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J. Phys. Chem. B 2008, 112, 2644-2648
The Chemistry of Cyanuric Acid (H3C3N3O3) under High Pressure and High Temperature W. Montgomery* Department of Earth & Planetary Science, UniVersity of California, Berkeley, 307 McCone Hall, Berkeley, California 94720-4767
J. C. Crowhurst and J. M. Zaug Lawrence LiVermore National Laboratory, Chemistry, Materials and Life Sciences Directorate, 7000 East AVenue, LiVermore, California 94551
R. Jeanloz Department of Earth & Planetary Science, UniVersity of California, Berkeley, 307 McCone Hall, Berkeley, California 94720-4767 ReceiVed: May 10, 2007; In Final Form: NoVember 15, 2007
We have studied cyanuric acid (H3C3N3O3) at static pressures up to 8.1 GPa and simultaneous temperatures up to 750 K, using primarily infrared absorption spectroscopy and visual observation. The corresponding phase diagram compares favorably with theoretical predictions of metastable organic materials. Two reactions were observed and characterized; both are irreversible. Below 2 GPa, melting is accompanied by a decomposition reaction, and upon cooling, cyanuric acid is not recovered. Above 2 GPa, heating results in a solid product recoverable at ambient conditions. Corresponding infrared spectra suggest that pressure leads to the formation of heterocycles of increasing complexity and biological potential, with the composition determined by the pressure of formation. Cyanuric acid is of interest at these conditions because it and its monomer, isocyanic acid, are “prebiotic” compounds found in stellar dust clouds, meteorites, and other remnants of the early Earth.
Introduction Cyanuric acid, H3C3N3O3 (1,3,5-triazine-2,4,6-trione), is a heterocyclic molecule of relatively high symmetry and low molecular weight compared with most other heterocycles. It contains all of the elements thought to be necessary for lifes H, C, N, and Osand has been recovered from meteorites.1 The effects of pressure are of interest both because primordial planetary material is exposed to high pressures (e.g., due to impact) and because prebiotic materials present within a young planet may be protected from destructive forces caused by collision and ionizing radiation. Knowledge of the pressures and temperatures at which cyanuric acid decomposes provides a guide to the conditions under which more complex molecules may be formed from cyanuric acid. Its phase diagram therefore has important implications for both the origin and survival of life on early Earth. At 273 K and ambient pressure, isocyanic acid (HNCO) polymerizes into a mixture of cyanuric acid and cyamelide, with the proportion of cyanuric acid formed increasing with higher temperature.2,3 At temperatures above 633 K, cyanuric acid decomposes to isocyanic acid. This is an inefficient decomposition, and additional compounds are produced, including a range of heterocyclic molecules, carbon dioxide, CO2, and ammonia, NH3.4,5 This decomposition is reported at pressures of a few hundred Torr,6 but very little is known about the effect of pressure on this reaction. Since isocyanic acid is commonly observed in the interstellar medium, meteorites, and comets,7-9 * To whom correspondence should be addressed.
and since the assembly of these materials into planets involves both increased temperature and pressures, the role of cyanuric acid in pre-planetary chemistry is likely to be significant. Cyanuric acid is a white crystalline powder at room temperature and pressure and has space group C2/n.10 The corresponding infrared spectrum was reported by Newman et al., who also made peak assignments.11 At ambient temperatures up to 4 GPa, Hamann et al. reported decreasing stretching frequencies and increasing bending frequencies in the N-H‚‚‚O bond, suggesting that the atoms grow closer and that intramolecular bonding is strengthened with pressure.12 In this paper, we examine in detail the pressure-temperature behavior of cyanuric acid up to 750 K and 8.1 GPa in a diamond anvil cell using Fourier transform infrared (FTIR) spectroscopy. We describe the phase diagram and compare it to that of benzene13 and to the theory presented by Brazhkin14 for the phase diagrams of metastable organic substances. Experimental Methods Pure cyanuric acid (99%) was obtained from Alfa Aesar. Samples were then loaded into a membrane-type diamond anvil cell containing type-II diamonds with 500 µm culets. To reduce possible catalytic effects or reactions, we used a pure Re gasket with a preindented thickness of ∼30 µm and a 200 µm diameter sample chamber to contain each sample. Samples were heated at rates of ∼0.1, 1, and >10 K/min using a Eurotherm 2408 electronic temperature-control system powering an external heating ring that surrounds the diamond cell. Experiments were performed at fixed pressure and variable temperature.
10.1021/jp073589y CCC: $40.75 © 2008 American Chemical Society Published on Web 02/12/2008
H3C3N3O3 under High Pressure and High Temperature We monitored the pressure during heating and cooling using the fluorescence lines of SrB4O7/Sm2+ (λ0-0)15 and/or ruby (Al2O3/Cr3+).16 Optimally, both pressure sensors were included, but some experiments only used one sensor due to loss of the second during loading. The temperature was monitored using type-K thermocouples from Omega Engineering secured against the gasket and one of the diamonds. The temperature precision was (0.5 K, but thermal gradients decreased absolute accuracy to ∼4 K at high temperature. The sample pressure was maintained to within 0.2 GPa throughout each experiment by monitoring the pressure gauge in situ and adjusting the DAC membrane pressure accordingly. To obtain a thin sample and avoid loss of signal due to high sample absorption, CsI was preloaded into the sample chamber. CsI is a common FTIR window for studies under ambient conditions and has been used at high pressures for the study of energetic materials.17 The melting curve of CsI is much higher than those measured for many organic species.18 CsI was chosen after a series of experiments demonstrated stability under the relevant pressure and temperature conditions. There is no evidence to suggest that cyanuric acid or its decomposition products react with CsI in the present experiments, although in the absence of experiments on cyanuric acid alone, we cannot rule the possibility out. Identical color changes (see below) were observed in thick samples prepared with no CsI in the sample chamber, and the FTIR spectra of recovered samples were comparable for similar pressures and temperatures. A thin sample of only cyanuric acid risks breaking the diamond anvils. We estimate the percentage of cyanuric acid by volume to be between 3 and 5%. We made spectroscopic measurements during both the heating and cooling cycles. Changes in the infrared absorption spectrum, as illustrated in Figure 1, combined with visual observation allowed us to determine the transition regimes of cyanuric acid as a function of pressure and temperature. FTIR spectra were obtained through a long working distance (26 mm) microscope designed to accommodate heated diamond anvil cells. Our FTIR system consists of a Bruker Optics Vector-33 interferometer and a glowbar source, in conjunction with an external sandwich array detector (single channel) composed of InSb and HgCdTe, for complete coverage of the spectral ranges of interest, 5005000 cm-1, at a resolution of 2 cm-1. Fluorescence signals are brought into a 0.8 m focal length spectrometer by fiber optics and dispersed onto a thermoelectrically cooled CCD multichannel array by means of an 1800 lines/mm grating. Raman measurements were made on samples after they had been depressurized and cooled to identify and eliminate reaction products. Raman spectra were excited with the 488 nm line of an Ar ion laser. The collection objective was a Mitutoyo nearIR-corrected 20× long working distance lens that provided a spot size of a few micrometers. The collection geometry was backscattering. Overall, the system we used was very similar to that described by Goncharov et al.19 After collection, FTIR spectra were analyzed by subtracting an atmospheric background collected before the experiments and a standard spectra collected for each pair of diamonds used. No attempts were made to subtract the effects of increased diamond background at elevated pressures and temperatures. Results A. Solid-Liquid Transformation. Figure 1 shows the FTIR spectra acquired at a fixed pressure of 1.1 GPa and a constant heating rate of 1 K/min from 400 to 750 K. At 654 K, all peaks associated with cyanuric acid disappeared, and the sample was
J. Phys. Chem. B, Vol. 112, No. 9, 2008 2645
Figure 1. Temperature evolution of FTIR spectra of 99% pure cyanuric acid during isobaric heating and cooling at 1.1 GPa from 423 to 750 K at a heating rate of 1 K/min. Initially, the sample was rapidly heated to 423 K. Each spectrum was collected in 127 s, and the complete heating-cooling cycle took 6-12 h. The entire sample changed appearance (from translucent to silvery) at 654 K, and all of the peaks associated with cyanuric acid disappeared. One peak at ∼3400 cm-1 suggests the transitory presence of an O-H bond. At 750 K, nodules of graphite (confirmed with micro-Raman measurement) formed, and a small C-H peak appeared. Upon cooling, the spectra are unchanged from those taken at high temperatures.
a clear liquid, as evidenced by the observed motion of the pressure sensors. At 750 K, small (∼50 µm) black nodules formed in the sample chamber. Raman spectra of these nodules collected after cooling showed features suggestive of the G band (1580 cm-1) and the D′ band (1620 cm-1) of microcrystalline graphite.20 The D band was obscured by the strong diamond peak at 1332 cm-1. A spectrum is provided as Supporting Information. According to previous research, isocyanic acid is one expected decomposition product of cyanuric acid.6 We carried out an experiment on cyanuric acid without CsI in the diamond anvil cell at very low pressure and at a heating rate of 1 K/min. The sample was sealed between two diamonds at ambient conditions with ultrapure argon gas. A perforated Inconel 718 gasket (which is softer and more chemically reactive than rhenium, used in our other experiments) was used to radially confine the sample and argon gas. We observed the disappearance of cyanuric acid peaks at 595 K, consistent with the reported melting and decomposition temperatures of cyanuric acid. This was followed by the formation of a number of products, including HCNO, CO2, and NH3. HNCO peaks disappeared by 734 K, consistent with previously reported thermal instability.2,5 The data from this experiment is presented in the Supporting Information Figure 2. It is possible that there are chemical reactions between CsI and cyanuric acid at ambient pressure and high temperatures; we cannot rule out the possibility. The spectra from these ambient pressure experiments are consistent with our less-wellresolved spectra from the
