Heteronuclear dipolar dephasing of deuterium quadrupole echoes

(6)Li NMR in lithium borate glasses

In anticipation of using fluctuations in the nuclear dipolar and quadrupolar interaction as a probe of lithium ion motion in lithium borate glasses, the static values of these interactions were measured using a variety of echo techniques. The static quadrupolar echo spectrum of (7)Li and a calculation of the dipolar interaction in crystalline Li(2)B(4)O(7) (same chemical composition as the glass under study) were used to estimate the strength of the two interactions. These indicate that the dipolar and quadrupolar interactions for (6)Li will be of similar size and the dipolar interaction will be dominated by the unlike spin interaction between the (6)Li and the (10)B, (11)B spins. An appropriate theoretical model is proposed and explicit expressions for the echo amplitude are calculated in terms of the dipolar and quadrupolar second moments. This single spin model takes into account the quadrupolar interaction but treats the dipolar interaction as an effective magnetic field. Experimental results are presented which show the essential validity of the model and measurements lead to reasonable values for the dipolar and quadrupolar second moments. The relative merits of the various echo techniques are discussed.

Slow Dynamics of Banana-Shaped Molecules: A Theoretical Approach to Analyze 2H-NMR T2 Relaxation Times

In the present work, we analyze pulsed deuterium NMR experiments performed on the isotropic and nematic phases of the banana-shaped liquid-crystalline mesogen 4-chloro-1,3-phenylene bis{4-4'-(11-undecenyloxy) benzoyloxy} benzoate (ClPbis11BB) selectively deuterated on the central ring. Starting from a previous evidence of unusual slow dynamics in the isotropic phase (Domenici V. et al., J. Phys. Chem. B 2005, 109, 769), a quantitative and model-supported analysis of the deuterium NMR data is performed here by accounting for slow-motional modulation of the magnetic anisotropies through the full solution of the stochastic Liouville equation. Focusing on the quadrupolar echo experiments performed in the nematic phase, the analysis of the transverse relaxation rate has been carried out by considering single-molecule motions and fluctuations of the local director. The main conclusions are: (a) director fluctuations are not relevant on driving the signal relaxation; (b) molecular reorientations about transverse axes control the dynamic regime of the signal relaxation and impose a full slow-motional treatment; (c) the small amplitude tumbling of the molecule within the wells of orientational potential occurs with characteristic times up to the microsecond. The outcome of our analysis has to be taken as indicative of very slow dynamics concerning out-of-plane motions of the molecules. Besides the specific application, this paper also offers the methodological tools to treat the pulsed deuterium NMR experiment in the slow-motional regime of reorientational motions and provides a detailed comparison with the usually employed fast-motional approximation.

Structural and Dynamic Aspects of Hydrogen-Bonded Complexes and Inclusion Compounds Containing α,ω-Dicyanoalkanes and Urea, Investigated by Solid-State 13C and 2H NMR Techniques

Solid-state 13C NMR and 2H NMR techniques have been used to investigate structural and dynamic properties of the 1,4-dicyanobutane/urea and 1,5-dicyanopentane/urea 1:1 hydrogen-bonded complexes and the 1,6-dicyanohexane/urea inclusion compound. The pure crystalline phase of urea has also been investigated. The 13C NMR studies have focused on 13C chemical shift anisotropy and second-order quadrupolar effects (arising from 13C-14N interaction) for the urea molecules and the cyano groups of the alpha,omega-dicyanoalkanes. Parameters describing these interactions are derived and are discussed in relation to the known structural properties of these materials. Comparison of 13C chemical shift anisotropies of the cyano carbons and rates of 13C dipolar dephasing suggest that 1,4-dicyanobutane and 1,5-dicyanopentane are effectively static, whereas 1,6-dicyanohexane has greater mobility. 2H NMR line shape analysis for the 1,4-dicyanobutane/urea-d4 and 1,5-dicyanopentane/urea-d4 complexes indicates that the only motion of the urea molecules that is effective on the 2H NMR time scale is a rapid libration about the C=O bond over an angular range of about 26 degrees . For the 1,6-dicyanohexane/urea-d4 inclusion compound, the 2H NMR line shape is consistent with a motion comprising 180 degrees jumps about the C=O bond at rates that are intermediate on the 2H NMR time scale. In addition, rapid libration about the C=O bond also occurs over an angular range of about 20 degrees . The dynamic properties of the urea molecules in these materials are compared with those of urea molecules in other crystalline environments.

Dynamics in synthetic oligonucleotides. A solid-state deuterium NMR study

Solid-state 2H NMR spectroscopy was used to investigate dynamics in the [methyl-2H]thymidine-labeled oligonucleotide [d(CGCGAAT*T*CGCG)]2. Quadrupole echo line shapes, spin-lattice relaxation, and quadrupolar echo decay rates were investigated as a function of hydration W (moles of water/moles of nucleotide) between 0 and approximately 30. The amplitude of the base motion, modeled as a fast four-site libration, or diffusion in a cone, increased slightly with higher levels of hydration. A slower component of motion about the helix axis appears at W approximately 10 and increases in rate and amplitude, leading to the intermediate rate line shape observed at W approximately 21.