Friday, August 03, 2007

Joel' Journal Update: ChemPhysChem

Probing the Evolution of Adsorption on Nanoporous Solids by In Situ Solid-State NMR Spectroscopy.
M. Xu, K. D.M. Harris, J.M. Thomas, D.E.W. Vaughan.
A rotor-spinning technique is developed for carrying out in situ studies of adsorption processes by solid-state NMR spectroscopy. The method is suitable for the study of adsorption at the liquid/solid interface, does not require any modification of the NMR instrumentation, and allows spectra to be recorded immediately from the start of the adsorption process (see picture).

Indirect Detection of Nitrogen-14 in Solid-State NMR Spectroscopy.
S. Cavadini, S. Antonijevic, A. Lupulescu, G. Bodenhausen
NMR spectra of 14N (spin I=1) are obtained by indirect detection in powders spinning at the magic angle. The method relies on the transfer of coherence from a neighboring spy nucleus with S=1/2, such as 13C or 1H, to single- or double-quantum transitions of 14N nuclei. The transfer of coherence can occur through a combination of scalar and residual dipolar splittings (RDS); the latter are also known as second-order quadrupole-dipole cross terms. The two-dimensional NMR spectra reveal powder patterns determined by second- and third-order quadrupolar couplings. These spectra depend on the quadrupolar coupling constant CQ (typically a few megahertz), on the asymmetry parameter Q of the 14N nucleus, and on the orientation of the internuclear vector rIS between the I (14N) and S (spy) nuclei with respect to the quadrupolar tensor. These parameters, which can be subject to motional averaging, can reveal valuable information about the structure and dynamics of nitrogen-containing solids. Application of this technique to various amino acids, either enriched in 13C or with natural carbon isotope abundance, with spectra recorded at various magnetic fields, illustrates the scope of the method.

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