J. Phys. Chem. 1989, 93, 2143-2147 the charge-transfer complex remains to be elucidated.
Conclusions A molecule's solvation region influences its rate of reaction, solubility, and other physicochemical processes. Supercritical fluid solvents are innately suited for the study of solvent effects, effectively bridging the gap between the gas phase and the liquid phase. The metal chelates studied show complex physicochemical behavior in pure and binary supercritical fluids as a function of temperature, modifier concentration, and density. The dynamics of the fluid on solvation for this system is, at best, poorly understood. The solvation structure (local composition) about the Fe"(1,lO-phenanthroline):+ chelate does not change significantly as indicated by the absence of density-dependent spectroscopic shifts and the minimal effect of bulk methanol concentration on the thermochromic shifts for the Fe"( l,lO-phenanthroline):+ complex. A limiting bulk methanol concentration is necessary to solvate the charged phenanthroline complex due to the low dielectric of pure CO,; whereas, the uncharged Fe"'(2,4-pentanedi0nate)~ is soluble in pure C 0 2due to the keto-enol equilibrium of the ligand.
2143
The Fe"'(2,4-~entanedionate)~chelate shows quite different spectroscopic behavior because of the keto-enol equilibrium and a chelate structure that allows the solvent access to the core of the chelate. The ability of methanol to interact with a limited number of specific sites on the metal chelate is demonstrated by the lack of a spectroscopic shift as a function of methanol concentration beyond a bulk concentration of 0.04 mol fraction. Depending on local density and local composition about the metal chelate, a blue shift can be seen for Fe"'(2,4-pentanedi0nate)~. Metal chelate solubility in supercritical fluid solvents suggests the ability to study charge-transfer molecules of interest in biologically related redox systems. Further work is being undertaken in this laboratory studying solvent effects on metal chelates with supercritical fluids and electron transfer reactions in supercritical fluids. Acknowledgment. We acknowledge the support of the US. Department of Energy, Division of Chemical Sciences, Office of Basic Energy Sciences, under Contract DE-AC06-76RLO 1830. Registry No. Fe(II)( l,10-phenanthroline)32t,14708-99-7;Fe(II1)(2,4-pentanedionate),, 14024-18-1.
Laser- Initiated Polymerization of Solid Formaldehyde Edward S. Mansueto, Chang-Yuan Ju, and Charles A. Wight* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (Received: July 13, 1988)
Polymerization of solid films of formaldehyde has been investigated by infrared absorption spectroscopy at 10 and 77 K. Samples are prepared by condensing gaseous mixtures of chlorine and formaldehydedirectly onto a CsI optical window cooled by a liquid nitrogen Dewar or closed-cycle helium refrigerator. The amorphous films are photolyzed with an excimer laser at 308 nm to dissociate the chlorine molecules, initiating a reaction that forms formyl chloride and HCI. The HC1 then induces the oligomerization of neighboring formaldehyde molecules in the solid and eventually leads to formation of poly(oxymethy1ene) (POM) upon warming. Photochemical quantum yields are reported for formation of the various products at different relative concentrations of formaldehyde and chlorine. The experiments reveal details of the initial steps in photochemical polymerization reactions within disordered solids. Some possible implications for the role of POM in the formation of comets and interstellar dust are briefly discussed.
Introduction Fundamental studies of chemical reactions in the solid state are important from the standpoint of learning how atoms and molecules can react in an environment that severely restricts molecular motion. We have recently reported several studies of reactions of chlorine with small hydrocarbons frozen as thin films Reactions are initiated by laser on an optical photolysis of a radical precursor (e.g., C12). Product yields and branching ratios are determined spectroscopically (mainly by infrared absorption and Raman scattering). Many of these results suggest that reactions in amorphous solids proceed via a random walk mechanism in which a radical may react only with one of its nearest neighbors in the solid. If all of its neighbors are unreactive species, the radical is trapped and the reaction sequence stops. The observed dependence of the photochemical quantum yields on the relative concentrations of chlorine and hydrocarbon is consistent with this simple picture of reaction dynamics in disordered solids. The present study was initiated in order to investigate solid-state chemistry involving only one type of reactant (Le., chain polymerization). Formaldehyde was chosen for these model studies because of its known propensity to polymerize at low tempera(1) Sedlacek, A. 1987, 109, 6223, (2) Sedlacek, A. (3) Sedlacek, A. (4) Sedlacek, A.
J.; Mansueto, E. S.;Wight, C. A. J . Am. Chem. SOC. J.; Wight, C . A. J . Phys. Chem. 1988, 92, 2821. J.; Wight, C. A. J . Chem. Phys. 1988, 88, 2847. J.; Wight, C. A. Laser Chem. 1988, 9, 155.
0022-365418912093-2143$01 SO10
ture~.~-' Formaldehyde forms a variety of polymers under different reaction conditions, many of which are brittle or unstable toward depolymerization or oxidation.* However, high molecular weight formaldehyde polymers and copolymers are also the basis for several commercially important plastics (Delrin, Celcon, and Bakelite, among others). Our investigation differs from previous studies of formaldehyde polymerization in several significant ways. We have used infrared and Raman spectroscopies to identify important species formed in the initial stages of the reaction. The emphasis, therefore, is on the molecular properties of the solid as it undergoes photochemical change. The technique is designed to produce disordered amorphous solid films of formaldehyde, and the results are interpreted on that basis. To initiate the reaction, we have utilized UV photolysis of small amounts of molecular chlorine doped into the solids (as opposed to y irradiation used in several previous studies). Therefore, the initial photochemistry is well-characterized and photochemical quantum yields have been determined for the reaction. Current interest in formaldehyde polymerization also stems from the recent observation of POM in the coma of comet Halley by a mass spectrometer aboard the spacecraft Giotto?Jo It has been ( 5 ) Goldanskii, V. I. Annu. Reu. Phys. Chem. 1976, 27, 85. (6) Goldanskii, V. I.; Frank-Kamenetskii, M. D.; Barkalov, I. M. Science 1973, 182, 1344. (7) Tsuda, Y. J. Polym. Sci. 1961, 44, 369. (8) Walker, J. F. Formoldehyde; Reinhold: New York, 1964.
0 1989 American Chemical Society
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Mansueto et al.
suggested that POM may serve as a “glue” that binds small particles of graphite and silicates into larger dust particles. Investigations of formaldehyde chemistry at very low temperatures are therefore crucial to the development of realistic models for formation of dust grains and comets in interstellar clouds.
Experimental Details Solid films of chlorine and formaldehyde were formed by premixing the gases in a glass vacuum manifold and depositing directly onto a cold substrate mounted at the base of a liquid nitrogen Dewar or closed-cycle helium refrigerator. Most experiments were performed at 77 K, but similar results have been obtained at temperatures as low as 10 K. The gaseous mixtures were prepared at room temperature in a darkened room and deposited at a rate of 7-8 Kmol/min. For IR absorption experiments, the substrate was a polished CsI window mounted in an oxygen-free high-conductivity copper retainer with indium gaskets for good thermal conduction between the window, retainer, and cold finger. Ultraviolet absorption spectra were obtained by using a Suprasil quartz window mounted in a similar fashion. Raman spectra were obtained by depositing samples directly onto a polished copper wedge. After the sample was deposited, spectra were obtained by placing the Dewar vessel into the sample chamber of an FTIR spectrometer (Biorad/Digilab Model FTS-40). Ultraviolet spectra were obtained with a Varian Model Cary 17 spectrometer, and Raman spectra were obtained by exciting with an Ar+ laser at 488.0 nm and collecting scattered light with a Spex double monochromator and phototube operated in photon counting mode. The samples were then photolyzed with an excimer laser at 308 nm. The fluence was normally limited to 1 mJ/cm2 per pulse, and each sample was irradiated with several thousand pulses at 10 pulses/s. The fluence of the laser was measured by passing the beam through an aperture of known size and measuring the average power with an absorbing disk calorimeter (Scientech Model 38-01). It was usually necessary to attenuate the laser output by passing it through several layers of Pyrex plate glass to reduce the risk of sample vaporization and/or transient heating. The total chlorine concentration was kept low so that the samples were optically thin at 308 nm (typically 0.05 absorbance unit with a maximum of 0.2). Working with optically thin samples makes quantitative determination of the photochemical yields more difficult. On the other hand, homogeneous photolysis of the samples is essential for proper interpretation of the reaction product infrared spectra. Following irradiation, spectra were obtained to determine the extent of reactant loss and product formation. For IR studies, integrated absorption intensities were obtained for characteristic bands of each species. Positioning of the chamber in the beam was sufficiently reproducible that no significant error resulted from this source. However, quantitative results could only be obtained from Raman spectra by comparing reactant and product band intensities to that of a photochemically inactive internal standard. For this purpose, a thin layer of SF6 was predeposited on the copper block before deposition of each chlorine/formaldehyde sample. Chlorine and hydrogen chloride were obtained from Matheson G a s Products and were purified by trap-to-trap distillation prior t o sample preparation. Formaldehyde was obtained by pyrolysis of commercial paraformaldehyde (J.T. Baker Chemical Co.) and was purified by trap-to-trap distillation. This sample polymerized to a low molecular weight polymer of formaldehyde which gave rise to a residual vapor pressure of several Torr of monomer at room temperature. Samples were prepared in the glass manifold by freezing C H 2 0 under liquid N2 and then warming until the desired quantity of monomer was obtained (typically 3 Torr in a 0.25-L volume). An appropriate quantity of purified chlorine (9) Mitchell, D.L.; Lin, R. P.; Anderson, K. A.; Carlson, C. W.; Curtis, D. W.; Korth, A.; Reme, H.; Sauvaud, J. A,; d’Uston, C.; Mendis, D. A. Science 1987, 237, 626. ( I O ) Huebner, W. F. Science 1987, 237, 628.
I
i
h
a)
Ij
_1
1 LJUL
I 11
1800
1400 1000 600 WAVENUMBERS Figure 1. Transmission infrared spectra of solid films of formaldehyde deposited on a CsI cold window under various conditions. Each tick mark on the vertical axis represents 1 absorbance unit (base IO). (a) An 11:l mixture of CH20:CI2deposited at 77 K prior to photolysis. (b) Same sample as (a) following photolysis with 5000 laser pulses at 308 nm (1.0 mJ/cm2 per pulse). (c) A 1O:l mixture of CH20:C12deposited and photolyzed at 10 K (5000 pulses, 1.0 mJ/cm2/pulse). (d) A 1O:l mixture of CH2O:HCI deposited at 10 K (not photolyzed). (e) The same sample as (a) and (b) following warming to room temperature.
gas was admitted to this volume, and the mixture was then deposited through a metering valve and deposition line onto the cold window. No evidence of water or other impurities was observed in the IR or Raman spectra of samples prepared in this way.
Results Product Identification. Figure l a shows the infrared spectrum of a thin film of chlorine in formaldehyde monomer before laser photolysis. Absorption bands attributable to formaldehyde are clearly The frequencies are red-shifted from the gas-phase fundamentals by typically 5-10 cm-I. The C-0 stretching vibrational frequency for C H 2 0 was red-shifted by 30 cm-’ from the gas-phase value. Chlorine is a homonuclear diatomic and is therefore IR inactive. Photolysis of this sample at 308 nm results in a decrease in the formaldehyde band intensities and the appearance of numerous new product bands (Figure lb). One of the products, CHC10, was identified by its characteristic absorption bands13at 720 and 1760 cm-l. The remaining product bands are assigned to an oligomer of f0rma1dehyde.I~ A series of bands in the 850-1 150-cm-’ region are characteristic of OC-0 linkages, and C-H stretching vibrations characteristic of CH2 groups are observed at 2800-3000 cm-’. A similar experiment performed at 10 K with a closed-cycle helium refrigerator produced results nearly identical with those at 77 K (Figure IC). Interestingly, no spectral evidence was found for C-Cl stretching vibrational modes (typically 600-750 cm-I) in the oligomer. A band at 680 cm-’ has been assigned to a 0-C-0 stretch/bend vibration of p~ly(oxymethylene).’~ This is significant because it indicates that C12is not involved in a chain-transfer step in the polymerization reaction despite the fact that it was present in relatively high concentration in some experiments. Also, we did not observe any direct evidence for the formation of HC1. When the sample was warmed to room temperature, the formaldehyde absorptions disappear completely. Bands associated (11) Harvey, K. B.; Ogilvie, J. F. Can. J . Chem. 1962, 40, 85. (12) Khoshkoo, H.; Nixon, E. R. Specfrochim. Acfn 1973, 29A, 603. (13) Niki, H.; Maker, P. D.; Breitenbach, L. P.; Savage, C. M. Chem. Phys. Lett. 1978, 57, 596. (14) Tadokoro, H.; Kobayashi, M.; Kawaguchi, Y.; Kobayashi, A,; Murahashi, S . J . Chem. Phys. 1963, 38, 703.
Polymerization of Solid Formaldehyde
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 2145
NUMBER OF LASER PULSES 1
0
3000
,
6000
TABLE I: Branching Ratios and Reaction Product Quantum Yields" following 308-nm Photolysis of CH20C12 Mixtures at 77 K
9000 I
1
CH20:Cl,b
RC
1.o 6.0 11 15
1.3 0.29 0.18
20
0.19 0.17
=u -5
0
5 10 15 TOTAL FLUENCE ( J / c d )
20
Figure 2. Semilogarithmic plot of the chlorine 517-cm-' Raman band intensity versus the extent of laser photolysis at 308 nm. The band intensities are measured relative to an SF6 internal standard and are normalized to the relative intensity of the CI., band prior to irradiation. The top horizontal axis shows the cumulative number of laser pulses while
the bottom horizontal axis shows the cumulative laser fluence. Each laser pulse has a measured incident fluence of 1.O mJ/cm2. The fluence in the sample is assumed to be twice this value due to reflection from the polished copper substrate. The line through the data represents an effective photochemical cross section of (1.6 i0.2) X cm2(base e). with 04-0 stretches of the oligomer grow in intensity, and the peaks in the 850-1 150-cm-I region coalesce into two principal bands near 900 and 1100 cm-l. The final room-temperature spectrum (Figure le) is substantially the same as that of the published spectrum of poly(oxymethylene).14 This film had to be removed from the window prior to the next experiment. Two control experiments were performed to help establish the mechanism of the observed photochemistry. First, a sample of pure formaldehyde monomer was deposited onto the CsI window and photolyzed with 10000 laser pulses at 308 nm (total fluence of 10 J/cm2). No diminution of the IR band intensities of C H 2 0 was observed, and no new product absorption bands were detected. Gaseous formaldehyde has a weak absorption cross sectionI5 at 308 nm, and matrix-isolated samples of formaldehyde have been dissociated in previous studies.I6 Dissociation channels leading to formation of H C O H and CO H2 are energetically accessible at 308 nm,Is but no evidence for formation of H C O or C O was found in the IR spectra. The second control experiment involved the deposition of premixed samples of formaldehyde and HC1 directly onto the CsI window at 10 K. This experiment, which was performed without photolysis, produced the spectrum shown in Figure Id. Obviously, the spectrum is substantially similar to that of the oligomer produced by photochemical reaction of chlorine with formaldehyde. This sample, when warmed to room temperature, also produced poly(oxymethy1ene). Quantum Yields. Important clues to the photochemical reaction mechanism were obtained by determining the quantum yields for consumption of reactants and for formation of the products. In a series of experiments, the concentration of molecular chlorine was monitored via its 517-cm-' Stokes band in the Raman spectrum. This band is red-shifted from the corresponding gasphase value by about 30 cm-l.'' The intensity of the band was always measured relative to the 775-cm-' band18 of SF, deposited as an internal standard. The relative intensity of the chlorine band diminished as a function of increasing numbers of laser pulses, as illustrated in Figure 2. The exponential decrease in chlorine concentration is consistent with an effective cross section for disappearance of C12of (1.6 f 0.2) X cm2 (base e). In this experiment, the effective laser fluence was assumed to be twice the measured incident fluence due to reflection from the polished
+
+
(15) McQuigg, R. D.; Calvert, J. G. J . Am. Chem. SOC.1969, 92, 1590. (16) Thomas, S. G., Jr.; Guillory, W. A. J . Phys. Chem. 1973, 77, 2469. (1 7) Herzberg, G. Molecular Spectra and Molecular Structure. I . Spectra of Diatomic Molecules; Van Nostrand Reinhold: New York, 1950. (18) Herzberg, G. Molecular Spectra and Molecular Structure. I I . Infrared and Raman Spectra of Polyatomic Molecules; Van Nostrand Reinhold: New York, 1945.
1.4 f 0.3 1.1 f 0.2 1.0 f 0.2 1.2 f 0.2 1.3 f 0.2 1.2 f 0.2
4.8 f 6.0 f 5.3 f 6.4 f 7.4 f 6.8 f
1.0 1.3
1.3 1.3
a3(CH2O)r 2.9 f 0.6 6.2 f 7.1 f 6.3 f 7.6 f
1.2 1.5 1.5
1.5 8.7 f 1.5 0.18 1.3 8.0 f 1.5 "Number of product molecules per 308-nm photon absorbed by the sample. *Mole ratio based on partial pressures prior to deposition. cProduct branching ratio from eq 2. dQuantum yield for formation of CHCIO from eq 3. eQuantum yield for average number of CH20 units consumed in formation of oligomer, from eq 4. IQuantum yield for reaction of CH20 (see text). 25 30
\
0.18
31(CHC10)d *.,(oligomer)' 1.6 f 0.3 1.3 f 0.3
1.3
copper substrate used in the Raman experiments. When similar samples of chlorine in formaldehyde are deposited on a quartz substrate, the absorption cross section at 308 nm is measured to be (1.4 f 0.2) X lo-'' cm2 (corrected for C H 2 0 absorption). This value assumes that 11% of the sample is deposited in a I-cm2area at the center of the window, based on the geometry of the window and deposition tube, and further assumes a unit sticking coefficient for gas molecules coming in contact with the cold surface. Interestingly, the gas-phase absorption cross s e c t i ~ nof' ~C12 ~ ~at~ 308 nm is 1.8 X lo-'' cm2. The fact that all of these values are the same to within our experimental uncertainty strongly suggests that the electronic transition dipole moment in C12 is only weakly perturbed by its solid-state environment and that the photodissociation quantum yield is essentially unity (Le., 1.2 f 0.2). The quantum yield for loss of formaldehyde was calculated two ways. For experiments in which a small fraction of chlorine molecules was photolyzed, the amount of sample deposited per unit area at the center of the window was calculated based on the sample size and the 11% geometrical factor discussed above. The fraction of formaldehyde reacted was determined by the relative intensities of the IR absorption bands before and after photolysis. These data show that in dilute samples of chlorine in excess formaldehyde 7.6 f 1.5 formaldehyde molecules were consumed for each photon absorbed by the sample, based on the measured absorption coefficient at 308 nm. In several experiments, samples were exhaustively photolyzed to dissociate nearly all of the chlorine molecules. The disappearance of CH2O proceeded exponentially to a limiting value, beyond which further photolysis was ineffective. Both the rate of disappearance and the limiting value were consistent with 7.6 f 1.5 formaldehyde molecules being consumed per photon absorbed by the sample. That is, in a 15:l mixture of CH20:C12the rate of disappearance of C H 2 0 was 7 times that of Clz on a mole to mole basis. About half of the formaldehyde remained unreacted even after exhaustive photolysis which destroyed nearly all of the chlorine. Under ordinary circumstances, the quantum yields for formation of the various products would be determined by measuring the relative intensities of characteristic IR absorption bands. Samples of the authentic compounds in excess formaldehyde would be deposited in order to determine the proportionality constants relating band intensities to concentrations. In this study it was not possible to follow this procedure for either of the two observed products, formyl chloride and formaldehyde oligomers. Several attempts were made to prepare samples of CHCIO, but all contained indeterminate amounts of HC1, CO, and oligomeric impurities and were therefore useless for quantitative product analysis. Clearly, it was not possible to prepare and deposit formaldehyde oligomers of known molecular weight. Furthermore, while we were ultimately able to determine the average number of monomer units in the oligomer (vide infra), we were not able to determine the molecular weight distribution, which would be (19) Calvert, J. G.; Pitts, J. N., Jr. Photochemistry; Wiley: New York, 1966. (20) Okabe, H.Photochemistry of Small Molecules; Wiley-Interscience: New York, 1978.
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The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
required for a detailed spectroscopic calibration of this type. The approximate branching between these two products (expressed as the relative number of C H 2 0 molecules consumed) was determined as follows. It was noted that the ratio R = [CHClO]/ [oligomer] increased with increasing mole fraction of C12 in the unphotolyzed samples, based simply on the relative intensities of characteristic IR absorption bands. These data are presented for a series of experiments in Table I. We assumed that the proportionality constants between band intensities and concentrations were independent of sample composition and fit the data to eq 1 , where AI represents the change in integrated intensity of the IR absorption bands, u represents the absorption coefficients, and the subscripts 1 , 2, and 3 refer to CHC10, oligomer, and C H 2 0 , respectively. The quantum yield for C H 2 0 (a3)was calculated from the absorption cross section of C4, the laser fluence, AI,, and the ratio [CH20]/[C12] in the unphotolyzed samples. Subsequently, branching ratios and quantum yields for the products were determined by eq 2-4, where a, and a2are R = [CHClO]/[oligomer] = u,AIl/(azAZ2) (2)
9,= a 3 / ( l + 1/R) aZ= a 3 / ( l + R )
(3) (4)
the quantum yields for CHClO and oligomer, respectively. The quantum yield for oligomer is expressed in terms of number of monomer units consumed per photon absorbed by the sample, such that the sum of quantum yields for CHClO and oligomer equals the quantum yield for loss of monomer (a, aZ= a3). As shown in Table I, the quantum yield for formation of CHClO is calculated to be somewhat greater than unity and does not change appreciably with increasing CHzO concentration. On the other hand, the number of monomer units consumed in forming the oligomer increases from 1.3 f 0.3 in 1: 1 mixtures of reagents to 6.8 f 1.3 in samples rich in formaldehyde.
+
Discussion The gas-phase reaction of formaldehyde and chlorine proceeds via a free-radical chain process13 depicted by reactions 5 and 6. C1'
+ CHzO
CHO'
+ Cl2
-+
+ CHO' CHClO + CI' HC1
In the solid state, radicals are always formed in pairs (initially two CI' atoms). In the absence of a specific mechanism to separate the radical pairs, geminate recombination is highly probable. Recombination reactions of this type are thought to be important in reactions of chlorine with small hydrocarbon^.'^ Initial attack of a chlorine atom on C H 2 0 may produce HCl, reaction 5, and subsequent recombination of the formyl radical with the partner chlorine atom forms the alkyl chloride as in reaction 7. In our HCO'
+ CI'
-
CHClO
(7)
experiments we can only determine average quantum yields and therefore cannot explicitly rule out the role of chain reactions. However, the overall quantum yield for formation of CHClO is not significantly greater than unity, and it is unlikely that chain processes play a significant role in the formation of the formyl chloride product in the solid state. An alternative mechanism involves direct reaction of electronically excited C12 molecules with CH20. The significant red shift of the CI2 and C-0 stretching vibrational frequencies (from their gas-phase values) suggests the presence of strong intermolecular forces in the ground state. This interaction invites the possibility of a direct mechanism on an excited-state potential surface. We turn now to the mechanism of formation of the oligomer. It is likely that the HCI formed in reaction 7 induces an ionic chain polymerization of formaldehyde. Two principal observations support this conclusion. First, although formation of formyl chloride via reactions 7 and 8 is consistent with previous gas-phase
Mansueto et al. and solid-state studies, no HCl is actually observed in the IR spectra. This product is normally observed in photochemically induced reactions of chlorine with simple alkanes. Second, codeposition of HCl and formaldehyde directly onto the cold window produces the oligomer even in the absence of photolysis. In these experiments it has not been possible to definitively establish whether the polymerization is cationic (initiated by protonation at the oxygen end of a nearby formaldehyde molecule) or anionic (perhaps via attack of C1- at the carbon end of CH20). Evidence for both mechanisms has been cited in solution-phase s t u d i e ~ . ~ ' -There ~ ~ is no evidence of formation of C-Cl bonds in the IR spectrum, and the lower quantum yields for formation of the oligomer a t high chlorine concentrations strongly suggest that C12 is ineffective as a chain-transfer agent, at least at the low temperature of our study. For these reasons we tentatively assume that the reaction is induced by proton transfer (cationic), although anionic processes cannot be definitively ruled out. Comparing our results with previous work, we find that the extent of photochemically induced oligomerization is quite modest. Goldanskii et aL5s6reported average chain lengths of lo3 at 4 K to lo5 at 80 K for y-irradiated formaldehyde crystals. The solids that we form by vapor deposition are almost certainly low-density amorphous glasses. The large differences in yield may be at least partly attributable to the random orientations of neighboring molecules and the relatively high intermolecular distances in our samples. The active (growing) end of the polymer chain is a -CH2+ group which may react with the oxygen end of nearby formaldehyde molecules. In sites where this group is surrounded by the carbon ends of its neighboring molecules, it is unable to react and is effectively trapped in the amorphous solid. The quantum yields indicate that this happens after an average of only 6.8 chain polymerization steps. In the study by Tsuda,' samples of frozen formaldehyde were y-irradiated with no apparent effect but polymerized violently upon warming. The final product in his experiments was an exceptionally high molecular weight formaldehyde polymer. Our samples also form poly(oxymethy1ene) (of undetermined molecular weight or other physical properties) when warmed slowly to room temperature. We suggest that the active species produced by y irradiation in Tsuda's work was a relatively short oligomer chain in which the active end of the growing polymer became trapped in a nonreactive site in the solid. Warming to temperatures near the melting point releases the trap and allows the polymer to grow freely. In our work, the solid films are typically 2-10 gm thick and are in good thermal contact with a massive heat sink. Therefore, the heat released in the polymerization (about 8.5 kcal/mol formaldehyde5v8)is not effective for heating the sample as it apparently was in Tsuda's experiments. We have observed no evidence of thermal runaway in the polymerization reaction during warmup, and the polymer film is homogeneous in appearance. The available laboratory data paint a very suggestive picture for the role of poly(oxymethy1ene) in the formation of comets. Clearly, it is possible to form oligomers of formaldehyde at temperatures as low as 10 K and in all likelihood at absolute zero via quantum mechanical tunneling. Formation of high molecular weight polymer molecules apparently occurs only in the crystalline material at very low temperatures. However, warming of amorphous solids following initiation of the polymer chain reaction can form reasonably stable material. The volatility of the polymer depends not only on the molecular weight but also on the particular end groups that terminate each chain reaction. This is a matter of some consequence in comets1° because the outer crust must be able to tolerate the elevated temperatures associated with orbits (21) Yokota, H.; Kondo, M.; Kagiya, T.; Fukui, K. J . Polym. Sci., Parr A-1 1968, 6,425.
(22) Yokota, H.; Kondo, M.; Kagiya, T.; Fukui, K. J . Polym. Sci., Parr A-1 1968. 6. 435.
(23) Yokota, H.; Kondo, M.; Kagiya, T.; Fukui, K. J . Polym. Sci., Parr A-1 1967, 5, 3129.
(24) MachPEek, Z . ; MejzlIk, J.; Plic, J. J . Polym. Sci. 1961, 52, 309. (25) Enikolopyan, N. S . J . Polym. Sci. 1962, 58, 1301.
J . Phys. Chem. 1989, 93, 2147-2151 near the sun so as to limit vaporization of the more volatile ices in the core (primarily H 2 0 ) . One possible scenario is that formaldehyde is condensed as a molecular solid (along with various other materials) while in the interstellar medium. Cosmic rays initiate short-chain polymerization sequences that eventually produce poly(oxymethy1ene) upon warming to 155 K or so (in the early stage of a solar encounter). During the solar encounter, the lower molecular weight poly(oxymethy1ene) chains undergo fragmentation and depolymerization. These fragments therefore
2147
exist in the coma and may be detected via mass s p e ~ t r o m e t r y . ~ Acknowledgment. We are grateful to Professor Cheves Walling for helpful discussions. This research is supported by the Air Force Astronautics Laboratory under Contract No. F04611-87-0023. Partial support was obtained through the Utah Laser Institute and Office of Naval Research under Contract No. N0014-86-K0710. Registry No. CH20,50-00-0.
Raman Spectra of Rapidly Quenched Glasses in the Systems Li,BO,-Li,SiO,-Li,P04 Li,B20,-LisSi207-Li,P207
and
Yoshiyuki Kowada,*it Masahiro Tatsumisago, and Tsutomu Minami Department of Applied Chemistry, University of Osaka Prefecture, Sakai, Osaka 591, Japan (Received: August 5, 1988)
Glasses with large amounts of lithium ions in the systems Li3B03-Li4Si04-Li3P04and Li4B2O5-Li6Si2O7-Li4P2O7were prepared by a rapid-quenching technique. Raman spectra of these glasses deconvoluted into Gaussian peaks assigned to several structural groups. The fractions of a given structural group were determined from the deconvoluted Gaussian peak area. The result suggested that nonbridging oxygens (NBO) were most preferentially formed in the phosphate groups, followed by the borate and the silicate groups. This preference order for NBO formation was consistent with the order of acidity, P205>> B2O3 > Si02, of glass-forming oxides in melts.
Introduction In recent years, rapid-quenching techniques have enabled us to prepare new glasses that could not be formed by usual meltcooling techniques.' For example, in the system Li20-B203 the glass-forming region was extended up to the composition with 70 mol % L i 2 0 by the rapid-quenching technique, although glasses had been prepared in the composition with 0-40 mol % L i 2 0 by the usual cooling. The structure of such glasses has been attractive because they contained extremely large amounts of lithium oxide as a network modifier.24 We previously reported the Raman spectroscopic studies of the rapidly quenched glasses containing as high as 67-70 mol % LizO in the systems Li20-B20,, Li20-Si02, and Li20-P205.5-7 The spectra revealed that such glasses with large amounts of L i 2 0 corresponding to the composition of lithium pyro oxo salts or lithium ortho oxo salts were not constructed by network structure but mainly by monomer and/or dimer anions. In this study, the glasses in the systems Li3B03-Li4Si04Li3P04,which consist of two or three ortho oxo salts, and in the systems Li4B2O5-Li6Si2O7-Li4P20,, which consist of two or three pyro oxo salts, were prepared by rapid quenching. The Raman spectra of these glasses were measured, and the mixing effects of two or three components on the glass structure are discussed in terms of the acidity of the components in melts.
measured intensities by a correction factor proposed by Long9 The peaks observed in measured spectra could be approximated by Gaussian curves for deconvolution.
Experimental Procedures The reagent-grade chemicals Li2C03,B2O3, S O 2 , and NH4H2PO4 were used as the starting materials. The glass samples were prepared by use of a rapid-quenching apparatus combining a thermal-image furnace and a twin roller.8 Raman spectra were obtained at 90° scattering geometries from these glasses with a JASCO N R - 1000 Raman spectrophotometer using the 5 145-A line of an Ar+ laser. The spectroscopic data were read by a microcomputer which was connected with the spectrophotometer. The background corrections were carried out with multiplying
(1)Sarjeant, R.T.; Roy, R. J . Am. Ceram. SOC.1967,50, 500. (2)Kamitsos, E.I.; Karakassides, M. A.; Chryssikos, G. D. J. Phys. Chem. i986,90,452a. (3)Kamitsos, E. I.; Karakassides, M. A,; Chryssikos, G. D. Phys. Chem. Glasses 1987,28,203. (4) Kamitsos, E.I.; Karakassides, M. A.; Chryssikos, G. D. J. Phys. Chem. 1987,91, 1073. ( 5 ) Tatsumisago, M.; Minami, T.; Umesaki, N.; Iwamoto, N. Chem. Lett. 1986, 1371. (6)Tatsumisago, M.; Takahashi, M.; Minami, T.; Tanaka, M.; Umesaki, N.;Iwamoto, N. Yogyo-Kyokai-Shi 1986,94,464. (7) Tatsumisago, M. Kowada, Y.; Minami, T. Phys. Chem. Glasses 1988, 29,63. ( 8 ) Tatsumisago, M.;Minami, T.; Tanaka, M. J. Am. Ceram. Soc. 1981, 64,C97. (9)Long, D.A. Raman Spectrscopy; McGraw-Hill: New York, 1977.
'Present address: Hyougo University of Teacher Education, Yashiro-cho, Kato-gun, Hyogo, 673-14Japan.
0022-365418912093-2147$01.50/0
Results I . Glass-Forming Regions. Figure 1 shows the glass-forming region by rapid quenching for the system Li3B03-Li4Si04-Li3P04 combining three lithium ortho oxo salts. Open and closed circles denote glassy and crystalline samples, respectively. Glasses are widely obtained by the rapid quenching, while no glasses could be formed by the usual melt-cooling technique for any composition in this system. The glass-forming region is spread in the Li3B03-richcompositions. The limit of the region lies near the line tied with two compositions, 80Li4Si04.20Li3P04 and 6OLi3BO3.40Li3PO4. The binary system Li4Si04-Li3P04 shows the narrow glassforming region, which is due to the formation of solid solution in this system; the solid solution formation raises the liquidus temperature and thus causes the difficulty of glass formation. Figure 2 shows the glass-forming region in the system Li,B205-LisSi207-Li4Pz07 combining three lithium pyro oxo salts. Also in this system, glasses could not be prepared by the usual meltcooling technique for any composition. The rapid-quenching
0 1989 American Chemical Society
