Complete Dipolar Decoupling of13C and Its Use in Two-Dimensional Double-Quantum Solid-State NMR for Determining Polymer Conformations
✍ Scribed by K. Schmidt-Rohr
- Publisher
- Elsevier Science
- Year
- 1998
- Tongue
- English
- Weight
- 238 KB
- Volume
- 131
- Category
- Article
- ISSN
- 1090-7807
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✦ Synopsis
A multiple-pulse technique for complete dipolar decoupling of ment, the directly detected dimension still exhibits 13 C-13 C directly bonded 13 C-labeled sites is described. It achieves signifidipolar splittings, which produce complex spectral patterns, cant spectral simplifications in a recently introduced two-dimenparticularly in the intermediate-coupling limit (5, 9). The sional double-quantum solid-state NMR experiment for determinutmost simplification of such two-dimensional patterns is ing torsion angles. Both homonuclear and heteronuclear dipolar desirable, especially for the analysis of torsion-angle districouplings are removed by combining a 13 C multiple-pulse sequence butions as encountered in amorphous polymers and other with continuous-wave irradiation on the protons. The 13 C sequence disordered solids. The required combination of high-power has a fundamental 10-pulse cycle which is a significantly modified heteronuclear and homonuclear decoupling during the detecmagic-sandwich-echo sequence. The crucial heteronuclear decoution period cannot be achieved by the conventional multiplepling is achieved by breaking the 360Њ ''inner'' pulses in the magic pulse sequences for homonuclear decoupling (10), such as sandwich into 90Њ pulses and spacing them by 1 H 360Њ pulse , , or BR-24 (13), since the lengths. Spectral artifacts typical of multiple-pulse sequences are pulses on the S spins reintroduce the I-S dipolar couplings.
eliminated by phase shifts between cycles. In contrast to many
In addition, for 13 C detection the sequence must provide other multiple-pulse decoupling sequences, the long window in the cycle is the dwell time and can be longer than the inverse dipolar particularly long windows before the detection of each point, coupling, which makes the sequence practical for direct detection because the ring-down time of the probe in 13 C NMR is even with long pulse ring-down times. A modification of the selonger than that in 1 H NMR.
quence to scale the chemical shift and increase the effective spec-
In this paper, we present a novel 13 C multiple-pulse setral width is also presented. The 1D and double-quantum 2D exquence combined with high-power proton decoupling that periments are demonstrated on polyethylene with 4% 13 C-13 C spin achieves these goals. The 13 C sequence is derived from the pairs. The potential of this approach for distinguishing segmental magic-sandwich-echo sequence originally introduced for 1 H conformations is illustrated by spectral simulations of the two-NMR (14). It is combined with high-power 1 H irradiation dimensional ridge patterns that correlate double-quantum and sinwhich removes the heteronuclear dipolar coupling during gle-quantum chemical-shift anisotropies.