ISSN:0021-2148    

DOI:10.1002/ijch.199200019

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

ISSN:0021-2148    

DOI:10.1002/ijch.199200020

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractThe principles of the rotor synchronized magic angle spinning (MAS) two‐dimensional exchange NMR, first proposed by Veeman and coworkers are reviewed, with particular emphasis on situations where chemical exchange in solids proceeds in concert with molecular reorientation. Calculations of cross peak intensities as function of the ratio between the chemical shift anisotropy and the spinning rate are presented for several cases. These calculations emphasize the advantage of using slow spinning rates (ωR<ωLΔσ) in such experiments when detailed information about mechanistic pathways in solids is sought. Three applications of the method to solid systems using carbon‐13 NMR are described. These include: (a) Trimethylsulfoxonium iodide, in which the molecules undergo 120°‐jumps about the molecular C3symmetry axis; (b) Tropolone, where the tautomeric hydrogen shift is found to be a consequence of the self diffusion within the crystal lattice, and in general accompanied by molecular reorientation. Here the two‐dimensional pattern is used to obtain information about the various mechanisms of the diffusion process; (c) Bullvalene, where a quantitative analysis of the cross peak intensities as function of the mixing time provides kinetic information on two independent processes, viz. symmetric threefold jumps and a concerted Cope rearrangement‐molecular reorient

ISSN:0021-2148    

DOI:10.1002/ijch.199200021

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractWe describe a new solid‐state nuclear magnetic resonance (NMR) technique for correlating anisotropic and isotropic chemical shifts in powdered samples. Two‐dimensional (2D) NMR spectra are obtained by processing signals acquired inindependentexperiments for different angles between the sample spinning axis and the Zeeman magnetic field. This 2D NMR approach can therefore resolve individual static anisotropic lineshapes according to their isotropic chemical shift frequencies, without use of sudden mechanical motions or multiple‐pulse irradiation schemes. Applications of the technique are illustrated with an analysis of the chemical shift anisotropy for the eight distinct13C sites in tyr

ISSN:0021-2148    

DOI:10.1002/ijch.199200022

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractIn this paper we discuss two methods to achieve high nuclear polarizations at room temperature using pulsed EPR techniques and triplet spins which are optically excited by a laser pulse. The results of our experiments, which are performed on a single crystal of napthalene doped with pentacene, show that an unusually high proton spin polarization can be achieved and that the transfer occurs in a time determined by the dipolar interaction between the electron spins of the pentacene guests and the proton spins on the surrounding napthalene molecules.

ISSN:0021-2148    

DOI:10.1002/ijch.199200023

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractIt is shown that odd‐order multiple quantum coherences can be generated and detected in a system of dipolar coupled spins in a solid. The odd‐order multiple quantum coherences can be detected in various ways. In one particular experiment positive‐ and negative‐order quantum coherences can be distinguished. The theory is demonstrated by experiments on ada

ISSN:0021-2148    

DOI:10.1002/ijch.199200024

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractWe derive an expression for the rate of spin diffusion between13C nuclei based on a simple model appropriate for organic solids. In this model, couplings between carbons and protons and among protons are included only as a source of fluctuating local magnetic fields at the carbon nuclei. Our results are essentially equivalent to the results of previous calculations in which the surrounding protons are treated quantum mechanically using memory function formalisms.

ISSN:0021-2148    

DOI:10.1002/ijch.199200025

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractWe present the results of an NMR investigation to study the decomposition of acetylene adsorbed on small Pt clusters similar to industrial catalysts. We begin by reviewing some of our earlier results where it was shown that acetylene converts primarily to vinylidene (CCH2) upon room temperature adsorption. It was proposed that the vinylidene gradually dehydrogenates between 450 and 650 K leaving C2surface species whose carbon‐carbon bond breaks at about 670 K. In this work we have shown that ethylidyne (CCH3) plays an important role in providing an alternate decomposition mechanism. In addition, although there is some evidence that CCH surface species are formed upon room temperature adsorption, no C2surface species were observed in our experiments. Therefore, although dehydrogenation prior to carbon‐carbon bond scission may occur, further experiments where the ethylidyne mechanism is eliminated are needed to completely unravel the decomposition mechan

ISSN:0021-2148    

DOI:10.1002/ijch.199200026

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractNutation spectroscopy of63Cu nuclei at zero static magnetic field is explored as a method for the determination of the quadrupole asymmetry parameters η of Cu sites in copper oxides, with special emphasis on high‐Tcsuperconductors. Since the nutation spectra reveal the actual radio frequency (rf) intensities experienced by the63Cu nuclei, the method also provides a verification of a quantitative Cu spin counting procedure based on integration of nuclear quadrupole resonance (NQR) echo‐signal intensities over wide frequency ranges. The paper presents numerical simulations of spectra and signal intensities that were obtained with a density‐matrix formalism whereby the spin dynamics is described as precessions in a 3D operator space. The quantitative effects of the echo refocusing pulse, rf amplitude fluctuations, and variations in η are examined. Examples of63Cu NQR nutation spectra are shown for randomly oriented powders of Cu2O and YB

ISSN:0021-2148    

DOI:10.1002/ijch.199200027

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractA gain in detection sensitivity of more than three orders of magnitude has been achieved in high resolution solid state2H NMR of monocrystalline systems by applying optical nuclear polarization (ONP) via excited triplet states. The method is demonstrated for crystals of fluorene doped with optically active guest molecules which have localized triplet states and for the1: 1 mixed crystal of anthracene/tetracyanobenzene with excitonic states. The factors determining the efficiency of ONP are discussed and compared to the case of proton spin systems.

ISSN:0021-2148    

DOI:10.1002/ijch.199200028

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY