Michael Pravica

Net NMR alignment by adiabatic transport of parahydrogen addition products to high magnetic field

Abstract A field-cycling method is demonstrated whereby the nuclear spin order of para-enriched H2 is inserted adiabatically into a product molecule formed by molecular addition. Intense NMR multiplets each of uniform phase are obtained, in contrast with previous results where antiphase multiplets of zero net area were obtained. The advantages of the present approach are discussed.

Characteristics of silicone fluid as a pressure transmitting medium in diamond anvil cells

The properties of a silicone fluid with initial viscosity of 1 cst as a pressure transmitting medium for diamond anvil cells have been determined by ruby R1 line broadening and R1-R2 separation measurements to 64 GPa at ambient temperature. By these criteria, the silicone fluid is as good a pressure medium as a 4:1 methanol:ethanol mixture at low pressures to about 20 GPa, and is better than the mixture at higher pressures. Although argon media are better than the silicone at pressures to 30 GPa, this silicone behaves as well as argon at higher pressures. Furthermore, the silicone is easier to load than argon and is almost chemically inert.

High-Pressure Far- and Mid-Infrared Study of 1,3,5-Triamino-2,4,6-trinitrobenzene

Synchrotron infrared measurements of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) have been performed in the far-IR and mid-IR spectral regions up to approximately 30 and approximately 40 GPa, respectively. For the far-IR experiment, no pressurizing medium was used, whereas KBr was utilized as a pressurizing medium for the mid-IR experiment. For both experiments, pressure was cycled and IR spectra were recorded at various pressures to ascertain sample survival. In the high frequency region (approximately 3000 cm(-1)) of the mid-IR spectra, the peak frequencies of the NH2 symmetric and antisymmetric vibrational modes steadily decrease with increasing pressure, indicating strengthening of intermolecular hydrogen bonding with pressure. In both experiments, no apparent phase transition was observed to the highest pressures studied.

Charge transfer in spinel Co3O4 at high pressures.

Charge transfer in cobalt oxide Co(3)O(4) in the spinel structure is evidenced by experimental results using x-ray diffraction (XRD), x-ray absorption near edge structure (XANES) spectroscopy, and Raman scattering at high pressures up to 42.1, 24.6 and 35.1 GPa, respectively. While the cubic structure was found to persist under pressure up to 42.1 GPa based on the XRD and Raman results, the mode Grüneisen parameter was calculated according to our Raman measurements. Our structural data refinement revealed a structural transition from the normal spinel structure at low pressures to a partially inverse spinel structure at pressures above 17.7 GPa. This transition may be caused by the interaction of charges between tetrahedral and octahedral sites via a charge transfer process. Evidence for the charge transfer process is further supported by changes of the pre-edge features in the XANES data.

A high-pressure far- and mid-infrared study of 1,1-diamino-2,2-dinitroethylene

High pressure infrared (IR) measurements of the insensitive explosive, 1,1-diamino-2,2-dinitroethylene (DADNE or FOX-7), have been performed in the far- and mid-IR spectral regions up to ∼28 GPa using a synchrotron source. Petroleum jelly and KBr were employed as the pressurizing media for the far- and mid-IR experiments, respectively. In both experiments, IR spectra were collected at various pressures both in compression and decompression to determine reversibility under pressure cycling. There is evidence for at least two and possibly three phase transitions in the samples at pressures near 2, 5, and10 GPa, respectively. In the high frequency region (∼3000–3300 cm−1), the NH2 symmetric and antisymmetric vibrational modes soften with increasing pressure, suggesting strengthening intermolecular hydrogen bonding, and then stiffen with further pressure increase. At higher pressures (above 5 GPa), we suspect progressive flattening of the zig zag structure characteristic of the α phase occurs, which was obser...

Note: A novel method for in situ loading of gases via x-ray induced chemistry.

We have developed and demonstrated a novel method to load oxygen in a sealed diamond anvil cell via the x-ray induced decomposition of potassium chlorate. By irradiating a pressurized sample of an oxidizer (KClO(3)) with either monochromatic or white beam x-rays from the Advanced Photon Source at ambient temperature and variable pressure, we succeeded in creating a localized region of molecular oxygen surrounded by unreacted sample which was confirmed via Raman spectroscopy. We anticipate that this technique will be useful in loading even more challenging, difficult-to-load gases such as hydrogen and also to load multiple gases.

Radiation-Induced Decomposition of PETN and TATB under Extreme Conditions

We conducted a series of experiments investigating decomposition of secondary explosives PETN and TATB at varying static pressures and temperatures using synchrotron radiation. As seen in our earlier work, the decomposition rate of TATB at ambient temperature slows systematically with increasing pressure up to at least 26 GPa but varies little with pressure in PETN at ambient temperature up to 15.7 GPa, yielding important information pertaining to the activation complex volume in both cases. We also investigated the radiation-induced decomposition rate as a function of temperature at ambient pressure and 26 GPa for TATB up to 403 K, observing that the decomposition rate increases with increasing temperature as expected. The activation energy for the TATB reaction at ambient temperature was experimentally determined to be 16 +/- 3 kJ/mol.

High pressure Raman spectroscopic study of structural polymorphismin cyclohexane

We have performed a Raman spectroscopic study of cyclohexane at high pressures up to 40GPa at ambient temperature and present evidence for one or two new phases of cyclohexane which appear around 10 and between 25 and 37GPa. The R1 and R2 lines of a ruby in the cyclohexane sample were well-resolved to our highest pressure, and the sample was highly transparent. Because cyclohexane is nonpolar, relatively inert, and a liquid at room temperature, it is an excellent and easy-to-use pressure medium, especially for reactive materials. At 40GPa, we heated the sample to 200 °C for several hours and then performed Raman measurements while decreasing pressure at ambient temperature. The material remained in its original liquid form demonstrating that the molecule is remarkably resilient.

Note: Experiments in hard x-ray chemistry: In situ production of molecular hydrogen and x-ray induced combustion

We have successfully loaded H(2) into a diamond anvil cell at high pressure using the synchrotron x-ray induced decomposition of NH(3)BH(3). In a second set of studies, radiation-assisted release of O(2) from KCLO(3), H(2) release from NH(3)BH(3), and reaction of these gases in a mixture of the reactants to form liquid water using x-rays at ambient conditions was observed. Similar observations were made using a KCLO(3) and NaBH(4) mixture. Depending on reaction conditions, an explosive or far slower reaction producing water was observed.

Measurement of the Energy Dependence of X-ray-Induced Decomposition of Potassium Chlorate

We report the first measurements of the X-ray induced decomposition of KClO3 as a function of energy in two experiments. KClO3 was pressurized to 3.5 GPa and irradiated with monochromatic synchrotron X-rays ranging in energy from 15 to 35 keV in 5 keV increments. A systematic increase in the decomposition rate as the energy was decreased was observed, which agrees with the 1/E(3) trend for the photoelectric process, except at the lowest energy studied. A second experiment was performed to access lower energies (10 and 12 keV) using a beryllium gasket; suggesting an apparent resonance near 15 keV or 0.83 Ǻ maximizing the chemical decomposition rate. A third experiment was performed using KIO3 to ascertain the anionic dependence of the decomposition rate, which was observed to be far slower than in KClO3, suggesting that the O-O distance is the critical factor in chemical reactions. These results will be important for more efficiently initiating chemical decomposition in materials using selected X-ray wavelengths that maximize decomposition to aid useful hard X-ray-induced chemistry and contribute understanding of the mechanism of X-ray-induced decomposition of the chlorates.