Publications

Abstract not provided.

Abstract not provided.

The use of rotational echo adiabatic passage double resonance (REAPDOR) solid-state nuclear magnetic resonance (NMR) to determine the site location of an adsorbed polar molecule in a zeolite cage is presented. Nitrogen-15 labeled ammonia is used as a probe molecule to characterize the initial adsorption site in 3A zeolite molecular sieves. The relative position of the ammonia adsorption site in the cage is determined by measuring the internuclear distance between the N on ammonia and both a Na cation site and an Al framework environment using {sup 15}N/{sup 23}Na and {sup 15}N/{sup 27}Al REAPDOR NMR experiments, respectively. The measured internuclear distances are similar to a specific ammonia adsorption site for the zeolite 4A ammonia sorption complex located using X-ray diffraction. Additional details regarding the ammonia hydrogen-bonding environment can be extracted from {sup 1}H/{sup 23}Na and {sup 1}H/{sup 27}Al REAPDOR NMR measurements.

Abstract not provided.

Abstract not provided.

Abstract not provided.

The production and aging of silicone materials remains an important issue in the weapons stockpile due to their utilization in a wide variety of components and systems within the stockpile. Changes in the physical characteristics of silicone materials due to long term desiccation has been identified as one of the major aging effects observed in silicone pad components. Here we report relaxation nuclear magnetic resonance imaging (R-NMRI) spectroscopy characterization of the silica-filled and unfilled polydimethylsiloxane (PDMS) and polydiphenylsiloxane (PDPS) copolymer (M9787) silicone pads within desiccating environments. These studies were directed at providing additional details about the heterogeneity of the desiccation process. Uniform NMR spin-spin relaxation time (T2) images were observed across the pad thickness indicating that the drying process is approximately uniform, and that the desiccation of the M9787 silicone pad is not a H2O diffusion limited process. In a P2O5 desiccation environment, significant reduction of T2 was observed for the silica-filled and unfilled M9787 silicone pad for desiccation up to 225 days. A very small reduction in T2 was observed for the unfilled copolymer between 225 and 487 days. The increase in relative stiffness with desiccation was found to be higher for the unfilled copolymer. These R-NMRI results are correlated to local changes in the modulus of the material

Abstract not provided.

Exploration of the fundamental chemical behavior of the AlCl{sub 3}/SO{sub 2}Cl{sub 2} catholyte system for the ARDEC Self-Destruct Fuze Reserve Battery Project under accelerated aging conditions was completed using a variety of analytical tools. Four different molecular species were identified in this solution, three of which are major. The relative concentrations of the molecular species formed were found to depend on aging time, initial concentrations, and storage temperature, with each variable affecting the kinetics and thermodynamics of this complex reaction system. We also evaluated the effect of water on the system, and determined that it does not play a role in dictating the observed molecular species present in solution. The first Al-containing species formed was identified as the dimer [Al({mu}-Cl)Cl{sub 2}]{sub 2}, and was found to be in equilibrium with the monomer, AlCl{sub 3}. The second species formed in the reaction scheme was identified by single crystal X-ray diffraction studies as [Cl{sub 2}Al({mu}-O{sub 2}SCl)]{sub 2} (I), a scrambled AlCl{sub 3}{center_dot}SO{sub 2} adduct. The SO{sub 2}(g) present, as well as CL{sub 2}(g), was formed through decomposition of SO{sub 2}CL{sub 2}. The SO{sub 2}(g) generated was readily consumed by AlCl{sub 3} to form the adduct 1 which was experimentally verified when 1 was also isolated from the reaction of SO{sub 2}(g) and AlCl {sub 3}. The third species found was tentatively identified as a compound having the general formula {l_brace}[Al(O)Cl{sub 2}][OSCl{sub 2}]{r_brace}{sub n}. This was based on {sup 27}Al NMR data that revealed a species with tetrahedrally coordinated Al metal centers with increased oxygen coordination and the fact that the precipitate, or gel, that forms over time was shown by Raman spectroscopic studies to possess a component that is consistent with SOCl{sub 2}. The precursor to the precipitate should have similar constituents, thus the assignment of {l_brace}[Al(O)Cl{sub 2}][OSCl{sub 2}]{r_brace}{sub n}. The precipitate was further identified by solid state {sup 27}Al MAS NMR data to possess predominantly octahedral A1 metal center which implies {l_brace}[Al(O)Cl{sub 2}][OSCl{sub 2}]{r_brace}{sub n} must undergo some internal rearrangements. A reaction sequence has been proposed to account for the various molecular species identified in this complex reaction mixture during the aging process. The metallurgical welds were of high quality. These results were all visually determined there was no mechanical testing performed. However, it is recommended that the end plate geometry and weld be changed. If the present weld strength, based on .003' - .005' penetration, is sufficient for unit performance, the end plate thickness can be reduced to .005' instead of the .020' thickness. This will enable the plug to be stamped so that it can form a cap rather than a plug and solve existing problems and increase the amount of catholyte which may be beneficial to battery performance.

The quantitative analysis of ammonia binding sites in the Davison (Type 3A) zeolite desiccant using solid-state {sup 15}N MAS NMR spectroscopy is reported. By utilizing 15N enriched ammonia ({sup 15}NH{sub 3}) gas, the different adsorption/binding sites within the zeolite were investigated as a function of NH{sub 3} loading. Using {sup 15}N MAS NMR multiple sites were resolved that have distinct cross-polarization dynamics and chemical shift behavior. These differences in the {sup 15}N NMR were used to characterize the adsorption environments in both the pure 3A zeolite and the silicone-molded forms of the desiccant.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

The effect of non-exponential and multi-exponential decay or relaxation behavior on the performance of the direct exponential curve resolution algorithm (DECRA) is investigated through a series of numerical simulations. Three different combinations of decay or relaxation behavior were investigated through DECRA analysis of simulated pulse gradient spin echo (PGSE) NMR diffusion spectra that contained the combination of two individual components. The diffusion decay behavior of one component was described by a single-exponential decay, while the second component was described by either (1) a multi-exponential decay, (2) a decay behavior described by the empirical Kohlrausch-Williams-Watts (KWW) relation or (3) a multi-exponential decay behavior correlated with variations in the NMR spectral line shape. The magnitudes and types of errors produced during the DECRA analysis of spectral data with deviations from a pure single-exponential decay behavior are presented. It is demonstrated that the deviation from single-exponential decay impacts the resulting calculated line shapes, the calculated relative concentrations and the quantitative estimation of the decay or relaxation time constants of both components present in the NMR spectra. Copyright © 2004 John Wiley & Sons, Ltd.

The quantitative analysis of microstructure and sequence distribution in polysiloxane copolymers using high-resolution solution {sup 29}Si NMR is reported. Copolymers containing dimethylsiloxane (DMS) and diphenysiloxane (DPS) monomer units prepared with either high vinyl content (HVM) or low vinyl content (LVM) were analyzed. The average run length (R{sub exp}), the number average sequence length (l{sub A}, l{sub B}), along with the various linkage probabilities (p{sub AA}, p{sub AB}, p{sub BA}, and p{sub BB}) were determined for different production lots of the LVM97 and HVM97 samples to address the lot variability of microstructure in these materials.

The γ-irradiated-oxidation of pentacontane (C50H102) and the polymer polyisoprene was investigated as a function of oxidation level using 17O nuclear magnetic resonance (NMR) spectroscopy. It is demonstrated that by using 17O labeled O2 gas during the γ-irradiation process, details about the oxidative degradation mechanisms can be directly obtained from the analysis of the 17O NMR spectra. Production of carboxylic acids is the primary oxygen-containing functionality during the oxidation of pentacontane, while ethers and alcohols are the dominant oxidation product observed for polyisoprene. The formation of ester species during the oxidation process is very minor for both materials, with water also being produced in significant amounts during the radiolytic oxidation of polyisoprene. The ability to focus on the oxidative component of the degradation process using 17O NMR spectroscopy demonstrates the selectivity of this technique over more conventional approaches. © 2001 Elsevier Science Ltd.

Abstract not provided.

Abstract not provided.

[Tl(OCH{sub 2}CH{sub 3})]{sub 4}, (1) was reacted with excess HOR to prepare a series of [Tl(OR)]{sub n} where OR= OCHMe{sub 2} (2, n = 4), OCMe{sub 3} (3, n = 4), OCH{sub 2}CMe{sub 3} (4, n = 4), OC{sub 6}H{sub 3}(Me){sub 2}-2,6 (5, n = {infinity}), and OC{sub 6}H{sub 3}(Pr{sup i}){sub 2}-2,6 (6, n = {infinity}). Single crystal X-ray diffraction was used to determine the structure of compounds ligated by more sterically demanding ligands. Compound 4 was found to adopt a cubane structure, while 5 and 6 formed linear polymeric structures. These compounds were additionally characterized by {sup 203,205}Tl solution and {sup 205}Tl solid state NMR. Compounds 1--4 were found to remain intact in solution while the polymeric species, 5 and 6, appeared to be fluxional. While variations in the solution and solid state structures for the tetrameric [Tl(OR)]{sub 4} and polymeric [Tl(OAr)]{sub {infinity}} may be influenced by the steric hindrance of their respective ligands, the covalency of the species is believed to be more an effect of the parent alcohol acidity.

Chemometric analysis of nuclear magnetic resonance (NMR) spectroscopy has increased dramatically in recent years. A variety of different chemometric techniques have been applied to a wide range of problems in food, agricultural, medical, process and industrial systems. This article gives a brief review of chemometric analysis of NMR spectral data, including a summary of the types of mixtures and experiments analyzed with chemometric techniques. Common experimental problems encountered during the chemometric analysis of NMR data are also discussed.

The authors have shown that the hydroperoxide species in {gamma}-irradiated {sup 13}C-polyethylene can be directly observed by {sup 13}C MAS NMR spectroscopy. The experiment was performed without the need for special sample preparation such as chemical derivatization or dissolution. Annealing experiments were employed to study the thermal decomposition of the hydroperoxide species and to measure an activation energy of 98 kJ/mol. EPR spectroscopy suggests that residual polyenyl and alkylperoxy radicals are predominantly trapped in interracial or crystalline regions, while the peroxy radicals observed after UV-photolysis of hydroperoxides are in amorphous regions.

Abstract not provided.

A molecular dynamics (MD) study of the lithium ultraphosphate glass series, xLi{sub 2}O{center_dot}(1{minus}x)P{sub 2}O{sub 5} (0 {le} x < 0.5) was used to investigate the changes in the Li environment with increasing modifier concentration. The results from the MD simulations indicate that no major structural variations in the Li coordination environment are observed. Changes in the type of oxygen coordinated to the modifier are observed and correlate with the T{sub g} minimum. Additionally, changes in the number of shared phosphorus vertices are observed with increasing modifier concentration, in support of recent models involving the role of the modifier in the extended range structure of phosphate glasses. Empirical calculations of the {sup 6}Li NMR chemical shifts directly from the MD simulation structures is also reported and compared to recent experimental solid-state NMR results.

A series of arylalcohols [H-OAr where OAr = OC{sub 6}H{sub 5} (OPh), OC{sub 6}H{sub 4}(2-Me) (oMP), OC{sub 6}H{sub 3}(2,6-Me){sub 2} (DMP), OC{sub 6}H{sub 4}(2-Pr{prime}) (oPP), OC{sub 6}H{sub 3}(2,6-Pr{prime}){sub 2} (DIP), OC{sub 6}H{sub 4}(2-Bu{prime}) (oBP), OC{sub 6}H{sub 3}(2,6-Bu{prime}){sub 2} (DBP) where Me = CH{sub 3}, Pr{prime} = CHMe{sub 2}, and Bu{prime} = CMe{sub 3}] were reacted with LiN(SiMe{sub 3}){sub 2} in pyridine (py) to generate the appropriate ``Li(OAr)(py){sub x}'' complex. The resultant products were characterized by single crystal X-ray diffraction as: [Li(OPh)(py){sub 2}]{sub 2} (1), [Li(oMP)(py){sub 2}]{sub 2} (2), [Li(DMP)(py){sub 2}]{sub 2} (3), [Li(oPP)(py){sub 2}]{sub 2} (4), [Li(DIP)(py){sub 2}]{sub 2} (5), [Li(oBP)(py){sub 2}]{sub 2} (6), and [Li(DBP)(py)]{sub 2} (7). Compounds 1--6 adopt a dinuclear, edge-shared tetrahedral complex. For 7, due to the steric crowding of the DBP ligand, only one py is coordinated yielding a dinuclear fused trigonal planar arrangement. Two additional structure types were also characterized for the DIP ligand as [Li(DIP)(H-DIP)(py)]{sub 2} (5b) and [Li{sub 2}(DIP){sub 2}(py){sub 3}] (5c). {sup 6,7}Li and {sup 13}C NMR solid state MAS spectroscopy indicated that the bulk powder was consistent with the crystalline material. Solution state NMR spectroscopy revealed a symmetric molecule existed in solution for 1--7.

Solid state {sup 31}P/{sup 27}Al and {sup 31}P/{sup 23}Na MAS NMR dipolar dephasing experiments have been used to investigate the spatial distribution of aluminum and sodium cations with respect to the phosphate backbone for a series of sodium aluminophosphate glasses, xAl{sub 2}O{sub 3}{center_dot}50Na{sub 2}O{center_dot}(50{minus}x)P{sub 2}O{sub 5} (0{le} x {le} 17.5). From the {sup 31}P/{sup 27}Al and {sup 31}P/{sup 23}Na connectivity data gathered, information about the medium range order in these glasses is obtained. The expanded connectivity data allows for better identification and interpretation of the new resonances observed in the {sup 31}P MAS NMR spectra with the addition of alumina. The results of the dipolar dephasing experiments show that the sodium-phosphate distribution remains relatively unchanged for the glass series, and that the addition of aluminum occurs primarily through the depolymerization of the phosphate tetrahedral backbone.

The author's laboratory continues to use NMR to investigate the structure and dynamics in amorphous materials, including the local structure of ultraphosphate glasses. Changes in the alkali environment in these phosphate glasses as a function of modifier concentration has recently been probed using {sup 6}Li and {sup 23}Na solid state NMR. Molecular dynamic (MD) simulations have also been performed in an attempt to gain additional insight into the variations of the local structure. Interestingly, although there are distinct variations in the Li coordination number as well as the Li-O bond lengths in the MD simulations (with a minimum or maximum in these parameters near the 20% Li{sub 2}O concentration), a linear change in the {sup 6}Li NMR chemical shift is observed between 5 and 50% Li{sub 2}O mole fraction. One would expect that such variations should be observable in the NMR chemical shift. In an attempt to understand this behavior the author has performed empirical calculation of the {sup 6}Li NMR chemical shift directly from the structures obtained in the MD simulations. It has been argued that the NMR chemical shift of alkali species can be related to a chemical shift parameter A, where A is defined as the summation of the shift contributions for all the oxygens located within the first (and possibly the second) coordination sphere around the cation. For the present case of Li phosphate glasses, the chemical shift correlates directly to the bond valence of the coordinating oxygen.

Abstract not provided.

Abstract not provided.

Phosphonate binding sites in guanidine and ammonium surface-functionalized silica xerogels were prepared via the molecular imprinting technique and characterized using solid state {sup 31}P MAS NMR. One-point, two-point, and non-specific host-guest interactions between phenylphosphonic acid (PPA) and the functionalized gels were distinguished by characteristic chemical shifts of the observed absorption peaks. Using solid state as well as solution phase NMR analyses, absorptions observed at 15.5 ppm and 6.5 ppm were identified as resulting from the 1:1 (one-point) and 2:1 (two-point) guanidine to phosphonate interactions, respectively. Similar absorptions were observed with the ammonium functionalized gels. By examining the host-guest interactions within the gels, the efficiency of the molecular imprinting procedure with regard to the functional monomer-to-template interaction could be readily assessed. Template removal followed by substrate adsorption studies conducted on the guanidine functionalized gels provided a method to evaluate the binding characteristics of the receptor sites to a phosphonate substrate. During these experiments, {sup 29}Si and {sup 31}P MAS NMR acted as diagnostic monitors to identify structural changes occurring in the gel matrix and at the receptor site from solvent mediated processes.

Monte Carlo (MC) simulations of phosphate tetrahedron connectivity distributions in alkali and alkaline earth phosphate glasses are reported. By utilizing a discrete bond model, the distribution of next-nearest neighbor connectivities between phosphate polyhedron for random, alternating and clustering bonding scenarios was evaluated as a function of the relative bond energy difference. The simulated distributions are compared to experimentally observed connectivities reported for solid-state two-dimensional (2D) exchange and double-quantum (2Q) nuclear magnetic resonance (NMR) experiments of phosphate glasses. These MC simulations demonstrate that the polyhedron connectivity is best described by a random distribution in lithium phosphate and calcium phosphate glasses.