Elsevier

Chemical Physics Letters

Volume 368, Issues 5–6, 24 January 2003, Pages 511-522
Chemical Physics Letters

Heteronuclear decoupling in NMR of Liquid Crystals using continuous phase modulation

https://doi.org/10.1016/S0009-2614(02)01808-0Get rights and content

Abstract

In this Letter we present a framework for the use of continuously phase modulated radio-frequency pulses for heteronuclear decoupling in NMR of Liquid Crystals. Within this framework, we found new sets of heteronuclear decoupling sequences using numerical optimisation. These sequences and supercycles are tested experimentally on a model liquid crystal, and their performance is compared with standard heteronuclear decoupling sequences.

Introduction

Broadband heteronuclear decoupling is essential for the observation of high-resolution NMR signals of nuclei coupled to abundant spins. Heteronuclear decoupling was first introduced for liquid-state NMR where it was used to remove heteronuclear J-couplings [1], [2], [3], [4]. Subsequently it has been used in solid-state NMR to remove heteronuclear dipolar couplings [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], where it has become the cornerstone technique for obtaining high-resolution cross-polarisation magic angle spinning (CPMAS) spectra of nuclei such as carbon-13 or nitrogen-15.

J-decoupling in liquids has been developed over the last thirty years using powerful theoretical tools [1], [2] to achieve today a quite stunning sophistication. For dipolar decoupling in the solid-state, on the other hand, continuous wave (CW) decoupling is still the most common method, and, despite some notable early attempts, only recently have pulse sequences been introduced which really improve on the performance of CW decoupling, with the TPPM sequence [8], [9] being the most commonly used for solids under MAS, and SPINAL [14] being suited to static samples (e.g., liquid crystals). However, there is reason to believe that there is still much room for improvement, notably in the power requirements for good decoupling and in the sensitivity of the sequences to experimental imperfections such as transmitter offset and rf inhomogeneity effects.

The main problem with developing decoupling methods in the solid-state is that dipolar couplings (H–H and C–H) cannot be considered small compared to the available rf fields. The success of theoretical methods in liquid-state NMR is closely related to the fact that the rf fields can be routinely 100 times larger than the J-couplings. In solids, the rf fields are typically only five times larger than the dipolar couplings and approximate methods must, therefore, be developed to much higher order to provide any useful insight. To avoid problems linked with approximate theoretical models, in this Letter we investigate decoupling in static solids using a computer modeling approach. Computer optimisations have had some success for decoupling in the liquid state [16]. We describe the approach we used to find new decoupling sequences that are tested experimentally on a liquid crystalline sample and found to be comparable in performance to the best existing methods.

Section snippets

The DUMBO approach

We have recently demonstrated that an approach based on computer modeling can be used with success to find homonuclear dipolar decoupling sequences [17]. This approach, which we have dubbed DUMBO for Decoupling Using Mind Boggling Optimisation, comprises four distinct steps: (i) a radio-frequency irradiation scheme is chosen which includes a variable parameter space, (ii) the model spin system for the problem is chosen, (iii) a target function for ideal behavior is defined, and (iv) the

Model spin system

The principle problem that we have encountered for the heteronuclear decoupling case is that very simple models for the spin system do not yield good decoupling sequences. For example, if we use a simple static two spin IS system (which would be the equivalent of the system we used in our previous work on homonuclear dipolar decoupling), we do not find any correlation between the numerical performance of sequences and their actual experimental behavior. A straightforward extension to include a

Optimisation procedure

Fig. 3 shows the results of using the model spin system in the optimisation procedure outlined in Section 3. A series of 106 combinations of Fourier coefficients (a set of 6*2 coefficients an,bn which define the phase) were randomly generated, and their q parameter numerically evaluated for a single nominal value of decoupling field strength, i.e., 100 kHz. Coefficients were determined for various values of the CH dipolar coupling constants, ranging from 0 to 10 kHz. The best sets of

Experimental results

All the experiments were performed on a 500 MHz Bruker DSX NMR spectrometer, using a static HX probe and a 5 mm coil diameter. The decoupling field was set to a nominal value of 50 kHz. In order to test the performance of the proton–carbon heteronuclear decoupling, we observe the carbon-13 CP spectrum of a sample of 4-pentyl-4cyanobiphenyl (5CB), with the decoupling sequence applied to protons during acquisition (see Fig. 1c). The temperature was controlled at 293 K, where 5CB is a nematic

Conclusion

In this Letter we have shown that heteronuclear dipolar decoupling sequences can be found for liquid crystalline samples using a numerical approach. Experimental implementation is straightforward, and the sequence with the best performance we have found so far, dubbed SDROOPY-1, is particularly robust with respect to the proton rf power setting, and, therefore, to rf inhomogeneity in the probe. Best decoupling is obtained for a nominal 9π cycle. The sequence has an absolute decoupling

Acknowledgements

We are grateful to Anne Lesage, Patrick Charmont, Bénédicte Eléna and Luminita Duma for stimulating discussions.

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