EMSL: Herman M Cho's Publications

Cho HM, WJ Shaw, VM Parvanov, GK Schenter, AJ Karkamkar, NJ Hess, CJ Mundy, SM Kathmann, JA Sears, AS Lipton, PD Ellis, and T Autrey. 2008. "Molecular Structure and Dynamics in the Low Temperature (Orthorhombic) Phase of NH3BH3." Journal of Physical Chemistry A 112(18):4277-4283. doi:10.1021/jp711696 Abstract Variable temperature 2H NMR experiments on the orthorhombic phase of selectively deuterated NH3BH3 spanning the static to fast exchange limits of the borane and amine motions are reported. New values of the electric field gradient (EFG) tensor parameters have been obtained from the static 2H spectra of Vzz = 5.509(±0.275)×1014 statvolt/cm2 and ! = 0.00±0.05 for the borane hydrogens and Vzz = 9.615(±0.481)×1014 statvolt/cm2 and ! = 0.00±0.05 for the amine hydrogens. The molecular symmetry inferred from the observation of equal EFG tensors for both the boron and amine hydrogens is in sharp contrast with the Cs symmetry derived from diffraction studies. The origin of the apparent discrepancy has been investigated using molecular dynamics methods in combination with electronic structure calculations of NMR parameters, bond lengths, and bond angles. The computation of parameters from a statistical ensemble rather than from a single set of atomic Cartesian coordinates gives values that are in close quantitative agreement with the 2H NMR electric field gradient tensor measurements and are more consistent with the molecular symmetry revealed by the NMR spectra. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program. The Pacific Northwest National Laboratory is operated by Battelle for DOE.

Farnan I, HM Cho, and WJ Weber. 2007. "QUANTIFICATION OF ACTINIDE ALPHA-RADIATION DAMAGE IN MINERALS AND CERAMICS." Nature 445:190-193. doi:10.1038/nature05425 Abstract There are large amounts of heavy alpha-emitters in nuclear waste and nuclear materials inventories stored in various sites around the world. These include plutonium and minor actinides such as americium and curium. In preparation for geological disposal there is a consensus that actinides that have been separated from spent nuclear fuel should be immobilised within mineral-based ceramics rather than glass. Over the long-term, the alpha-decay taking place in these ceramics will severely disrupt their crystalline structure and reduce their durability. A fundamental property in predicting cumulative radiation damage is the number of atoms permanently displaced per alpha–decay. Currently, this number is estimated as 1000-2000 atoms/alpha decay event. Here, we report nuclear magnetic resonance, spin-counting experiments that measure close to 5000 atoms/alpha decay event in radiation damaged natural zircons. New radiological NMR measurements on highly radioactive, 239Pu zircon show damage similar to that created by 238U and 232Th in mineral zircons at the same dose, indicating no significant effect of dose rate. Based on these measurements, the initially crystalline structure of a 10 wt% 239Pu zircon would be amorphous after only 1400 years in a geological repository. These measurements establish a basis for assessing the long-term structural durability of actinide-containing ceramics based on an atomistic understanding of the fundamental damage event.

Cho HM. 2006. "2-D Homonuclear Correlation and Separated Local Field Experiments for Solids with Strong Homonuclear Dipolar Couplings." Solid State Nuclear Magnetic Resonance 29(1-3):219-226. Abstract An experiment for obtaining two-dimensional homonuclear correlation spectra of nuclei in solids in the presence of strong homonuclear dipolar couplings is described. The experiment utilizes a multiple-pulse homonuclear decoupling sequence with an effective precession axis parallel to the rotating frame z-axis during the evolution and detection periods. A multiple-pulse sequence that suppresses chemical shift and heteronuclear dipolar coupling evolution and scales the static homonuclear dipolar coupling is proposed for the mixing period. The evolution during the mixing period is analogous to the dynamics of the mixing period in solution-state TOCSY experiments, and can be interpreted as the oscillatory exchange of longitudinal magnetization between coupled spins. For nuclides with large gyromagnetic ratios, the static homonuclear dipolar interaction will be substantially larger than the mechanisms used to develop internuclear correlations in solution state 2-D experiments, which should make it possible to establish correlations over much longer distances and with significantly shorter mixing times. Extensions to separated local field experiments are discussed.

Cho HM, AR Felmy, RN Craciun, JP Keenum, NK Shah, and DA Dixon. 2006. "Solution State Structure Determination of Silicate Oligomers by 29Si NMR Spectroscopy and Molecular Modeling." Journal of the American Chemical Society 128(7):2324-2335. doi:10.1021/ja0559202 Abstract Evidence for nine new solution state silicate oligomers has been discovered by 29Si NMR homonuclear correlation experiments of 29Si-enriched samples. In addition to enhancing signal sensitivity, the isotopic enrichment increases the probability of the 29Si–29Si two-bond scalar couplings that are necessary for the observation of internuclear correlations in 2-D experiments. The proposed assignments are validated by comparisons of experimental and simulated crosspeaks obtained with high digital resolution. The internuclear connectivity indicated by the NMR data suggests that several of these oligomers can have multiple stereoisomers, including conformers and/or diastereomers. The stability of these oligomers and their possible stereoisomers have been investigated by electronic structure calculations.

Felmy AR, HM Cho, DA Dixon, Y Xia, NJ Hess, and Z Wang. 2006. "The Aqueous Complexation of Thorium with Citrate under Neutral to Basic Conditions." Radiochimica Acta 94(4):205-212. doi:DOI 10.1524/ract.2006.94.4.205 Abstract The aqueous complexation of thorium with citrate was investigated under neutral to basic conditions and over a broad range of ionic strengths. The solubility data for ThO2(am) as a function of citrate concentration indicate the presence of stable species with citrate-to-metal ratios of between two to three. The dependence of the ThO2(am) solubilities on hydrogen ion concentration can also be readily explained by the classical assumption of hydrolysis of the central Th(IV) ion to form mixed thorium-hydroxide-citrate complexes. 13C NMR spectra of the species in solution confirm that the citrate-to-metal ratio of the species in solution is between two and three and show that the citrate attaches to the metal in a bidentate fashion through oxygens on the -carboxylate and -alkoxyl groups, rather than through the carboxylate groups. The 13C NMR spectra, as well as a density functional theory (DFT) electronic structure study of the presumptive complexes, suggests that the associated α-hydroxyl proton can be displaced during complex formation. These findings indicate an alternative explanation for the observed changes in solubility as a function of hydrogen ion concentration, the displacement of protons from the citrate alkoxyl groups via metal binding. Removal of protons from the alkoxyl groups or hydrolysis of the central Th(IV) cannot be distinguished by thermodynamic measurements, however the species with the α-hydroxyl proton removed (i.e., ThOH(Cit)25- and Th(Cit)38-) would appear to better represent the microscopic binding. Apparent equilibrium constants for the solution phase reactions of these species and the hydrous thorium oxide have been calculated as a function of ionic strength.

Harel E, and HM Cho. 2006. "A General Numerical Analysis of Time-Domain NQR Experiments." Journal of Magnetic Resonance 183(2):308-314. doi:10.1016/j.jmr.2006.06.033 Abstract We introduce a general numerical approach for solving the Liouville equation of a quadrupolar nuclide that we show can be used to analyze time-domain NQR exper- iments. A computer-based treatment is necessitated by the dimensionality of the Liouville space, which precludes analytical, closed form solutions for I > 3/2. Ac- curate simulations of experimental nutation curves, forbidden transition intensities, powder and single crystal spectra, and off-resonance irradiation dynamics can be computed with this method. We also examine the validity of perturbative approx- imations where the signal intensity of a transition is proportional to the transition moment between the eigenstates of the system, thus providing a simple basis for determining selection rules. Our method allows us to calculate spectra for all values of the asymmetry parameter, ղ, and sample orientations relative to the coil axis. We conclude by demonstrating the methodology for calculating the response of the quadrupole system to amplitude- and frequency-modulated pulses.

Wang Z, AR Felmy, Y Xia, O Qafoku, W Yantasee, and HM Cho. 2005. "Complexation of Cm(III)/Eu(III) with Silicate in Basic Solutions." Radiochimica Acta 93(12):741-748. Abstract The complexation of Cm(III) and Eu(III) with dissolved silica was studied by time resolved laser fluorescence spectroscopy (TRLFS) in basic solutions over a range of total silica concentrations and ionic strengths (NaNO3). In highly basic solutions, both the fluorescence spectra and lifetime data indicate the formation of Eu(III)/Cm(III) complexes with oligomeric silicates as well as hydroxide groups and/or nitrate in the presence of concentrated NaNO3. At high silica concentration the inner-sphere complexation caused the shift of the fluorescence spectral maximum for Cm(III)(aq) from 594 nm to up to 607 nm and a significant increase of the hypersensitive 5D0 → 7F2 band around 615 nm relative to the non-hypersensitive 5D0 → 7F1 band at 592 nm for Eu(III). At the same time, the fluorescence lifetime increased from 68 s to up to 202 s for Cm(III) in 0.1 M NaNO3 and from 115 s to 1.8 ms for Eu(III) in 3.0 M and 5.0 M NaNO3, consistent with the removal of 6 or more water molecules upon silicate complexation. Linear correlations between the spectral intensity of Cm(III) complexes and the concentrations of the dissolved silicates suggest that Cm(III) complexation with the silicate dimer, Si2O2(OH)22-, may play a role.

Cho HM, WA De Jong, BK McNamara, BM Rapko, and IE Burgeson. 2004. "Temperature and Isotope Substitute Effects on the Structure and NMR Properties of the Pertechnetate Ion in Water." Journal of the American Chemical Society 126(37):11583-11588. Abstract The uniquely well-resolved 99Tc NMR spectrum of the pertechnetate ion in liquid water poses a stringent test of the accuracy of ab initio calculations. The displacement of the 99Tc chemical shift as a function of temperature has been measured over the range 10-45 °C for the three isotopomers Tc(16O)4ˉ, Tc(16O)3(18O)ˉ, and Tc(16O)3(17O)ˉ at natural oxygen isotope abundance levels, and in addition the temperature dependence of the Tc-O scalar coupling was determined for the Tc(16O)3(17O)ˉ isotopomer. Values for these parameters were computed for three different theoretical models, which include both an unsolvated ion approximation and treatments of the solvated ion based on the COSMO approach. According to these analyses, the temperature- and isotope-dependent shifts in energy observed in the 99Tc NMR spectra can be quantitatively explained by changes in the average Tc-O bond length of the order of 1 x 10ˉ4 Å, which are induced by a redistribution of vibrational state populations and solvent-ion interactions. Vibrational energies and Tc-O bond lengths derived from these models also compare favorably with previous experimental determinations.

Farnan I, HM Cho, WJ Weber, RD Scheele, N Johnson, and AE Kozelisky. 2004. "High-Resolution Solid-State Nuclear Magnetic Resonance Experiments on Highly Radioactive Ceramics." Review of Scientific Instruments 75(12):5232-5236. Abstract A triple containment magic-angle spinning rotor insert system has been developed and a sample handling procedure formulated for safety analyzing highly radioactive solids by high resolution solid state NMR. The protocol and containment system have been demonstrated for magic angle spinning (MAS) experiments on ceramic samples containing 5-10 wt% 239Pu and 238Pu at rotation speeds of 3500 Hz. The technique has been used to demonstrate that MASNMR experiments can be used to measure amorphous atomic number fractions produced during accelerated internal radioactive decay. This will allow incorporated ν-emitters with short half-lives to be used to model the long-term radiation tolerance of potential ceramic radioactive waste forms. It is believed to be the first example of MASNMR spectroscopy on samples containing fissionable isotopes.

Hunger S, HM Cho, JT Sims, and DL Sparks. 2004. "Direct Speciation of Phosphorus in Alum-Amended Poultry Litter: Solid-State 31P NMR Investigation ." Environmental Science and Technology 38(3):674-681. Abstract Amending poultry litter (PL) with aluminum sulfate (alum) has proven to be effective in reducing water-soluble phosphorus (P) in the litter and in runoff from fields that have received PL applications; it has therefore been suggested as a best management practice. Although its effectiveness has been demonstrated on a macroscopic scale in the field, little is known about P speciation in either alumamended or unamended litter. This knowledge is important for the evaluation of the long-term stability and bioavailability of P, which is a necessary prerequisite for the assessment of the sustainability of intensive poultry operations. Both solidstate MAS and CP-MAS 31P NMR as well as 31P{27Al}- TRAPDOR were used to investigate P speciation in alumamended and unamended PL. The results indicate the presence of a complex mixture of organic and inorganic orthophosphate phases.Acalcium phosphate phase, probably a surface precipitate on calcium carbonate, could be identified in both unamended and alum-amended PL, as well as physically bound HPO4 2-. Phosphate associated with Al was found in the alum-amended PL, most probably a mixture of a poorly ordered wavellite and phosphate surface complexes on aluminum hydroxide that had been formed by the hydrolysis of alum. However, a complex mixture of organic and inorganic phosphate species could not be resolved. Phosphate associated with Al comprised on average 40 ( 14% of the total P in alum-amended PL, whereas calcium phosphate phases comprised on average 7 ( 4% in the alum-amended PL and 14 ( 5% in the unamended PL.