. H.; Bronson, J. C.; Schmidt, L. C. Rev. Sci. Instrum. 1980,51, 891-5.
pumped on for several hours. The absence of a blue color indicated that no N203(the form of NO in excess N204) remained in the N204. Pressures were measured by the ruby-fluorescence method using an R1 line shift' of 1.322 kbar/cm-l. The line position of a ruby standard at 1 bar was checked periodically. The laser power for the pressure measurement was kept below 2 mW at the cell to minimize local heating of the ruby. Because N204reacts with the epoxy which is normally used to hold the diamonds in their backing plates, it was necessary to secure the diamonds with a less reactive indium seal. This seal was not gas tight, however, and we were obliged to glue a temporary Mylar window over the aperture behind each diamond table to prevent leakage of air into the cell. It was also necessary to passivate the chamber walls by filling the cell once with N204and then pumping it out again. Presumably, N204 reacted with fresh surfaces in the BeCu backing plates to form lowboiling NO which caused a vapor block in the inlet capillary, preventing further N204from condensing into the diamond cell. Once the initial fill was pumped out, N204 condensed readily at 273 K into the cell. While some N204 leakage occurred around the diamonds into the outer volume between the diamond tables and the temporary Mylar windows, no contamination of the sample took place inside the gasket. Once loaded, the cell was pressurized and then used in the spectroscopic experiments. Raman spectra were obtained on a SPEX Model 1403 3/4-mdouble monochromator by using a back-scattering technique and an elliptical collecting mirror. The resolution was 3 cm-' and, typically, ten spectra were signal averaged. A Spectra-Physics Model 171 argon-ion laser was adjusted to give the 4880-A line with a power of less than 30 mW a t the sample. All measurements were performed at room temperature. Results A spectrum of the fluid phase of N204 at 2.3 kbar is shown in Figure la. The intense peak at 265 cm-' has been assigned to the N-N stretch (vg, AJ by previous worker^.^,^ The scissor mode (v2, Ag) appears at 811 cm-l, (7) Barnett, J. D.; Block, S.; Piermarini, G. J. Reu. Sci. Instrum. 1973, 44, 1-9.
(8) Begun, G. M.; Fletcher, W. H. J . Mol. Spectrosc. 1960,4, 388-97.
0022-3654/83/2087-5065$01.50/00 1983 American Chemical Society
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The Journal of Physical Chemistry, Vol. 87, No. 25, 1983
I
i
I l l
_u
"
l!l
Letters
(c) 76 k b a r
(b)
I
16 k b a r
(a) 2. 3 l t b a r
500 500
1000
1000
Flgure 1. Raman spectra of fluid N,O, and a-N204: (a) fluid at 2.3 kbar; (b) solid at 16 kbar; (c) solid at 76 kbar.
while the in-phase symmetric NO2 stretch (ul, Ag)appears at 1385 cm-l. There are also two peaks at 507 and 1733 cm-l (the rock us, B,, and in-phase antisymmetric NO2 stretch u5, Blg, respectively) that are seen in the higher pressure solid phase. All of these features are completely consistent with the previously reporteds values for planar nitro-N204. At around 4.6 kbar and room temperature, N204freezes into a solid, hereafter referred to as a-N204,which does not extinguish under crossed polarizers. On this basis and coupled with the fact that its Raman spectrum agrees with that of the known low-temperature, low-pressure form,8 we believe that a-Nz04exhibits the same structure, having space group Im3 with six molecules per unit cell.'O The spectrum of a-N204shows a splitting in the fundamental at 293 cm-' that increases to 34 cm-' at 76 kbar, as shown in Figure 1, b and c. Since factor-group splittings of this magnitude are rarely observed, we assume that these two bands are actually two, nearly coincident fundamentals. We attribute the lower frequency peak to the N-N stretch (v3, Ag) while the higher frequency peak is tentatively assigned to the Raman-active NOz wag (us, Bzg). This latter assignment, however, is at odds with one reported previously.8 Factor-group splittings are, in fact, observed for all of the other bands at high pressure and are in the range of 1-3 cm-l. As shown in Figure 1, a-c, all of the Raman modes shift to higher frequency with increasing pressure. Raman spectra of low-pressure a-N204,obtained with 30 mW of 4880-A light, showed features characteristic of the fluid phase, indicating that laser heating produced some local melting. This was also apparent upon subsequent visual examination. A t 10.8 kbar, prolonged exposure to 4880-A radiation resulted in the formation of a pale blue spot in the solid. The blue color faded slowly and disappeared after 2 h. On the basis of its color and the (9) Hisatsune, I. C.; Devlin, J. P.; Wada, Y. J. Chem. Phys. 1960, 33, 714-9. (10) Wyckoff, R. W. G. "Crystal Structures"; Wiley: New York, 1963; Vol. 1, 2nd ed, pp 371-2.
Figure 2. Raman spectra of P-N204at 11.6 kbar (no analyzer was used): (a) electric vector of laser perpendicular to crystal long axis; (b) electric vector of laser parallel to crystal long axis.
Raman peaks observed during its formation, we tentatively identify the blue transient as N203. Photolytic production of N203from a-N204 will be discussed in detail in a later report. Extensive laser irradiation or the use of higher laser power resulted in the appearance of a new crystalline form which we refer to as P-N204. This form is colorless and insensitive to further laser irradiation. The well-formed crystals produced by laser irradiation extinguished cleanly every 90° under crossed polarizers in contrast to the isotropic nature of a-Nz04. Thus, P-N204 is not cubic. The Raman spectra of P-N204 are quite different from those of a-Nz04 as is apparent from a comparison of Figures 1 and 2. For example, the 295-cm-' feature is not split in P-Nz04,the phonon regions are very different, and P-N204 shows highly polarized Raman spectra (Figure 2). Indeed, in one crystalline orientation, the N-N stretch at 307 cm-' almost disappears. While the Raman spectra of the two forms of Nz04are sufficiently distinct to ascribe different structures to them, they nevertheless retain the fundamental characteristics of planar nitro-N2O4molecular crystals. Unlike a-NZ04,which is stable to at least 76 kbar, P-N204 undergoes a reversible first-order phase transformation with a large hysteresis over the pressure range 15-30 kbar. As shown in Figure 3, the new phase has Raman peaks at 2234, 1345, 1056, and 721 cm-'. These frequencies are consistent11J2 with the ionic structure NO+N03-. The 2234-cm-' band is the N-0 stretch for NO+ while the 1345, 1056-, and 721-cm-l features correspond, respectively, to the antisymmetric stretch, symmetric stretch, and in-plane deformation for the nitrate ion. Once formed, NO+N03is stable to our highest pressure of 76 kbar. Spectral features of the various forms of Nz04are summarized in Table I. The transformation of the a-N204to NO+N03- does occur at crystal occlusions and faults. Thus the multicrystalline environment that results upon raising the (11) Goulden, J. D. S.; Millen, D. J. J. Chem. SOC.1950, 2620-7. (12) Laane, J.; Ohlsen, J. R. Prog. Inorg. Chem. 1980, 27, 465-513.
The Journal of Physical Chemistry, Vol. 87, No.
25, 1983 5067
TABLE I: Observed Raman Modes (cm-') of t h e Three Forms of N,O, at High Pressure a-N,O,
1 bar, 238 Ka
P-N,O,; 1 6 kbar, 21.3 kbar. 300 K 300 K
1723.5
1732.7
1725
1383.2
1389.6
1397.5
811
816
820.3
491
507.1 296.1 287.5
506.5
I
i
i ;L,*,
,
L
i y 2000
1500
(in-phase NO, antisym str, B,,)' v 1 (in-phase NO, sym str, A ) v , (in-phase scissor, A ) v 6 (NO, roc%, B l f ) v 8 (NO, wag, B,, v 3 (N-N str, Ag)
v5
k0,
274.8
phonon
91.2 62 NO'NO;;
VNO+
antisym str, E ' ) (NO,', sym str, A,') v , (NO;, in-plane def, E')
v 3 (NO;, u,
phonon
500
1000
CM-' Figure 3. Low-frequency and high-frequency spectra of NO+N03- at 31 kbar. Peaks due to residual N,04 are labeled. Diamond first-order peak is at 1333 cm-' and divides the low- and highfrequency regions. The relative scales of the two spectra are arbitrary.
pressure of fluid Nz04invariably results in the formation of the salt at pressure higher than 20 kbar. It is only well-formed single a-N204crystals that can survive to high pressure.
Discussion Earlier X-ray and electron diffraction studies1°J3 have shown that the most stable form of N204is the planar, nitro-(02N-N02)molecular structure. Besides this form, matrix isolated Nz04 shows the nitrite-(ONO-N02) structure as well as nonplanar analogues of both the nitro and nitrite formsae Since the various molecular forms of Nz04 are readily distinguished by their spectra,12 it is certain the Nz04 in both a- and @-N204occurs as the planar nitro molecule. The existence of the &phase, however, has not been previously reported. The most striking result reported here is that @-Nz04 undergoes a reversible phase transformation to the ionic NO+N03- form. To our knowledge, this is the first observation of a pressure-induced phase transformation between a molecular crystal and an ionic crystal. The ionic species NO+ and NO3- have been recognized for many years as contributing to the solution chemistry of Nz04. It is knownl1J2 that numerous ionic solids with the formulas NO+X- and M+N03- can be obtained from Nz04 solutions by the addition of appropriate counterions. The Raman bands of NO+N03-have been observed in anhydrous nitric acid solutions of Nz04, and their frequencies are in good agreement with those reported here.'l In light of the known transformation of N205 to NO2+NO3- when solidified a t low pressure,14 the stability of NO+N03- is not surprising. The molecular-ionic crystal phase transformations in both N205and N204are accompanied by a volume decrease brought on by electrostric(13) McClelland, B. W.; Grundersen,G.; Hedberg, K. J. Chern. Phys. 1972,56, 4541-5. (14) Teranishi, R.;Decius, J. C. J. Chern. Phys. 1954, 22, 896-900.
306.6 118.9 99.2c
8O.lc 3 1 kbar
2234 1344.5 1055.9 721.2 133.1 62.3
'
a Reference 8. The vibrational assignments are based o n t h e planar nitro form of N,O,; t h e terms symmetric and antisymmetric refer t o t h e NO, fragment while in-phase and out-of-phase refer to the relationship between motions of t h e t w o NO, fragments. Error limits for each of t h e observed Raman modes are estimated t o be i 0.5 cm-I. The t w o phonon modes a t 99.2 and 80.1 cm-' are polarized perpendicular t o the N-N stretch, v j .
tion.15 At high pressure, processes giving a decrease in volume are naturally favored, and reaction rates involving negative volumes of activation increase with increasing pressure. We believe that the increased density of ionic N204over that of molecular Nz04is the driving force for the covalent-to-ionic phase transformation at high density. The stability of a-Nz04with respect to the formation of ionic NO+NO, indicates that a substantial barrier must exist between the two forms. This is remarkable in view of the fact that both a- and P-N204are comprised of nitro-N,O,. It is possible that the nitrite-Nz04is important in the transformation to NO+N03-, and that this intermediate is more readily accessed in the P-phase. A simple electron transfer and bond scission would be sufficient to create the salt as ON+-ONO2- from the nitrite form. We believe, however, that a suitable packing arrangement of the N204molecules could favor a concerted mechanism resulting in the same transformation. Thus, the formation of NO+N03- may be an example of a pressure-induced topochemical transformation, much like the photoinduced topochemical reactions that are known to occur for 9cyanoanthracene16 and 1-chl~roanthracene.'~ The highly polarized Raman spectrum observed in 0N204is supportive of a topochemical transformation. In the cubic (Im3) form of a-N204,the N-N bonds are oriented perpendicular to each cube face. The strong polarization of the N-N stretch in @-N204 suggests that the N-N bond axes are all aligned in the same direction. Such molecular alignment may be a critical factor in lowering the activation barrier in the formation of NO+N03-. (15) Benson, S. W. "The Foundations of Chemical Kinetics"; McGraw-Hill: New York, 1960, pp 510-7. (16) Cohen, M. D.;Cohen, R.; Cohen, R.; Lahav, M.; Nie, P.L. J. Chern. SOC.,Perkin Trans. 1973, 1095-1100. (17) Desvergne, J. P.;Thomas, J. M.; Williams, J. 0.;Bonas-Laurent, H. J . Chern. SOC.,Perkin Trans. 1974, 363-8. (18) Boldman, F.; Jodl, H. J. Chern. Phys. Lett. 1982, 85, 283-6.
J. Phys. Chem. 1983, 87,5068-5070
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In conclusion, we have identified three crystalline forms of N204. We believe that a-N204is identical with the low-temperature crystal already reported. The P-N204 form is new, and we are currently working to obtain its crystal structure. We know, however, that this form is not cubic, that it has a significant N-N bond alignment lacking in a a-N204, and that it converts readily to the ionic structure. The third form of Nz04is the ionic NO+N03-, which is apparently the thermodynamically favored structure at high density. It is obtained upon either rapidly increasing the pressure of fluid N204or increasing the pressure of P-N204above 20 kbar. Indeed, it is the only well-formed single a-N204crystals that can survive at
higher than 20 kbar. Work is now in progress to identify the crystal structures of these various solids by X-ray diffraction. N o t e Added in Proof. We have recently become aware of work by Boldman and Jodl'' also concerning the production of NO+N03- from N204. In this work the metastable ionic solid was trapped in a low-temperature neon matrix. Acknowledgment. This work was performed under the auspices of the US.Department of Energy. Registry No. Nz04, 10544-72-6.
Far-Infrared Laser Magnetic Resonance Detection of F02 F. Temps, H. Gg. Wagner, Max-Pianck-Institut fur Stromungsforschung, D-3400 Giittingen, F.R.G.
P. B. Davies," D. P. Stern, Department of Physical Chemistry, University of Cambridge, Cambridge CB2 IEP, England
and K. 0. Chrisle Rockweii International, Rocketdyne Division, Canoga Park, California 9 1304 (Received: September 14, 1983)
New far-infrared laser magnetic resonance (LMR) spectra have been detected in the reactions of fluorine atoms with O2 and O3 These are assigned to the FOz radical based on chemical and kinetic results and on a qualitative spectroscopic investigation. Thermal decomposition of 02SbF6,a known source of FOz, also yielded the same spectra.
Introduction The FO radical has not been as extensively studied as the other diatomic halogen oxides. The first structural parameters for the radical in the gas phase were determined only recently from the 10-pm laser magnetic resonance spectrum of the 211 ground state.l Subsequent photoelectron spectroscopy yielded ionization potentials,2 and improved vibrational and rotational parameters have been determined from infrared diode laser spectroscopy by McKellar et al.3 Prior to these investigations the microwave spectrum had been searched for unsuccessfully by gas-phase electron paramagnetic resonance spectro~copy.~The relatively high concentrations of FO measured mass spectrometrically5 and the much enhanced sensitivity of far-infrared laser magnetic resonance over microwave spectroscopy led to the present search for FO spectra by LMR. During this investigation strong, previously unreported spectra were detected at many laser frequencies. Based on chemical and qualitative spectroscopic evidence the carrier of these (1) A. R. W. McKellar, Can. J. Phys., 57, 2106 (1979). (2) J. M. Dyke, N. Jonathan, J. D. Mills, and A. Morris, Mol. Phys., 40, 1177 (1980). (3) A. R. W. McKellar, C. Yamada, and E. Hirota, J.Mol. Spectrosc., 97, 425 (1983). (4) D. H. Levy, J. Chem. Phys., 56, 1415 (1972). (5) H. Gg Wagner, C. Zetzsch, and J. Warnatz, Ber. Bunsenges. Phys. Chem., 76, 526 (1972).
0022-365418312067-5068$0 1.5010
TABLE I: Source Reactions and Far-IR Laser Lines Used to Detect F O , Spectraa
-
~ _ _ _ _ ~ _ _ _ source reaction _ _ ____I I _
wavelength, fim
119 170 354 3 83
419 433 502 513 634 635 742 a
laser gas
F
CH,OH CH,OH CD,OD
CH;F, HCOOH HCOOH C A F HCOOH C,H,Cl
C,H;B~ HCOOH
+
F+O,+
0,
X X X
X X X
M X X X X X X X X X X
X indicates spectra observed.
spectra is identified as the FOz radical.
LMR Spectra and Assignment The LMR spectrometers operated on a large number of far-infrared molecular laser lines excited by optical pumping with flowing gas C 0 2 lasers. Further details of these instruments have been published e l ~ e w h e r e .With ~~~ ~~~
~
(6) A. W. Preuss, F. Temps, and H. Gg Wagner, MPI fur
Stromungsforschung,report 18, Gottingen, 1980. (7) D. P. Stern, Ph.D. Thesis, University of Cambridge, 1983.
0 1983 American Chemical Society
