Friday, August 03, 2007

Joel' Journal Update: PCCP

Quantifying hydrogen-bonding strength: the measure of 2hJNN coupling in self-assembled guanosines by solid-state 15N spin-echo MAS NMR.
T.N. Pham, J.M. Griffin, S. Masiero, S. Lena, G. Gottarelli, P. Hodgkinson, C. Filip, S.P. Brown.
2hJNN hydrogen-bond mediated J couplings are measured in the solid state for two synthetic deoxyguanosine derivatives by 15N MAS NMR spin-echo experiments. The use of rotor-synchronised Hahn-echo pulse train (RS-HEPT) 1H decoupling, with a duty cycle of 6%, allows spin-echo durations out to 200 ms, hence enabling the accurate determination of J couplings as small as 3.8 Hz. A single-crystal X-ray diffraction structure exists for the shorter alkyl chain derivative dG(C3)2: the observation of significantly different 2hJNN couplings, 6.2 ± 0.4 and 7.4 ± 0.4 Hz, for the two resolved N7 resonances is to be expected given the NHN hydrogen-bonding distances of 2.91 and 2.83 for the two distinct molecules in the asymmetric unit cell. For the longer alkyl chain derivative, dG(C10)2, for which there is no single-crystal diffraction structure, a 15N refocused INADEQUATE spectrum (Pham et al., J. Am. Chem. Soc., 2005, 127, 16018–16019) has demonstrated the presence of N2–HN7 intermolecular hydrogen-bonds indicative of a quartet-like structure. The 2hJNN hydrogen-bond mediated J coupling of 5.9 ± 0.2 Hz is at the lower end of the range (5.9–8.2 Hz) of 2hJNN couplings determined from solution-state NMR of guanosine quartets in quadruplex DNA. A full discussion of the determination of error bars on the fitted parameters is given; specifically, error bars determined by a non-linear fitting (using the covariance matrix) or in a Monte-Carlo fashion are found to give effectively identical results.

Spatial distribution of lithium ions in glasses studied by 7Li{6Li} spin echo double resonance.
S.P. Puls and H. Eckert.
This manuscript introduces 7Li{6Li} spin echo double resonance (SEDOR) spectroscopy as a novel approach for studying the spatial distribution of lithium ions in solid electrolytes. Theoretical simulations using density operator theory as well as experimental validation on the model compound lithium carbonate reveal that this method affords a selective measurement of 7Li–6Li heteronuclear dipole–dipole couplings. Dipolar second moments characterizing internuclear lithium–lithium interactions have been measured in lithium silicate (Li2O)x(SiO2)1–x, (0.1 x 0.4) and lithium borate (Li2O)x(B2O3)1–x, (0.1 x 0.3) glasses. The results indicate that the spatial distributions of the lithium ions in these two glass systems are decidedly different. In the lithium silicate glass system, the results give clear evidence of strong cation clustering for x 0.3, providing quantitative support for a previously proposed model of a one-dimensional channel structure. In contrast, in the lithium borate glass system, the cations seem to be more or less randomly distributed. Nevertheless, an observed superlinear dependence of M2(7Li–6Li) as a function of ion concentration indicates subtle changes of the lithium arrangement principles, which are discussed in relation to the previously proposed ring structure of borate glasses.

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