Friday, March 05, 2010

J. Phys. Chem. B. and J. Phys. Chem. C, Volumes 114, Issues 9

LiBH4 in Carbon Aerogel Nanoscaffolds: An NMR Study of Atomic Motions

David T. Shane†, Robert L. Corey†‡, Charlie McIntosh†, Laura H. Rayhel†, Robert C. Bowman, Jr.§, John J. Vajo, Adam F. Gross and Mark S. Conradi*†

J. Phys. Chem. C, 2010, 114 (9), pp 4008–4014
DOI: 10.1021/jp9107365

Hydrogen NMR of LiBH4 in the pores of carbon aerogel nanoscaffolds shows the coexistence of motionally narrowed and broad components. The fraction of mobile, diffusing hydrogen, already evident at room temperature, increases continuously with temperature. Thus, a broad distribution of environments is present, as in some ball-milled hydrides. With decreasing pore size from 25 to 13 nm, the narrowed fraction increases, suggesting that the narrow resonance is from the most defective regions, the grain boundaries. The broad component eventually exhibits narrowing in the same temperature window as for bulk material, confirming the bulk-like structure of those regions. Hole-burning measurements reveal magnetization exchange between the broad and narrow resonance lines, confirming the close spatial proximity of the atoms in each line. The solid−solid transition is clearly evident in 7Li line shapes, with a 10−15 °C depression from the bulk. More rapid decay of the quadrupolar satellite signals in spin echoes, compared to the central transition, is due to lithium atoms diffusing between differently oriented nanocrystallites. Our results suggest that crystallites in neighboring pores have similar orientations but are incoherent for diffraction. Remarkably, the T1 data of hydrogen and 7Li are continuous in the vicinity of the transition, in contrast with the bulk T1 data, suggesting that some rapid lithium motion remains below the transition.

Metal Carbonation of Forsterite in Supercritical CO2 and H2O Using Solid State 29Si, 13C NMR Spectroscopy

Ja Hun Kwak, Jian Zhi Hu*, David W. Hoyt, Jesse A. Sears, Chongming Wang, Kevin M. Rosso and Andrew R. Felmy

J. Phys. Chem. C, 2010, 114 (9), pp 4126–4134
DOI: 10.1021/jp1001308

Ex situ natural abundance magic angle spinning (MAS) NMR was used for the first time to study fundamental mineral carbonation processes and reaction extent relevant to geologic carbon sequestration (GCS) using a model silicate mineral forsterite (Mg2SiO4)+supercritical CO2 with and without H2O. Run conditions were 80 °C and 96 atm. With H2O but without CO2, 29Si MAS NMR reveals that the reaction products contain only two peaks of similar intensities located at about −84.8 and −91.8 ppm, which can be assigned to surface Q1 and Q2 species, i.e., SiO4 tetrahedra sharing one and two corners with other tetrahedra, respectively. Using scCO2 without H2O, no reaction is observed within 7 days. Using both scCO2 and H2O, the surface reaction products for silica are mainly Q4 species (−111.6 ppm) accompanied by a lesser amount of Q3 (−102 ppm) and Q2 (−91.8 ppm) species. No surface Q1 species were detected, indicating the carbonic acid formation and magnesite (MgCO3) precipitation reactions are faster than the forsterite hydrolysis process. Thus, it can be concluded that the Mg2SiO4 hydrolysis process is the rate limiting step of the overall mineral carbonation process. 29Si NMR combined with XRD, TEM, SAED, and EDX further reveals that the reaction is a surface reaction with the Mg2SiO4 crystallite in the core and with condensed Q2, Q3, and Q4 species forming highly porous amorphous surface layers. 13C MAS NMR unambiguously identified a reaction intermediate as Mg5(CO3)4(OH)2·5H2O, i.e., the dypingite.

Solid State 2H NMR Analysis of Furanose Ring Dynamics in DNA Containing Uracil

Monica N. Kinde-Carson†, Crystal Ferguson§, Nathan A. Oyler‡, Gerard S. Harbison† and Gary A. Meints*§

J. Phys. Chem. B, 2010, 114 (9), pp 3285–3293
DOI: 10.1021/jp9091656

Abstract: DNA damage has been implicated in numerous human diseases, particularly cancer, and the aging process. Single-base lesions, such as uracil, in DNA can be cytotoxic or mutagenic and are recognized by a DNA glycosylase during the process of base excision repair. Increased dynamic properties in lesion-containing DNAs have been suggested to assist recognition and specificity. Deuterium solid-state nuclear magnetic resonance (SSNMR) has been used to directly observe local dynamics of the furanose ring within a uracil:adenine (U:A) base pair and compared to a normal thymine:adenine (T:A) base pair. Quadrupole echo lineshapes, T1Z, and T2e relaxation data were collected, and computer modeling was performed. The results indicate that the relaxation times are identical within the experimental error, the solid lineshapes are essentially indistinguishable above the noise level, and our lineshapes are best fit with a model that does not have significant local motions. Therefore, U:A base pair furanose rings appear to have essentially identical dynamic properties as a normal T:A base pair, and the local dynamics of the furanose ring are unlikely to be the sole arbiter for uracil recognition and specificity in U:A base pairs.

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