Floquet-van Vleck analysis of heteronuclear spin decoupling in solids: The effect of spinning and decoupling sidebands on the spectrum

We investigate the effect of magic angle spinning on heteronuclear spin decoupling in solids. We use an analytical Floquet-van Vleck formalism to derive expressions for the powder-averaged signal as a function of time. These expressions show that the spectrum consists of a centerband at the isotropic frequency of the observed spin, omega(0), and rotational decoupling sidebands at omega(0) +/- omega(1) +/- m omega(r) where omega(1) is the decoupling field strength and omega(r) is the rotation frequency. Rotary resonance occurs when the rotational decoupling sidebands overlap with the centerband. Away from the rotary resonance conditions, the intensity of the centerband as a function Of omega(r)/omega(1) is simply related to the total intensity of the rotational decoupling sidebands. Notably, in the absence of offset terms it is shown that as omega(1) decreases, the centerband intensity can decrease without any associated broadening. Furthermore, the centerband width is shown to be independent of spinning speed, to second order for the effects we consider. The effects of I spin chemical shift anisotropy and homonuclear dipolar couplings are also investigated. The analytical results are compared to simulations and experiments. (c) 2005 Elsevier Inc. All rights reserved

Solution-state NMR studies of the surface structure and dynamics of semiconductor nanocrystals

H-1 and C-13 nuclear magnetic resonance (NMR) relaxation studies of thiophenol-capped CdS nanocrystals are presented. The transverse and longitudinal relaxation times were investigated as a function of nanocrystal radius, and the transverse relaxation time was also studied as a function of temperature. Both proton and carbon T-2 values were found to increase with nanocrystal radius, contrary to initial expectations. This effect is explained in terms of motion of the thiophenol with respect to the nanocrystalline surface. Theoretical expressions for relaxation due to anisotropic motion are developed based on both bridging and terminal bonding configurations of the thiophenol ligands, and the data are fit to these models. The data are found to be consistent with thiophenol ligands bound in a terminal fashion to a single Cd atom. The temperature dependence of the proton T-2 value is also suprising. T-2 is found to decrease with increasing temperature, and the size of this change scales with the nanocrystal radius. This is explained in terms of an extra component of relaxation due to thermally excited electrons

Enhanced sensitivity in RIACT/MQ-MAS NMR experiments using rotor assisted population transfer (vol 150, pg 71, 2001)

The rotor assisted population transfer (RAPT) sequence is used to enhance the sensitivity of the RIACT(II) experiment for spin-3/2 quadrupolar nuclei. A detailed theoretical analysis of the polarizations that contribute to different types of MQ-MAS experiments is provided. In particular, two polarization pathways are distinguished for the creation of triple-quantum coherence. The existence of these pathways is experimentally demonstrated by comparing the sensitivities of different sequences with and without RAPT preparation. (C) 2001 Academic Press

NMR-STUDIES OF THE SURFACE-STRUCTURE AND DYNAMICS OF SEMICONDUCTOR NANOCRYSTALS

H-1 NMR studies of thiophenol capping groups on cadmium sulfide nanocrystals demonstrate that the coverage of the capping molecule depends on the size of the nanocrystal. Data are presented which show that as the size of the nanocrystal increases, the coverage of thiophenol decreases. In addition, information about the overall tumbling of the nanocrystal and the motion of the capping groups relative to the surface can be obtained from linewidth studies, indicating that the rotation of the capping groups is hindered in the smaller nanocrystals (r almost-equal-to 12 angstrom) and becomes less so in larger nanocrystals (r almost-equal-to 20 angstrom). The coverage data are related to the electronic properties of this important class of compounds

IH significantly decreases glutamate transporters, MAP2 and GFAP immunoreactivity.

<p>EAAT1, EAAT2, GFAP and MAP2 immunoreactivity in slices exposed to 7 days RA, SH or IH. EAAT1 and EAAT2 expression was unchanged by SH while significantly reduced in IH. MAP2 and GFAP expression decreased in both SH and IH.</p

Intermittent hypoxia decreases cell viability and impairs glutamate response.

<p>Propidium iodide staining (A) and quantification of PI positive cells presented as mean <u>+</u> SD (B) of slices exposed to 7 days RA, SH or IH at baseline (BL), after 200 µM glutamate, and 10 mM glutamate, followed by a second 200 µM glutamate challenge. n = 12–18. *: At all concentrations RA^- < SH^- & IH^- (p<.001). ⁺ At 10mM: SH^- < IH^- (p<.05). ^# At 10 mM & 200 µM#2: BL < SH^- & IH^- (p<.05 and p<.001 respectively).</p

Long term sustained or Intermittent hypoxia decreases cell viability.

<p>Propidium iodide (red) and Fluorescein diacetate (green) staining of slices exposed to 7 days RA, SH or IH (A) without and (B) with 100 µM ceftriaxone (n = 4-6).</p

Graphic representation of glutamate transporters, MAP2 and GFAP immunoreactivity with or without ceftriaxone.

<p>Quantification of EAAT1, EAAT2, GFAP and MAP2 immunofluorescence per unit area of slices exposed to 7 days RA, SH or IH in presence (+ve) or in absence (-ve) of 100 µM ceftriaxone. Data are presented as mean immunofluorescence <u>+</u> SD. n =  4-13 for RA⁺, IH⁺, SH⁺; n = 5–11 for RA^-, IH^-, SH *: IH^- or SH^- - (p≤0.01). ⁺: IH⁺ < SH⁺ (p≤0.01).</p

Ceftriaxone prevents hypoxia-induced cell death and improves tolerance to excitotoxicity.

<p>Propidium iodide staining (A) and quantification of PI positive cells presented as mean <u>+</u> SD (B) of slices exposed to 100 µM ceftriaxone during 7 days RA, SH or IH at baseline (BL), after 200 µM glutamate, and 10 mM glutamate, followed by a second 200 µM glutamate challenge. baseline (BL) n = 9–12, ^# At 10 mM & 200 µM#2: RA⁺; IH⁺; SH⁺> their respective BL (p<.01).</p

Ceftriaxone effect on cell tolerance to glutamate is significantly greater in IH-exposed slices.

<p>Propidium iodide staining quantification of ceftriaxone (+) treated slices exposed to RA, SH and IH at baseline (BL), after 200 µM glutamate, and 10 mM glutamate, followed by a second 200 µM glutamate challenge. n = 12–18 for RA^-, SH^-, IH^- and n = 9-12 for RA+, SH⁺, IH⁺; IH^- > IH⁺: *At BL (p = .005), ^&At 10 mM (p<.001) & ∧At 200 µM#2 (p = .005).</p