J . Phys. Chem. 1986, 90, 6726-6731
6726 Conclusions
The present investigation has reported on the kinetics of the reaction of metastable N(2D,2P)atoms with NO2. Previous investigations on the ground-state N(4S) NOz reaction have yielded a spread of rate constants apparently due to low [NO2]/ [N] ratios initiating rapid catalytic cycles involving N and NOz. Under the conditions of our experiments a [NO,]/[N] ratio in the range of lOI3 is achieved which virtually eliminates this problem in the kinetic measurement. The overall rate constant of reaction was evaluated as 3.3 X cm3 molecule-' s-l (293 K). This value is an upper limit because the rate constant used for the N + N2 exchange reaction, the competitive probe in this system, may actually represent both reaction and quenching channels for the atom-molecule encounter. In any event, the above rate constant for metastable N(2D,ZP) NO, reaction is a t least an order of magnitude smaller than reported values on the ground-state N(4S) + NO2 reaction. In
+
+
future studies, it will be interesting to see if the present experimental methods applied to the N(4S) NO, reaction will indeed yield a smaller rate constant. Branching ratios for the distribution of channels la:lb:(lc Id) were evaluated as 1.00:0.65:0.27. Previous studies have yielded direct evidence only for channel la. The remaining channels were postulated from reaction stoichiometry. The present investigation demonstrates additional validation for this channel, and also provides direct evidence for some reaction channel or channels yielding N2, presumably by I C and Id.
+
+
Acknowledgment. This research was carried out at Brookhaven National Laboratory under contract DE-AC02-76CH00016 with the U S . Department of Energy and supported by its Office of Basic Energy Sciences. Registry No. N, 17778-88-0; NO2, 10102-44-0; Ne, 7440-01-9; N,, 7727-37-9.
Chemkal and Phase Transformatlons of Cyanogen at High Pressures Choong-Shik Yo0 and Malcolm Nicol* Department of Chemistry and Biochemistry, University of California. Los Angeles, California 90024 (Received: May 8, 1986)
Chemical and phase transformations of CzNzat 293 K and pressures as high as 12 GPa have been studied by Raman and Fourier-transform infrared spectroscopy. Three phases of the monomer have been identified. C2N2I freezes at 0.3 GPa and transforms to another solid, C2Nz11, at 0.5 GPa. At 2 GPa, C2N2I1 transforms to a third solid phase of the monomer, C2N2111. Between 3.5 and 10 GPa, C2N2reversibly converts to what is identified as a linear chain of dimers, (-(C2N2)2-)m. Above I O GPa, further irreversible reactions convert these chains to a very chemically and thermally stable material that can be recovered at atmospheric pressure. Vibrational spectra indicate that the linear chain is a poly(2,3-diiminosuccinonitrile) and the stable product is a paracyanogen ladder of fused pyrazine rings that may be highly cross-linked.
Introduction
Pressure accelerates the polymerization of unsaturated molecules. Thus, organic polymer chemistry a t high pressures,',z including applications to electroconducting polymers and high strength material~,33~ is of considerable interest. Chemical reactions at high pressure have been observed with many types of unsaturated chemical bonds including those in alkene^,^,^ alkyne^,^ nitriles: carbonyls: and carbon sulfides.IO Indeed, all unsaturated molecules are expected to be unstable at high pressure with respect to associative, cross-linking reactions which form denser, more saturated species." Very detailed information concerning these reactions can be obtained by combining diamond-anvil cell methods, which generate high, quasi-hydrostatic pressures at which even the strongest multiple bond should be unstable,', with spectroscopic and other sub-microanalytical technique^.'^,^^ At the very high pressures (1) Katz, A. M.; Schiferl, D.; Mills, R. L. J . Phys. Chem. 1984,88, 3176. (2) Babare, L. B.; Dremin, A. N.; Perskin, S. V.; Yakovlev, V. V. Proceedings the First International Symposium on Explosive Cladding Institute of Chemical Physics: Moscow, 1971; pp 239-57. (3) Heiman, R. B.; Kleiman, J.; Salansky, N. M. Carbon 1984, 22, 147. (4) Bundy, F. P. J. Geophys. Res. 1980, 85, 6930. (5) Mea, R.; German, A. L.; Heikens, D. J. Polym. Sci. 1977, 15, 1765. (6) Holmes, W. S.; Tyrrall, E. Trans. Faraday Soc. 1956, 52, 47. (7) Rice, J. E.; Okamoto, Y. J . Org. Chem. 1981, 46, 446. (8) Yakushev, V. V.;Nabatov, S. S.; Yakusheva, 0.B. Dokl. Akad. Nauk. SSSR 1973, 214, 879. (9) Dremin, A. N.; Babare, L. V. J . Phys. 1984, 45, C8-177. (IO) Ginsberg, A. P.; Lundberg, J. L. Inorg. Chem. 1971, 10, 2079. (11) Nicol, M.; Yen, G . Z. J . Phys. 1984, 45, C8-163. (12) McMahan, A. K.; LeSar, R. Phys. Reu. Left. 1985, 54, 1929. (13) Agnew, S. F.; Swanson, B. I.; Jones, L. H.; Mills, R. L.; Schiferl, D. J . Phys. Chem. 1983, 87, 5065.
used in this work, polymerizations tend to occur as bulk solid reactions that proceed without participation of a solvent or catalytic initiators. Reactivity and active centers are created by a combination of mechanical deformation, topological arrangements of the solid reagents, and reduction of the free energy of activation. The reactions either proceed a t constant pressure or, because transport and steric considerations often are important at intermediate stages of the reactions, occur in stages as the pressure increases. Cyanogen, C2N2,is one of the simpler carbonitriles, a class of molecules whose common feature is a strong carbon-nitrogen triple bonds. Cyanogen also is a relatively energetic m o l e c ~ l e and ' ~ can be polymerized to an electrically conducting material.16 Much of the chemistry, spectroscopy, and industrial applications of the nitriles near atmospheric pressure have been explored,I7 and the behaviors of some nitriles at high densities have been determined. Most halonitriles and alkyl nitriles trimerize symmetrically to aromatic 1,3,5-trisubstituted triazines at ambient]*and high static pressures. The trimerizations can be promoted by specific solvents and catalyst^,'^ and the product yields also depend strongly on the group attached to the nitrile moiety. Cyanogen has not been reported to trimerize, although it is known to polymerize at high temperatures.,O (14) Yin, G.2.;Nicol, M. J . Phys. Chem. 1985,88, 1171. (15) Knowlton, J. W.; Prosen, E. J. J. Res. Natl. Bur. Srand. 1951, 46, 4x9. .. .
(16) Whangbo, M.; Hoffman, R.; Woodward, R. B. Proc. R. SOC.London, See. A 1976, A366, 23. (17) Cordain, B. Coord. Chem. Reu. 1982, 47, 165. (18) Cook,A. H.; Jones, D. G . J . Chem. Soc. 1941, 278. (19) Cairns, T. L.; Larcher, A. W.; McKusick, B. C. J . Am. Chem. SOC. 1952, 74, 5633. (20) Perret, A,; Krawczynski, A. Bull. SOC.Chim. 1932, 51, 622
0022-3654/86/2090-6726$01.50/00 1986 American Chemical Society
Chemical and Phase Transformations of Cyanogen
The Journal of Physical Chemistry, Vol. 90, No. 25, 1986 6727
The polymer chemistry of liquid acrylonitrile under shock loading provides an interesting model for the chemistry of cyanogen. At 4.3 GPa, acrylonitrile is reported to react to form a linear chain polymer which converts to a ladder-type polymer upon further compression to 8.6 GPaS2l Similar reactions might be expected to occur under hydrostatic compression, although this chemistry has not been studied. In the work described in this report, cyanogen and its reaction products were examined at pressures as high as 12 GPa by Raman and Fourier-transform infrared (FTIR) spectroscopy and direct visual observations. Three phases of solid cyanogen were found a t pressures between 0.3 and 3.5 GPa. At higher pressures, reversible and irreversible reactions were discovered. The spectra and possible structures of the products are discussed.
(A)
2.1
GPa
Experimental Section
Two types of diamond-anvil high-pressure cells were used for this work. One type was a new design that combines features such as Holzapfel's sapphire window for supporting one of two diamond anvils with 0.5-mm-diameter culets22and Mao and Bell's clamp system for compressing cryogenic fluidsz3in a way that is convenient to use with Mills's indium-dam technique for loading highly toxic g a s e ~ . ~This ~.~ cell ~ was used for most of the Raman spectroscopy. A more compact cell of Bassett's designz6with type IIa diamond anvil with 0.8-mm-diameter culets was used for the infrared work. Since cyanogen reacts with many epoxy cements, the diamonds were mounted in these cells with Varian TorrSeal which, after hardening, was covered with indium. Pressures were measured by the ruby luminescence method2' using a R,-line shift of 2.746 GPa/nm. In order to minimize possible photochemistry or thermal reactions during the pressure measurements, the power of the focussed laser at the sample was kept below 10 mW during pressure measurements. Inconel gaskets with 0.2- to 0.3-mmdiameter holes were used to confine the samples between the diamond anvils. The loading procedure involved mounting an empty gasket on one culet and surrounding the gasket with an indium dam that sealed an evacuable volume between the gasket and the culet of the second diamond. Atmospheric gases and residual moisture were removed from this volume by evacuating it to less than 20 Pa and then filling it with cyanogen (Matheson, 98.5% without further purification) a t room temperature to approximately atmospheric pressure three or four times. Because cyanogen is highly toxic, the evacuated gases were oxidized by passing them through a saturated solution of Borax before they were ventilated to the atmosphere. A sample of liquid cyanogen then was condensed into the volume enclosed by the indium dam by cooling the cell to between 248 and 252 K with a dry ice-ethanol mixture. The second diamond was then advanced to trap some of the liquid inside the gasket, and the excess cyanogen was evacuated from the dam and oxidized with the saturated Borax solution. Raman spectra were obtained with Spex 1400 double monochromator with a photon-counting detection system. The spectra were excited with radiation from a Spectra-Physics Model 165 argon ion laser. The 514.5-nm line was used for most of the spectra. However, each feature was also identified in at least one spectrum taken with a second excitation line, typically 488.0 nm, or in an anti-Stokes spectrum. FTIR spectra were obtained with a Matheson Cygnus 25 spectrometer equipped with a microbeam condensor and corner cube. The diamond-anvil cell was mounted in the Matheson spectrometer on a x-y-z translation mount and was aligned by maximizing the interferogram voltage. Typically, (21) Babara, L. V.;Dremin, A. N.; Perskin, S. V.;Yakovlev, V. V. Dokl. Akad. Nauk S S R 1969, 184, 1120. (22) Hirsch, K. R.; Holzapfel, W. B. Reu. Sci. Instrum. 1981, 52, 52. (23) Mao, H. K.; Bell, P. W. Science 1978, 200, 1145. (24) Liebenberg, D. H. Phys. Lett. 1979, 73A, 74. (25) Mills, R. L.; Liebenberg, D. H.; Bronson, D. H.; Schmidt, L. C. Rev. Sci. Instrum. 1980, 51, 89 1. (26) Bassett, W. A,; Ming, L. C. In The Physics and Chemistry of Minerals and Rocks, Strens, R. G., Ed.; Wiley: New York, 1973; p 366. (27) Barnett, J. D.; Block, S.; Piermarini, G. J. Reu. Sci. Instrum. 1973, 44, 441.
0.1
G Pa
0.5 C ? a
t
I
0
50
1
I
1
-
100 150 200 450 W A V E N U M BE R
550
C M-1 Figure 1. Raman spectra of three solid phases of C2N2in the regions of (A) the lattice vibrations and (B) the trans-bending mode, u4.
TABLE I: Raman Frequencies of Different Solid Phases of C2N2in a Reeion of Lattice Vibration orthorombic," C2N2I, C2N2 I1 C2N2 111
-
0 GPa 44 (45) s 57 (59) m 74 (76) w 77 (80) s 83 (85) s 107 (110) m 111 (115) m
0.5 GPa
0.7 GPa
du/dpb
2.1 GPa
dv/dpb
40 w 49 w
47 s 55 m
45 w
84 s
84s
21.8
120m
112s
20.0
80 m 102s 123 w 142m
7.8 6.8 13.2
Frequencies are based on the previous works of solid C2N2at 77 K in ref 31, and at 20 K in ref 29 with parentheses. of du/dp are all in cm-'/GPa. (I
lo4 scans were collected with 8-cm-I resolution to obtain a reasonable signal-to-noise ratio.
Results At room temperature, nominally 293 K, C2N2freezes at 0.3 GPa. None of the molecular vibrational frequencies change abruptly when C2N2freezes, but the transformation can be detected visually and by the appearance of strong bands at 47 and 84 cm-' and a weaker band at 120 cm-I in the Raman spectrum, as illustrated in Figure 1A. The wavenumbers of these and other features of the Raman spectra of this and other phases of C2N2 are summarized in Table I. The solid that forms at 293 K, designated C2NzI, is stable over only a small range of pressures; and crystals of this phase often regrow or reorient in response to small temperature changes or exposure to laser excitation. Thus, the pressure dependences of the spectra and single-crystal X-ray diffraction patterns of CzN2I have not been determined; and it is not known whether C2N2I is the same as the orthorhombic phase that freezes at low temperatures and pressure.z8 However, (28) Parkes, A. S.; Hughes, R.E. Acta Crystallogr. 1963, 16, 734.
The Journal of Physical Chemistry, Vol. 90, No. 25, 1986
6728
Yo0 and Nicol
200
P
I
< B
150
K W
m 5 3 2 W
>
a-
a
ji
0
I
3
I\ I1
U
a
100
T
I
500
I
I
I
I
I
I
I
I
I
1000
I
I
1500
r
-
2200
1 2500
WAVENUMBER I
I
I
1
3
4
5
6
PRESSURE
/GPa
I
1
2
Figure 2. A plot of the wavenumbers of the Raman bands of the lattice modes of C2N2I1 and C2N2111.
this identification is suggested by similarities of the lattice-mode regions of the Raman spectra of C2N2I at 293 K and 0.3 GPa and the orthorhombic phase near 20 K.29 Compression of pure, unreacted C2N2produced two other solid phases which were distinguished by reversible changes of crystal morphology, Raman spectra of the lattice modes, and pressure dependences of vibrational frequencies as well as abrupt shifts of lattice mode frequencies and splitting of the doubly degenerate bending mode, as shown in Figure 1A and B. These phases are designated CzNz I1 and C2N2111. Phase I1 is stable between 0.5 and 2.0 GPa, and occurs as a few large crystals that can be easily annealed to even larger crystals at temperatures between 3 13 and 323 K. The transformation from phase I1 to phase 111 at 2.0 GPa can be detected visually by the appearance of a highly polycrystalline sample as well as changes of the vibrational spectra. Phase I11 is stable between 2.0 and 3.5 GPa. Preliminary X-ray diffraction data suggests that phase I1 has a monoclinic structure, with 4 molecules per unit cell. However, the space group and molecular arrangement have not yet been determined. The pressure dependences of the wavenumbers of the vibrational bands differ among the solid phases, and many bands shift abruptly at the phase boundaries. The lattice phonon spectrum of C2N2 I1 at 0.7 GPa consists of two dominant peaks at 84 and 112 cm-', and the double-degenerate bending mode appears as a single peak over the entire stability range of this phase. At the transition from I1 to 111, the wavenumber of the most intense lattice mode decreases abruptly by 10 cm-', new weak features appear in the lattice-phonon spectrum, and two bands of approximately equal intensities but split by 5 cm-I appear in the bending-mode region of the Raman spectrum. This splitting may not have been observed (29) Andrews, B.; Anderson, A,; Torrie, B. J . Raman Spectrosc. 1984, 15, 61.
Cm
-'
Figure 3. Typical Raman spectra of (a) C2N2111 at 2.9 GPa, (b) pDISN at 5.6 GPa, and (c) p-DISN at 9.0 GPa.
in the appropriate combination bands of the infrared spectra because the spectra were collected at relatively low resolution. (At higher resolution, Fabry-Perot interference patterns created by the diamond surfaces dominated the infrared spectra.) Within the resolution of the Raman spectra, typically 3 cm-I, the splitting of the bending mode does not change between 2.0 and 3.6 GPa. The frequencies of the lattice phonons of these three phases of solid C2N2are relatively sensitive to pressure as illustrated in Figure 2 for the stronger and more clearly resolved modes. The frequencies of the phonons are typically 55-60% greater at 3.5 GPa than at 0.7 GPa. These large changes suggest that these crystals are very compressible and the intermolecular separations are significantly reduced at higher pressures. Over the same range of pressures, the internal modes change much less. Figure 3A shows the Raman spectrum of the internal modes of C2N2111 at 2.9 GPa, which is typical of other spectra. The wavenumbers of the major features of this spectrum, the CEN stretching mode (2354 cm-'), C-C stretching mode (864 cm-]), and trans-bending modes (510 and 515 cm-]), differ from the Raman spectrum of the liquid30and solid3' by no more than 30 and 20 cm-], respectively. The pressure dependence of the wavenumber of the CEN stretching mode, 7 cm-'/GPa (Figure 4), is not unusual for strong intermolecular bonds.32 The infrared spectrum of C2N2I1 at 0.9 GPa shown in Figure SA is in similarly close agreement with results at lower pressures.30 Although the v3 fundamental and ~ 1 - ~ combination 5 are obscured by the strong diamond absorption, several combination bands can be observed, including v2 - v 5 at 610 cm-', v4 u5 at 753 cm-', v , + v 5 at 2585 cm-', and u j + u4 at 2672 cm-I. The pressure dependences of the wavenumbers of the v 1 u5 and v 3
+
+
(30) Herzberg, G. infrared and Raman Spectra of Polyatomic Molecules; Van Nostrand: Princeton, 1945, p 293. (31) Richardson, P. M.; Nixon, E. R. J . Chem. Phys. 1968, 14, 4276. (32) Nicol, M.; Ebisuzaki, Y.; Ellenson, W. D.; Karim, A. Rev. Sci. Instrum. 1972, 43, 1368.
Chemical and Phase Transformations of Cyanogen
The Journal of Physical Chemistry, Vol. 90, No. 25, 1986 6729
7-----
a
oc
4
0
b
W
0
z a
m
m m
h
e
W
> -
IU
2330d'
C
A
.k
W
oc
TABLE 11: Raman Frequencies of CzNz and Its Pressure-Induced Products at Different Pressures Raman frea, cm-' C2N2 0 GPa" 503 s (509, 510, 513)
510s
856 w
845 w
2331 vs 2340 vs (2340, 2343, 2344)
cni'
TABLE III: Infrared Frequencies of C2N2and Its Pressure-Induced Products at Different Pressures infrared freq, cm-I
u2,
c-c
711 s 785m
2y4
1410m 2378 m C=N stretching
u4 combinations are shown in Figure 6. From the 8 cm-'/GPa shift of the Y, u5 infrared combination band and the 7 cm-'/GPa shift of the v 1 Raman band, the pressure dependence of the us mode can be estimated to be a modest 1 cm-'/GPa. Above 3.5 GPa, dramatic changes occur in both the Raman and infrared spectra as illustrated in Figures 3b and 5b, respectively. At 3.5 GPa, the wavenumbers of the lattice modes changes only slightly, the C=N stretching mode abruptly decreases by 10 om-', and new features develop in the Raman and infrared spectra. With increasing pressure, these new features in the Raman spectrum become much more intense and the lattice modes weaken and broaden until, at 7 GPa, they are indistinct. The effect of extended laser excitation on the infrared spectrum has not been studied. The wavenumbers of the bands of these vibrational
+
600
trans bending
"Frequencies refer to the values for solid C2N2 at 77 K in ref 31, and the values in parentheses are the ones at 20 K in ref 29. Descriptions are based on C2N2.
+
1000
u4,
stretching 1123 w 1157 w 1249 w 1439s 2364 vs
1400
descriptionsb
(850)
1023 w
1 10
Figure 5. Typical Fourier-transform infrared spectra of (a) paracyanogen at 10.0 GPa, (b) p-DISN at 5.0 GPa, and (c) C2N2I11 0.9 GPa.
673 vw 709s 781 m 875 w
2250
W A V E N UMBER
(C2N2)m (C2NZ)m 0.7 GPa 5.6 GPa 9.0 GPa 404 w 513 s 519 s
--rT 2850
C2N2 0 GPa" 234 s (244)
(C2N2)n (C2N2)m 0.9 GPa 5.5 GPa 10.0 GPa descriptionsb 246 257 us, cis bendingC
(637)
610 m
732 s (746)
753 vs
1105w 1266vw 2158 s (2165, 2168)
410 w 448 m 510m 618 m 664 m 7 6 4 ~ 869 w 1058 m 11121n 1251 w 1400w 1544 w
432 m 510m 648 m ~764 s
~4
or v4
+ u,
+ VS
888 w 1058 sh 1158s 1274s 1367 sh 1560s
2162
2562 s 2585 s (2574, 2591) 2662 m 2672 m (2670, 2673, 2674)
u2 - u j
Ujd
2370m 2615 s
2386 m 2648 m
v,
+ v5
2687 m'
2702 w
v,
+ u4
a Frequencies refer to the values for solid C2N2 at 77 K in ref 3 1, and the values in parentheses are the ones at 20 K in ref 29. Descriptions are based on the C2N2. Frequencies was estimated by the values of v2, v4 in Raman and vz - v5, v4 + v5 in infrared. Frequency of C2N2was estimated by the values v4 in Raman and v3 + v4 in infrared, because of the strong absorption of the diamond.
6730 The Journal of Physical Chemistry, Vol. 90, No. 25, 1986
Yo0 and Nicol
TABLE I V Vibrational Assignment for the Fundamental Mode of (C2N2), at 5.5 GPa" symmetryb mode and c 2h type descriptions ujr R symmetric C = C stretching A, symmetric C=N stretching symmetric C-CN stretching symmetric N-N stretching skeletal C-C stretching C-C=N bending (in phase and in plane) C=N-N bending (in phase and in plane) skeletal deformation (in phase and in plane) skeletal deformation (in phase and in plane) C-CEN bending (in phase and out of plane) C=N-N bending (in phase and out of plane) twisting mode (in phase and out of plane) umbrella mode (in phase and out of plane) C-CEN bending (out of phase, out of plane) C=N-N bending (out of phase, out of plane) scissor mode (out of phase, out of plane) antisymmetric C=N stretching antisymmetric C=N stretching antisymmetric C-CN stretching antisymmetric N-N stretching C=N-N bending (out of phase and in plane) C-C=N bending (out of phase and in plane) skeletal deformation (out of phase and in plane) skeletal deformation (out of phase and in plane)
freq, cm-' 2364 1439 875 781 709 513 404
764 410 257 519
2370 1540 1058 869 664 618 510 448
"Dimeric linear C2N2was chosen as a unit. bBased on dimeric unit, diiminosuccinonitrile, of p-DISN.
spectra of C2N2under several characteristic conditions are compared in Table I1 and 111. The appearance of the sample does not change abruptly near 3.5 GPa; however, the sample develops a uniform pale yellow color which intensifies as pressure increases. This color change implies that, in addition to the changes in the vibrational spectra, a moderately strong, near-ultraviolet electronic transition also develops. The color changes occur independently from exposure to laser excitation (of either the Raman spectra or the ruby) and are reversible with large hysteresis (1 GPa) of pressure, as long as the pressure of the sample of CzNzdoes not exceed 6 GPa. The uniformity of the coloring (even when the sample was subjected to the focussed laser used to excite the ruby) contrasts with the behaviors of CO and other materials which photopolymerize at high pressures and suggests that the transformations occur in the bulk sample. Above 6 GPa, the C2N, samples develop deep yellow-to-brown hue which deepen slowly with increasing pressure until 10 GPa, where the samples turn black rapidly. Absorption losses in the colored samples cause the quality of the Raman spectra to deteriorate seriously, as shown by the 9.0-GPa spectrum in Figure 3c, and obscure many details of the spectra. However, features can be identified near 711, 785, 1410, and 2378 cm-I. These changes of the Raman spectra are not reversible. The color changes do not interfere with collecting infrared spectra, and the positions of the bands in the infrared spectra do not change as the color intensifies between 3.5 and 10 GPa. Above 10 GPa, other very strong absorptions develop in the C=N region of the infrared spectra, such as that in Figure 5c. These changes appear to be irreversible, and a solid product with a metallic black luster can be recovered after reducing the pressure to ambient. Mass spectrometric analysis failed to detect residual cyanogen gas in samples that were opened after 24 h at 12 GPa. Thus, on this time scale, cyanogen is completely and irreversibly converted to polymer. The polymerization kinetics under other conditions are now being studied and will be reported el~ewhere.~'This polymer is extremely stable thermally and chemically. No decomposition products were detected by mass spectroscopy when the sample was heated to 623 K on a solid sample probe; and no evidence of solubility was found for acetone, benzene, CS,, alcohols, or chloromethanes. Discussion Bridgman observed that polymorphism is ubiquituous in solids at high pressures;33 and, below 3.5 GPa, cyanogen is typical in
I 0
1
2
I
4 PRESSURE
6
8
I
/GR
Figure 6. A plot of the wavenumbers of the infrared combination bands, u3 + u4 and v l + v5.
this respect. At high pressures, unsaturated molecules also should be driven to polymerize by the decrease of the free energy of the system associated with the reduction of the molar volume.34 The interesting observation here is that the expected polymerization of cyanogen can be followed through at least two stages, beginning with the reversible chemical reaction from C2N, I11 above 3.5 (33) Bridgman, P. W.; Conant, J. B. Proc. Natl. Acad. Sci. U.S.A. 1929, 15, 680. (34) Weale, K. E. Chemical Reactions at High Pressures; F. N. & E. Spon: London, 1967; p 239.
Chemical and Phase Transformations of Cyanogen
The Journal of Physical Chemistry, Vol. 90, No. 25, 1986 6731
GPa. Infrared and Raman spectra of the product of this reversible reaction suggest that this step follows eq 1 to form a linear chain
PDISN
species,a polymer, poly(2,3-diiminosu&nonitrile) (pDISN), with a dimeric repeating unit. These measurements, however, do not indicate the lengths of the p D I S N chains. Densification probably is the driving force, and the molecular arrangement of the monomers in the C2N2I11 crystal may determine the structure of the polymer. Both the backbone and side groups of p-DISN remain unsaturated. However, at these intermediate pressures, energetic considerations, topological factors, or a mechanistic step with a high activation energy appears to prevent further reaction. Identification of the product as poly(2,3-diiminosuccinonitrile) is based upon the following changes of the vibrational spectra that accompany the reaction: (a) The vibrational spectra of the product are relatively simple. Most of the modes of C2Nz persist in the infrared and Raman spectrum; however, many modes that are inactive in the Raman or infrared spectrum of the monomer appear in both spectra of the product but at slightly different positions. These observations suggest that the product is built of units that contain two C,N2 groups that are related by inversion symmetry. For example, the infrared peak at 2370 cm-' clearly is a nitrile stretching mode and is shifted by 7 cm-l from the Raman peak. (b) The Raman peak at 1439 cm-' is undoubtedly the -C=Nstretching mode. For most immines, this mode would be expected between 1550 and 1600 ~ m - ' . However, ~~ the frequency of the -C=N- stretch depends strongly upon the substituent attached to the nitrogen. The -C=N-H stretch of diiminosuccinonitrile (DISN), for example, is at 1623 cm-' at atmospheric pressure, while the -C=N-C1 mode of dichloro-DISN is at 1543 ~ 1 1 7 - l . ~ ~ This mode also has a strong negative pressure dependence (-9 cm-'/GPa) which means that it occurs at even lower wavenumbers a t high pressures. (c) The chain-bending and -stretching modes of a p-DISN polymer are expected to occur between 400 and 600 and 700 and 800 cm-I, respectively. The prominant Raman bands at 709 and and =N-N= stretches of the 781 cm-' may be the =C-Cpolymer chain, respectively. This assignment of the =C-C= stretch agrees well with those of DISN (745 cm-') and dichloro-DISN (717 ~ m - l ) . ~ ~ Table IV shows a plausible assignment of the vibrational spectra of the chain polymer at 5.5 GPa (5.6 GPa for Raman) based on DISN as the repeat unit. Most of the peaks have been assigned; however, weak features near 1150 cm-' (1 123, 1157, and 1249 cm-' in the Raman and 11 12 and 1400 cm-' in infrared) cannot be assigned as fundamentals or obvious combinations for this structure. These bands may result from a minor product with an aromatic ring such as a pyrazine nitrile, defects in the polymer chain, or terminal groups. (35) Dollish, F. R.; Fateley, W. G.; Bentley, F. F. Characteristic Raman Frequencies of Organic Compounds; Wiley: New York, 1973. (36) Powell, D. L.; Popovic, L.; Kloeboe, P.; Nielsen, C. J. Spectrochim. Acta 1980, 36A, 29.
DISN is a highly versatile polyfunctional reagent?' from which many heterocycles have been synthesized through condensation and displacement reactions. 4 2 cycloadditions by the -N= C-C=N- heterodiene moiety of DISN and displacement of the nitriles3* are well-known examples of these reactions. Similar reactions can be expected to occur for p-DISN; however, the rate at which they will occur in the polymeric solid should depend strongly upon the distances between the adducts. Cycloaddition should occur more rapidly as the separations between the adduct decrease and, therefore, at higher pressures. Thus, a plausible, although very idealized, mechanism for the high pressure transformation of p-DISN to the black paracyanogen polymer is illustrated in eq 2. There is no evidence to suggest that the reaction is as ordered as the equation suggests.
+
Y
N
P D I S N
PARACYANOGEN
(2)
Two different (C,N,), structures have been reported for paracyanogen, namely a chain39 and a ladder.40 The infrared spectrum of the chain polymer39 has been reported, but not that of the ladder polymer. The infrared spectra of the ladder polymer is expected to be similar to that of the chain polymer, but the nitrile vibrations should be weak or absent. The infrared spectra of the polymer a t 10.0 GPa in Figure 5c are, indeed, similar to the published spectrum of chain paracyanogen except for the weakness of the nitrile modes. Thus, the spectra strongly suggest that predominant structural element of the high-pressure product is the ladder paracyanogen. In summary, several aspects of the phase relationship and chemical instability at high pressures of the unsaturated molecule, cyanogen, have been described in this report. Near room temperature, three solid phases of occur at pressures below 3.5 GPa. Cyanogen polymerizes to a linear-chain poly(2,3-diiminosuccinonitrile) at intermediate pressures (3.5-10 GPa) and to a paracyanogen ladder of pyrazine rings above 10 GPa. Topochemically assisted densification seems to be the main driving force for these reactions.
Acknowledgment. Support provided for this work by grants from the U S . National Science Foundation (DMR83-18812) and Los Alamos Branch of the University of California Institute of Geophysics and Planetary Physics (No. 028) are gratefully appreciated. Registry No. PDISN (SRU), 104619-15-0; NCCN, 460-19-5; paracyanogen, 25215-76-3. (37) Begland, R. W.; Hartter, D. R. J . Org. Chem. 1972, 37, 4136, and references cited therein. (38) Fukunaga, T.; Begland, R. W. J . Org.Chem. 1984, 49, 813. (39) Peska, J.; Benes, M. J. Collect. Czech. Chem. Commun. 1966, 31, 243. (40) Bircumchaw, L. L.; Tayler, F. M.; Wiffer, D. H. J. Chem. SOC.1954, 931. (41) Yoo, C . - S . ;Nicol, M. J . Phys. Chem., this issue.
