Biomedical subjects

Limin Shao

Publications and source records attributed to Limin Shao.

5 recordsLinked to original sources

PubMed2006

Triggered ligand release coupled to framework rearrangement: generating crystalline porous coordination materials.

A robust 3-D porous structure of formula [Ln2(PDC)3(DMF)2](infinity) has been constructed from lanthanide cations (Ln = Er3+ or Y3+) and the non-linear anionic bridging ligand, pyridine-3,5-dicarboxylate (PDC2-) in dimethylformamide (DMF). The solvated framework polymers {[M2(PDC)3(DMF)2].n(solv)}(infinity) (M = Er, Y) undergo a solid-state, crystal-to-crystal reaction upon heating and are converted via loss of both sorbed and coordinated solvent and rearrangement of the framework core to give a desolvated and porous form with retention of structural integrity. This structural transfer is the first crystallographically characterized system with lanthanide metal ions. These porous products are shown to be effective absorbants for H2, N2, and benzene.

Journal Article↗

PubMed2006

CAESURA: measurement of slow molecular dynamics by solid-state nuclear magnetic resonance chemical shift anisotropy modulation amplification.

An alternative magic angle spinning (MAS) exchange NMR experiment based on chemical shift anisotropy (CSA) amplification is described. The CSA amplification experiment correlates a standard MAS spectrum in the omega(2) dimension with a sideband pattern in omega(1) in which the intensities are identical to those expected for a sample spinning at some fraction 1N of the actual rate omega(r). In common with 2D-PASS, the isotropic shift appears only in the omega(2) dimension, and long acquisition times can be avoided without loss of resolution of different chemical sites. The new CSA amplification exchange experiment provides information about the time scale and geometry of molecular motions via their effect on the sideband intensities in a one-dimensional pattern. The one-dimensional patterns from different chemical sites are separated across two frequency dimensions according to the isotropic shifts.

Journal Article↗

PubMed2006

Quantitative vapor-phase infrared spectrometry of ammonia.

Reference spectra of ammonia from four sources are compared. Low-resolution spectra (i.e., spectra for which the spectrometer resolution is numerically greater than the full-width at half-height of the rotational lines) were obtained from the Environmental Protection Agency (EPA) web site and from Infrared Analysis, Inc. High-resolution (0.12 cm-1) spectra were obtained from the National Institute for Standards and Technology (NIST) and Pacific Northwest National Laboratory (PNNL). Two protocols were used to validate the EPA and Infrared Analysis spectra, with one requiring that Beer's law be obeyed by the low-resolution spectrum and the other that Beer's law be obeyed by the high-resolution spectrum. In all cases, the second protocol gave a significantly better spectral match. It is shown that the path-integrated concentrations for the low-resolution reference spectra were in error by as little as 4% to as much as an order of magnitude, presumably because of the effect of adsorption on the cell walls. Measured absorptivities of the NIST and PNNL spectra were different by approximately 2.6% and it is believed that the effect of adsorption on these spectra is small. When the same protocols were used to test the reference spectrum of methane, the calculated path-integrated concentration was only approximately 2% different from the one that was calculated from the NIST reference spectrum, suggesting that the data processing protocols provide accurate data.

Ammonia↗

PubMed2005

Chemical shift anisotropy amplification with high amplification factor and improved sensitivity.

An improved version of the recently proposed chemical shift anisotropy amplification experiment is described. The original experiment correlates a fast magic angle spinning spectrum in the omega2 dimension with a sideband pattern in omega1 in which the intensities mimic those for a sample spinning at a fraction of the rate omegar/N. Advantages of the experiment include the use of standard methods to extract the principal tensor components from the omega1 sideband patterns and the small number of t1 increments required. The improved version described here permits large amplification factors N to be obtained without resort to prohibitively long sequences of pi-pulses and allows sensitivity to be maximized by eliminating the need to store the magnetization along the z-axis for t1. Amplification factors up to 32 are demonstrated experimentally.

Algorithms↗

PubMed2004

Chemical shift anisotropy amplification.

A new NMR experiment which allows a measurement of the chemical shift anisotropy (CSA) tensor under magic angle spinning (MAS) is described. This correlates a fast MAS spectrum in the omega2 dimension with a sideband pattern in omega1 in which the intensities mimic those for a sample spinning at a fraction of the rate omega r/N, and these sidebands result from an amplification by a factor N of the modulation caused by the CSA. Standard methods can be used to extract the principal tensor components from the omega1 sideband patterns, and the nature of the experiment is such that the use of a large number of t1 increments can be avoided without compromising the resolution of different chemical sites. The new experiment is useful for accurately measuring narrow shift anisotropies.

Algorithms↗