High-pressure far-infrared spectroscopic studies of hydrogen bonding in formic acid.

Simple molecules such as HCOOH, or formic acid, are suggested to have played important roles in planetary physics due to their possibility for high pressure and temperature chemistry under impact conditions. In this study, we have investigated the effect of pressure (up to 50 GPa) on H-bonding and reactivity of formic acid using synchrotron far infrared spectroscopy. Based on the pressure-induced changes to H-bond ν(O-H···O) stretching and γ(O-H···O) deformations, we observe significant reorganization of H-bonding network beginning at ~20 GPa. This is in good agreement with reports of symmetrization of H-bonds reported at 16-21 GPa from X-ray diffraction and Raman spectroscopy studies as well as molecular dynamics simulations. With further increase in pressure, beyond 35 GPa, formic acid undergoes a polymerization process that is complete beyond 45 GPa. Remarkably, upon decompression, the polymeric phase reverts to the crystalline high-pressure phase at 8 GPa.

The phase diagram of ammonium nitrate.

The pressure-temperature (P-T) phase diagram of ammonium nitrate (AN) [NH(4)NO(3)] has been determined using synchrotron x-ray diffraction (XRD) and Raman spectroscopy measurements. Phase boundaries were established by characterizing phase transitions to the high temperature polymorphs during multiple P-T measurements using both XRD and Raman spectroscopy measurements. At room temperature, the ambient pressure orthorhombic (Pmmn) AN-IV phase was stable up to 45 GPa and no phase transitions were observed. AN-IV phase was also observed to be stable in a large P-T phase space. The phase boundaries are steep with a small phase stability regime for high temperature phases. A P-V-T equation of state based on a high temperature Birch-Murnaghan formalism was obtained by simultaneously fitting the P-V isotherms at 298, 325, 446, and 467 K, thermal expansion data at 1 bar, and volumes from P-T ramping experiments. Anomalous thermal expansion behavior of AN was observed at high pressure with a modest negative thermal expansion in the 3-11 GPa range for temperatures up to 467 K. The role of vibrational anharmonicity in this anomalous thermal expansion behavior has been established using high P-T Raman spectroscopy.

Mass-spectroscopic observations of glycine subjected to strong shock loading.

We have made time-of-flight mass-spectroscopic observations of 85/15 wt % water/glycine solutions and of crystalline alpha-glycine subjected to strong shock loading. The shockwaves were produced by placing the materials in contact with detonating solid explosives. In the solution observations, we have done experiments with glycine molecules composed of ordinary isotopes and with molecules labeled with (13)C, (15)N, and D atoms. The primary reason for conducting this research was to examine whether glycine molecules can survive exposure to strong shock loading, e.g., as might occur in the entry of a meteor into the earth's atmosphere. Our results show that glycine molecules can withstand the rigors of shock environments that generate pressure and temperature up to 180 kbar and 3200 K. Glycine in a 85 H(2)O/15 glycine wt % solution (i.e., one molecule of glycine to ca. 24 H(2)O molecules) exists primarily in its zwitterionic form. In both the solution and crystal experiments, we observed zwitterionic dimers, trimers, and, possibly, tetramers, after the materials were shocked. This implies that the solvating water molecules in the solution experiments must reside on the exterior of groups of solvated glycine molecules. We report quantum-chemical calculations, using density functional theory, that predict that two glycine zwitterions are bound together by ca. 15.72 kcal when immersed in an Onsager model of water. Our observations allow us to place lower-bound estimates on the lifetime of glycine zwitterions under our conditions. We have examined our data to determine whether dipeptide formation has occurred and found no evidence that it has. Compressible fluid-mechanical calculations were performed to estimate the pressures, temperatures, and the time scales present in the experiments.

Phase transition and chemical decomposition of hydrogen peroxide and its water mixtures under high pressures.

We have studied the pressure-induced phase transition and chemical decomposition of hydrogen peroxide and its mixtures with water to 50 GPa, using confocal micro-Raman and synchrotron x-ray diffractions. The x-ray results indicate that pure hydrogen peroxide crystallizes into a tetragonal structure (P4(1)2(1)2), the same structure previously found in 82.7% H(2)O(2) at high pressures and in pure H(2)O(2) at low temperatures. The tetragonal phase (H(2)O(2)-I) is stable to 15 GPa, above which transforms into an orthorhombic structure (H(2)O(2)-II) over a relatively large pressure range between 13 and 18 GPa. Inferring from the splitting of the nu(s)(O-O) stretching mode, the phase I-to-II transition pressure decreases in diluted H(2)O(2) to around 7 GPa for the 41.7% H(2)O(2) and 3 GPa for the 9.5%. Above 18 GPa H(2)O(2)-II gradually decomposes to a mixture of H(2)O and O(2), which completes at around 40 GPa for pure and 45 GPa for the 9.5% H(2)O(2). Upon pressure unloading, H(2)O(2) also decomposes to H(2)O and O(2) mixtures across the melts, occurring at 2.5 GPa for pure and 1.5 GPa for the 9.5% mixture. At H(2)O(2) concentrations below 20%, decomposed mixtures form oxygen hydrate clathrates at around 0.8 GPa--just after H(2)O melts. The compression data of pure H(2)O(2) and the stability data of the mixtures seem to indicate that the high-pressure decomposition is likely due to the pressure-induced densification, whereas the low-pressure decomposition is related to the heterogeneous nucleation process associated with H(2)O(2) melting.

Mass spectroscopic observation of shock-induced chemistry in liquid CS2.

We have observed, via time-of-flight mass spectrometry, 13 chemical species more massive than CS2 produced by shocking liquid CS2 to very high pressure/temperature. The stoichiometry of three of these species is uniquely determined from the 12CS2 experiments; these species are C2S2, C3S2, and C4S2. The stoichiometry of the other 10 structures cannot be uniquely determined from 12CS2 experiments. However, by redoing the experiments using isotopically labeled CS2 (i.e., 13CS2), we determined the stoichiometry of nine of the remaining structures. The nine structures are Sn (n = 3-8) and CS3, C2S5, and C4S6. A structure with mass 297.1 amu was also observed in the 12CS2 experiments but was not detected in the 13CS2 experiments. This structure must be C6S7, C14S4, or C22S; given the low carbon content of the other observed carbon species, it is probably C6S7. The shockwaves to which the CS2 molecules were subjected were produced by the detonation of high mass-density solid explosives. The explosives used were either a plastic bonded form of cyclotetramethlylene tetranitramine or pure hexanitrostilbene. Numerical compressible fluid-mechanical simulations were done to estimate the pressures, temperatures, and time scales of the processes that occurred in the shocked CS2. The results obtained in the present experiments are related to earlier work on CS2's chemical reactivity that used both shockwave methods and static techniques to produce very high pressure.

Effect of deuteration on the diameter-effect curve of liquid nitromethane.

The detonation properties of liquid nitromethane [CH(3)NO(2)] are probably the most thoroughly studied of any condensed-phase explosive. Because it is homogeneous (i.e., lacks hot-spot phenomena), it provides a window into the underlying chemical processes induced by a passing shock or detonation wave-such information is submerged in the complex fluid mechanics when heterogeneous explosives are detonated. In this paper, we provide experimental data and data analysis of the effect that deuterating nitromethane's methyl group has on some aspects of the processes that occur in the detonating liquid material. In the experimental part of this study, we report diameter-effect curves (i.e., inverse charge internal radius vs steady detonation speed) for pure CH(3)NO(2) and pure CD(3)NO(2) confined in right-circular cylinders of C-260 brass. Large differences in the infinite-medium (i.e., plane wave) detonation speed and in the failure diameter of the two materials are observed. Interpretations of the observations based on physical and chemical theory are given. The observed large decrease in deuterated nitromethane's infinite-medium detonation speed, relative to the protonated material, is interpreted in terms of the Zeldovitch, von Neumann, and Doering theory of steady-state detonation. We also estimate the relative size of the steady plane-wave reaction-zone length of the two materials. We interpret the observed increases in NM's failure diameter and its steady one-dimensional chemical-reaction-zone length due to deuteration in terms of the quantity of NM aci ion present. The new results are placed in the context of earlier work on detonating liquid nitromethane.