Double resonance solid-state

The majority of double-resonance solid-state NMR experiments involving spin-1/2 nuclei use transfer of nuclear polarization via dipolar cross polarization (CP) to enhance polarization of the diluted spins S with small gyromagnetic ratio ys and significant longitudinal relaxation time T at the expense of abundant spins I with large y, and short 7 [215]. Typically, CP is used in combination with MAS, to eliminate the line broadening due to CS A, as well as with heteronuclear decoupling. To achieve the / S CP transfer, a (n/2)y pulse is applied at the I spin frequency,... 

NMR-spectroscopic investigations of dendrimers can provide comparable information. Thus Wooley et al. used REDOR (Rational Echo Double-Resonance) solid state NMR experiments for the study of the density distribution and back-folding of terminal groups in fifth-generation poly(benzyl ether) dendrimers... 

Mani, R., Tang, M., Wu, X., et al. (2006) Membrane-bound dimer structure of a beta-hairpin antimicrobial peptide from rotational-echo double-resonance solid-state NMR. Biochemistry, 45(27), 8341-8349. 

P-23 - Characterization of acidic sites in zeolites by heteronuclear double resonance solid state NMR... 

A solid-state nuclear magnetic resonance (NMR) experiment was carried out in 4 mm double bearing rotor made from Zr02 on a Bruker DSX 200 MHz spectrometer with resonance frequency at 75.468 MHz. The pulse length was 3.5 ps and the contact time of IH-13C CP was 2-5 ms. 

Rotational-echo double-resonance (REDOR)(75,79) is a new solid-state NMR technique which is sensitive to through-space carbon-nitrogen interactions between selectively 13C and 15N-enriched sites separated by up to 5A (20-22). The parameter directly measured in a REDOR experiment is the heteronuclear dipolar coupling constant DCN, which is in itself proportional to the inverse third power of the intemuclear distance, rCN. It is this dependence on (icn)3 which accounts both for REDOR s ability to accurately measure short distances and its insensitivity to longer-range interactions. As a technique which can probe, in detail, intermolecular interactions over a distance range of 5A, REDOR is well suited to studying the distribution of small selectively-labeled molecules in polymer delivery systems. 

Rotational echo double resonance (REDOR) [86] is one of the most important techniques developed for solid-state NMR spectroscopy. The theory of REDOR is... 

Numerous new developments and applications of solid state NMR techniques have emerged. Multidimensional NMR methods are able to probe connectivity patterns of zeolite framework structures and solve ambiguities in line assignments [27], high-resolution techniques for quadrupolar nuclei have been developed [31-34], and powerful double-resonance methods permit the study of spatial... 

Crosslinked polymer networks formed from multifunctional acrylates are completely insoluble. Consequently, solid-state nuclear magnetic resonance (NMR) spectroscopy becomes an attractive method to determine the degree of crosslinking of such polymers (1-4). Solid-state NMR spectroscopy has been used to study the homopolymerization kinetics of various diacrylates and to distinguish between constrained and unconstrained, or unreacted double bonds in polymers (5,6). Solid-state NMR techniques can also be used to determine the domain sizes of different polymer phases and to determine the presence of microgels within a poly multiacrylate sample (7). The results of solid-state NMR experiments have also been correlated to dynamic mechanical analysis measurements of the glass transition (1,8,9) of various polydiacrylates. 

Another type of DOUBLE ENDOR, called special TRIPLE , has been introduced by Dinse et al.90 to study proton hf interactions of free radicals in solution. In a special TRIPLE experiment two rf fields with frequencies vp + Av and vp — Av are swept simultaneously. For systems with Tln < T,i this leads to a considerable signal-to-noise improvement and to TRIPLE line intensities which are directly proportional to the number of nuclei with the same hf coupling constant. It should be remembered, however, that in transition metal complexes in the solid state the resonance frequencies are not, in general, symmetrically placed about the free proton frequency vp and that the condition Tln < Tj,i is not always fulfilled. 

The ENDOR techniques, of course, are not confined to studies of transition metal complexes. A fast growing interest on electron nuclear double and multiple resonance experiments is also noticed in other fields of natural sciences, such as radical, radiation and polymer chemistry, solid state physics, biophysics and mineralogy. 

The experimental methods in ENDOR spectroscopy have been extensively described by Kevan and Kispert4) in their monograph, Electron spin double resonance spectroscopy, and by Leniart18 in a recent paper. In this section we shall briefly review the instrumentation used in solid state ENDOR and describe the technical details of some new experimental methods. 

The solid-state Si SPE NMR spectra of SBA-15 and the titania surface-coated SBA-15 (Ti-SBA-15) are in accord with this expectation. The spectrum of SBA-15 displays a broad as)mimetric peak at 109 ppm (Q" sites) with shoulders at —101 ppm (Q sites) and 90 ppm(Q sites) in the area ratio 79 19 2. The NMR spectrum of Ti-SBA-15 (one layer) shows a reduction of the band intensity relative to the intensity. The normalized Q Q Q site populations become 85 13 2. No asymmetry is observed in the Q site band. Repetition of the monolayer deposition to form a double layer of titania on silica yields a material whose Si NMR spectrum is indistinguishable from that of the Ti-SBA-15 with a monolayer coverage. As expected, the titania-insulated silica resonances are unperturbed by the second titania layer. 

Samoson, A., and Lippmaa, E. (1989) Synchronized double-rotation NMR spectroscopy. Solid State Magn. Reson., 84, 410-416. 

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