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J . Phys. Chem. 1984, 88, 3176-3179
barrier exists or unless they are not exothermic. Proton transfers between a particular ion and a series of neutrals generally exhibit efficiencies close to unity until the reactions become endothermic, at which point the reactions become very slow or do not proceed at all. Such kinetic observations have been used to determine relative acidities.'8-21 Our kinetic data strongly suggest that the equilibrium gas-phase acidity of HF lies between neopentyl alcohol and benzyl alcohol. In our previous discussion of photochemical branching fractions,l5 we noted that the slow kinetics of reaction 1 could be explained by a barrier, and the size of the branching fractions for the proton-bound complex by an energy difference between the two decomposition channels greater than that indicated by the thermochemistry; the latter could be caused by rotational excitation of the complex. A simpler explanation now suggests itself the thermochemistry is in error by about 2 kcal/mol, which explains not only the branching fraction and kinetics, but also the long(18) Bohme, D. K.; Mackay, G. I.; Tanner, s.D. J . Am. Chem. SOC.1979,
101, 3724-30.
(19) Tanner, S. D.; Mackay, G. I.; Bohme, D. K. Can. J. Chem. 1981,59, 1615-21. (20) Bohme, D. K.;Rakshit, A. B. Mackay, G. I. J. Am. Chem. SOC.1982, 104, 1100-1. (21) Meot-Ner, M. J . Am. Chem. SOC.1982, 104, 5-10.
standing difference between RO-H bond energies as determined by kinetic and spectroscopic methods. The gas-phase acidity scale has been developed from many equilibrium measurements and anchored by as many trustworthy absolute values as possible. It seems likely that the 2 kcal/mol discrepancy is due to the difficulty in studying reactions of HF; therefore, only the neopentyl alcohol-HF equilibrium constant is suspect. We propose that the present set of relative acidities of aliphatic alcohols be used in conjunction with the absolute acidity of methanol, as determined by kinetic7 and spectroscopic12 measurements, to form a new set of alcohol acidities. These values are listed in Table I. Methanol is chosen as the anchor point because its 0-H bond energy (104.0 0.2 kcal/mol) and alkoxide electron affinity (36.2 f 0.5 kcal/mol) are both known to excellent precision. The resulting heat of deprotonation is 381.4 f 0.6 kcal/mol. We feel that the proposed changes will produce values that better describe the behavior of these alcohols and will serve as a more reliable anchor for the lower end of the acidity scale. Thus, it is likely that other acidities which lie near H F will have to be revised as well.
*
Acknowledgment. We are grateful to the National Science Foundatior, for support of this work and for a graduate fellowship (C.R.M.). We also thank the donors of the Petroleum Research Fund, administered by the American Chemical Society.
New Phases and Chemical Reactions in Solid CO under Pressure Allen 1. Katz: David Schiferl, and Robert L. Mills* University of California, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received: April 9, 1984)
The Raman scattering of solid carbon monoxide was studied in a diamond cell from 15 to 297 K at pressures from 1.0 to 5.8 GPa. At low temperature a transition occurs near 3.4 GPa from the known a-phase (space group P&3) to a new e-phase (structure unknown), rather than to the predicted y-phase (P42/mnm).A transformation from /3-CO (space group P63/mmc) into a new &phase (Pm3n?) was found near 5.2 GPa at room temperature. Above about 4.6 GPa and 80 K, CO reacts photochemically when irradiated with visible laser light. The photoreactivity may be associated with the formation of a yellow polymer, which can be recovered at zero pressure.
Introduction It is well-known that the condensed phases of carbon monoxide and nitrogen exhibit striking similarities. The molecules CO and N2are isoelectronic, have the same size and weight, and condense into liquids with almost identical molar volumes. Liquid CO and N, freeze at triple points of 68.15 and 63.14 K, respectively, forming a disordered hexagonal solid, the 0-phase, which on further cooling transforms into a molecularly ordered cubic structure, the a-phase. The @-a crystallographic transition occurs at 61.6 K for C O and 35.6 K for N,, and is driven primarily by electric quadrupole-quadrupole interactions. The structure of P-N2at low pressure has been determined',, by single-crystal X-ray diffraction to have space group P63/mmc, with the molecules tilted from the hexagonal c axis by about 56' and either precessing or statically disordered. The nature of the disorder in hexagonal 0-CO at low pressure is ~ n k n o w n . ~Recently, however, Cromer et al.435showed by X-ray measurements in diamond cells that both CO and N, freeze at room temperature near 2.5 GPa into P-solids with almost the same molecular disorder and molar volume. The molecults in these high-pressure crystals are statically disordered and tipped from the hexagonal c axis by 54O for N, and 49O for CO. 'Associated Western Universitiesappointee during 1984. Present address: Arizona State University, Tempe, AZ.
0022-3654/84/2088-3176$01.50/0
In the a-phase, it is now generally accepted that the homonuclear molecules of N2 occupy centrosymmetric positions6 in space group Pa3, while heteronuclear CO, lacking inversion symmetry, packs7 as P213, with the molecules only very slightly shifted* from the Pa3 arrangement. In a-CO the molecules exhibit a large amount of end-for-end di~order,~ which becomes effectively frozen inlo well above 5 K, where transition to the ordered state would be expected." For this reason, the Raman spectrum of a-CO shows marked differences7 from that of a - N z . (1) Streib, W. E.; Jordan, T. H.; Lipscomb, W. N. J . Chem. Phys. 1962, 37, 2962. (2) Jordan, T. H.; Smith, H. W.; Streib, W. E.; Lipscomb, W. N. J. Chem. Phys. 1964, 41, 756. ( 3 ) Vegard, L. Z . Phys. 1934,88,235. (4) Cromer, D. T.; Schiferl, D.; LeSar, R.; Mills, R. L. Acta Crystallogr., Sect. C 1983, 39, 1146. ( 5 ) Schiferl, D.; Cromer, D. T.; Ryan, R. R.; Larson, A. C.; LeSar, R.; Mills, R. L. Acta Crystallogr., Sect. C 1983, 39, 1151. (6) Venables, J. A.; English, C. A. Acta Crystallogr., Sect. E 1974,30, 929. (7) Anderson, A.; Sun, T. S.; Donkersloot, M. C. A. Can. J . Phys. 1970, 48, 2255. (8) Hall, B. 0.; James, H. M. Phys. Reu. E 1976, 13, 3590. (9) Clayton, J. 0.; Giauque, W. F. J . Am. Chem. SOC.1932, 54, 2610. (10) Walton, J.; Brookeman, J.; Rigamonti, A. Phys. Rev. E 1983, 28, 4050. (11) Melhuish, M. W.; Scott, R. L. J . Phys. Chem. 1964, 68, 2301.
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 3177
Letters Ordered cubic a-N2 (Pa3) transform^'^,'^ into ordered tetragonal y-N2 (P4,lrnnm) at temperatures between 0 and 40 K and corresponding pressures from 0.35 to 0.45 GPa. In contrast, no such transition has been observed in solid CO by X-ray studies13 up to 0.5 GPa or by compressibility measurement^'^ up to 1.0 GPa. The stability of a - N 2 at low pressure has been des~ribed’~-’’ in terms of interactions between axial quadrupoles, whereas y-N2 stability at high pressure has been ascribed18 to nonaxially symmetric quadrupoles. Felsteiner and LitvinI9 have shown, from symmetry considerations of quadrupoles only, that it is impossible for CO to have a structure similar to y-N2. Raich and Mills,zo however, using a parametric, shape-dependent, hard-core potential, similar to one that gave a satisfactory account of y-N2, predicted that CY-CO should be stable up to 4.8 GPa, where transformation to a y phase should occur. When pressurized to about 4.5 GPa at room temperature, hexagonal &N2transforms21,22into a fourth structure, disordered cubic 6-N, (Pm3n). The phase boundary of this highest-pressure form has been traced down to helium temperature by Raman spectroscopy in a diamond cell.23 Cromer et al.4 reported that C O at 4.2 GPa and room temperature undergoes a photochemical reaction when exposed to 10 mW of 19435-cm-’ argon laser light. It was suggested that the structure of this photosensitive phase of CO might also be Pm3n, by analogy with 6-N2which appears at about the same pressure. Previous experiment^^,^^-^^ have shown that the Raman-active y-, ,and 6-Nz are suffivibrational and phonon modes of CY-, /Iciently different to allow identification of these crystal forms from spectroscopic measurements alone over large regions of the phase diagram. An analogous situation is expected for CO, although the spectra may be of lower quality because of the smaller Raman scattering cross section in CO. The present study was carried out to establish the P-T phase diagram of solid carbon monoxide from about 15 to 300 K and 1.O to 5.8 GPa, using Raman scattering in a diamond cell. We find that a y-phase does not exist in pure CO at low temperature. Instead, a transition from a - C O to an unknown structure takes place somewhat below the a-y transition pressure calculated by Raich and Mills.” We also find that solid CO above 5.2 GPa at room temperature shows a splitting in the vibron Raman peak, which is similar to that in 6-N2 and characteristic of the Pm3n structure. Above about 4.6 GPa and 80 K, the sample is photosensitive to low-intensity laser light. Finally, we report the exciting discovery that high-pressure CO forms a polymer-like material that can be recovered at zero pressure.
Experimental Section A Merrill-Ba~sett~~ diamond-anvil cell, fitted with a precompressed Inconel X750 gasket, was filled by condensing high-purity CO gas into it, using the indium-dam technique.26 Special precautions were taken to preclude the entry of liquid-nitrogen coolant into the dam. Mass spectrometric analysis of the gas showed 0.04% Ar and 0.01% C 0 2 as the only detectable impurities. From infrared spectra the Nz content of the gas was crudely estimated to be below 1%. When filled, the diamond cell was mounted on the cold finger of a Janus Model S/T helium-flow cryostat. Temperatures from 15 to 300 K could be set to within ~
~~
~
(12) Mills, R. L.; Schuch, A. F. Phys. Reo. Lett. 1969, 23, 1154. (13) Schuch, A. F.; Mills, R. L. J . Chem. Phys. 1970,52, 6000. (14) Stevenson, R. J . Chem. Phys. 1957, 27, 673. (15) Jansen, L.; deWette, F. W. Physica 1956, 22, 644. (16) Kobin, B. C. J. Chem. Phys. 1960, 33, 882. (17) Nagai, 0.;Nakamura, T. Prog. Theoret. Phys. (Kyoto) 1960,24,432. (18) Felsteiner, J.; Litvin, D. B.; Zak, J. Phys. Rev. B 1971, 3, 2706. (19) Felsteiner, J.; Litvin, D. B. Phys. Rev. B 1971, 4, 671. (20) Raich, J. C.; Mills, R. L. J. Chem. Phys. 1971, 55, 1811. (21) LeSar, R.; Ekberg, S. A.; Jones, L. H.; Mills, R. L.; Schwalbe, L. A.; Schiferl, D. Solid State Commun. 1979, 32, 131. (22) Cromer, D. T.; Mills, R. L.; Schiferl, D.; Schwalbe, L. A. Acta Crystallogr., Sect. B 1981, 37, 8. (23) Buchsbaum, S.; Mills, R. L.; Schiferl, D. J. Phys. Chem. in press. (24) Medina, F. D.; Daniels, W. B. J . Chem. Phys. 1976, 64, 150. (25) Merrill, L.; Bassett, W. A. Rev. Sci. Instrum. 1974, 45, 290. (26) Mills, R. L.; Liebenberg, D. H.; Bronson, J. C.; Schmidt, L. C. Reu. Sci. Instrum. 1980, 51, 891.
f l K by varying the flow of liquid helium and by regulating an electrical heater on the cold finger. Pressures were measured by the frequency shift of in situ fluorescing ruby p ~ w d e r , ~assuming ~ * ~ * the relation
P = 380.8([vo(T)/vp(T)1~ - 1)
(1)
where P is the pressure in GPa, vp( T ) is the frequency of the ruby R, line at pressure P and temperature T, and vo(T) is the corresponding frequency at the same T and zero pressure. Equation 1 is valid at room temperature and its low-pressure linear approximation has been checked to 1.O GPa at helium t e m p e r a t ~ r e . ~ ~ The quantity yo( T ) was previously determined for our particular sample of Although pressures could be measured with the sample cold, it was necessary to remove the cell from the cryostat to make pressure changes. Both the ruby-fluorescence pressure measurements and observations of the Raman spectra were made with a Spex 1403 spectrometer. The resolution was 3 cm-’ and, typically, several spectra were signal averaged. Above 4.6 GPa at temperatures from 80 to 297 K, pure CO rapidly undergoes photochemical reaction when exposed to any of the visible lines of argon or krypton lasers. Because the reaction is somewhat slower for red light, the 15 454-cm-I Kr line was used for all room-temperature runs above 4.6 GPa. The laser power, focussed on a spot of the sample about 30 p m in diameter, was kept below 200 mW. Under this condition, we could measure Raman spectra from the CO sample in the 15 min or so before photoreaction interfered with the signal. At 15 K photochemical reaction was not observed even after a 10-h exposure to 20 492cm-l Ar laser light.
Results In pure, unreacted CO, we discovered two new solid phases, one at 3.4 GPa along an isotherm at 15 K and the other at 5.2 GPa and room temperature. To confirm that the transformations into these forms were reversible, the samples were cycled in T and P to regenerate the known a- and 0-phases of CO from which the new phases had been formed. Since the new CO phase at 297 K has a Raman spectrum similar to that of 6-Nz,we designate it 6-CO. The spectrum of the phase observed above 3.4 GPa at 15 K has no obvious counterpart in any of the N, structures, and we call this form e C 0 . Typical Raman spectra in the stretching-mode region for cy-, 0-, 6-, and 6-CO are shown, respectively, in parts a, b, c, and d of Figure 1. In Figure 2a we show the 50-150-cm-I spectral region for low-temperature a-CO and in Figure 2b we show the same region for E-CO. No lattice modes were observable in the higher-temperature 0- or 6-CO phases. The pressure dependence of the stretching-mode peaks at 15 and 297 K is shown in Figure 3a. We present the lattice frequencies at 15 K as a function of pressure in Figure 3b. Surprisingly, room-temperature CO transforms at about 5 GPa into a polymer-like solid. This material appears first as a clear and colorless film, but at higher pressure or with time it can take on an intense yellow color. The exact conditions required to produce the yellow form have not been determined. One sample, which had been pressurized to 5.4 GPa, exhibited both clear and yellow regions after it was decompressed to 1.8 GPa (below the normal melting pressure of CO) and allowed to stand for 2 weeks. The solid polymer can be removed from the diamond cell on release of pressure. The yellow form persists for months in air, although it gradually darkens. When examined by infrared spectroscopy, an exposed sample exhibited a sharp absorption edge at 3700 cm-’, below which it was completely opaque, except for a small window near 2200 cm-l. At room temperature both the clear and yellow solids react photochemically when irradiated with (27) Forman, R. A.; Piermarini, G. J.; Barnett, J. D.; Block, S. Science 1912, 176, 284. (28) Barnett, J. D.; Block, S.; Piermarini, G. J. Reu. Sci. Instrum. 1973, 44, 1. (29) Noack, R. A,; Holzapfel, W. B. In “High Pressure Science and Technology, Sixth AIRAPT Conference;” Timmerhaus, K. D.; Barber, M. S., Eds.; Plenum: New York, 1979; Vol. 1, p 748.
3178
The Journal of Physical Chemistry, Vol. 88, No. 15, 1984
Letters
2160 -
1 I
2
3
5
6
PRESSURE ( G P O )
Figure 3. Raman frequencies in CO as a function of pressure. (a) Stretching modes. Open circle, 297 K present measurement; open triangle, room-temperature gas value; closed circle, 15 K, present measurement; closed triangle, low temperature, ref 7. (b) Lattice modes. Closed circle, 15 K, present measurement; closed triangle, low-temperature, ref 7. Bars indicate full width a t half-maximum.
low-intensity visible light from krypton or argon lasers. The reaction, which takes place with the sample either inside or outside the diamond cell, gives brownish colored products with no discernable Raman signature.
Raman sh 1 f t [ern-'] Figure 1. C O stretching-mode features for the four solid phases of unreacted CO: (a) CY-COat 2.3 GPa and 15 K; (b) @-COat 2.7 GPa and 297 K; (c) 6-CO at 5.0 GPa and 297 K; and (d) e-CO at 4.3 GPa and 15 K.
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tL?
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u
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, 50.00
100.00
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~
150.00
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Figure 2. Lattice-mode region for the low-temperature phases of CO: (a) CY-COat 2.3 GPa and 15 K; (b) e-CO at 4.3 GPa and 15 K.
Discussion The statistical error in the spectra of Figures 1 and 2 is relatively large because we were obliged to keep the laser intensity low and the scans short to minimize photodecomposition of the sample. Nevertheless, the features we report are unmistakably present. In Figure 1 we see that both a-CO and @-COhave a single stretching-mode peak, 6-CO has two such features, and 4 0 , curiously, has four. The low-frequency regions of a-CO and e-CO are also quite different. Figure 2 shows only one broad peak in a-CO, while there are four discernable lattice features in e-CO at 4.3 GPa. Below 4.0 GPa, however, the two peaks at the highest frequencies in e-CO merge into one and cannot be resolved. The Raman-active lines that we observe in cy- and 0-CO under pressure have the same character as those reported7 at P = 0. At 15 K, CO is a solid at all pressures, and the frequencies of the low-temperature peaks in Figure 3, a and b, form smooth extensions of the zero-pressure values. The plot in Figure 3a at 297 K, however, corresponds to fluid CO up to about 2.5 GPa and solid CO above this pressure. The vibrational frequency at first decreases with pressure and then rises. There appears to be no noticeable discontinuity in v at freezing, and we interpret this as indicating that the @-phasehexagonal crystal field has little additional influence on the vibrons a t 2.5 GPa. We assume that the splittings in the vibrational spectra that we observe in Figure 1 c and d, are due to different molecular site symmetries in the 6-CO and e-CO lattices and, therefore, cannot be fully explained until the crystal structures have been determined in detail. We note that the twin peaks in 6-CO are reminiscent of those in 6-Nz which belongs to cubic space group Pm3n. The relative intensities of the two peaks in 6-CO are roughly the same as those2' in &NZ. In CO, however, the splitting
The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 3179
Letters
8(Prn3") ?
?. -
/
/
--
4
/--
I
/
/
,
P(PB,/rnmc) / /
/
/
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i -
TEMPERATURE (K) Figure 4. Preliminary phase diagram of CO. Open circle, a-phase
spectra, Figures la and 2a; open hexagon, @-phasespectrum, Figure lb; open square, &phase spectrum Figure IC; closed circle, e-phase spectra, Figures Id and 2b; closed hexagon, &phase X-ray data, ref 4; heavy solid line, a-@and @-fluid phase boundary NMR data, ref 30; heavy dashed line, phase-line segment, present work; light dashed line, phase-line, interpolation; dotted-line arrow, ref 14, compressibility run showing aphase only. of 6 cm-' is only about half that2' for N2 a t 5 GPa. From an analysis of C O spectra, visual observations, and data of others, we have constructed the provisional phase diagram shown in Figure 4. The heavy solid lines are phase boundaries determined up to about 0.2 GPa by N M R measurements30 in i3C160.Similarly, the heavy dashed lines represent transitions determined by present Raman and past X-ray4 scattering studies along 15 K and room-temperature isotherms. The light dashed curves connecting the phase-line segments in Figure 4 are reasonable interpolations, based on similar lines from the N 2 phase diagram. The melting curve of CO is almost identical with that of Nz. We emphasize that Figure 4 is offered as guidance for future experiments which are obviously needed to fill in the missing phase boundaries. Although CO and N2 have been shown to be close molecular analogues at low pressure, the present results reveal striking (30) Fukushima, E.; Gibson, A. A. V.; Scott, T. A. J . Low Temp. Phys. 1977, 28, 151. (31) Snow, A. W.; Haubenstock,H.; Yang, N.-L. Macromolecules 1978, 11, 11.
differences between the behavior of solid C O and N, at high pressure. Raich and Millsz0calculated that a-CO would be stable up to about 4.8 GPa, where a transition to y-CO was expected to occur, by analogy to a similar transition in N2. As shown in Figure 4, a-CO does indeed transform, but at 3.4 GPa and into an entirely new t-structure with a Raman spectrum quite different from that of y-N2. We have demonstrated, however, that a y-phase can be stabilized in solid solutions of N2 in CO, although the required concentration of N 2 has not yet been determined. While room-temperature N 2 is chemically stable23up to at least 50 GPa, C O is photoreactive above 4.6 GPa. In mixtures of CO and N2, C O similarly photoreacts. On the other hand, C O at 15 K is quite resistant to photoreaction at our highest experimental pressures. Some inferences concerning the nature of the new high-pressure phases in C O can be drawn from the present Raman spectra, but X-ray diffraction studies now underway are needed to determine unambiguously the crystal structures. We are also carrying out experiments to characterize the interesting polymer that we have synthesized in diamond cells at high pressure. Although this new material is fairly stable under ambient conditions, diagnostic studies are made difficult by the sample's minute size and its sensitivity to photoreaction. One possible explanation of our observations is that, above about 4.6 GPa and 80 K, CO begins to disproportionate chemically, forming carbon suboxide (C302), which then polymerizes. The infrared spectrum of poly-(C302), synthesized at ambient pressure, has been reported3I to have a sharp absorption edge at 3700 cm-' and a maximum in transmission at 2200 cm-', features we have seen in our sample. The disproportionation and polymerization are probably accelerated by higher pressure and temperature. Monomeric C3Oz and/or its polymer may be carbonized by visible laser irradiation. It is clear that future studies of pure CO above 4.6 GPa will require a cryostat in which the pressure can be increased at low temperature to prevent polymerization and photochemical reaction. Acknowledgment. We thank G. F. Mortensen and L. H. Jones for carrying out analyses to characterize the purity of our CO samples. Jones also ran an infrared spectrum on a recovered solid sample. We are indebted to our colleague, M. F. Nicol, for suggesting that the new solid might be polymerized carbon suboxide. We acknowledge other useful discussions with R. LeSar and S. F. Agnew. A.I.K. expresses appreciation for financial support provided by Associated Western Universities. Work was sponsored by the Los Alamos National Laboratory Center for Materials Science and was performed under the auspices of the U. S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences. Registry No. CO, 630-08-0.
