Homonuclear recoupling and heteronuclear decoupling in solid state NMR

Abstract

Nuclear magnetic resonance is a technique with a wide range of applications in biological and chemical sciences. Developments in the methods for recoupling and decoupling in solid-state nuclear magnetic resonance are of fundamental importance for its use in studying the biomolecules. In this thesis, work on developing such pulse sequences is presented. For recoupling, symmetry-based pulse sequences (CN n and RN n )are well known and frequently used. Exponentially modulating recoupling technique is a sequence which make use of a small oscillating eld in addition to a comparatively strong rf eld to recouple e ectively at low power irradiation. Inspired by strong points of these two techniques, here we present a new method EXPORTCN. This o ers an e ective recoupling at low power and at high spinning conditions. Some selection rules, based on the pulse sequence parameters are derived similar to symmetry based sequences, allowing us to select particular interactions to recouple. In an alternative settings these sequences may also be used for decoupling and along these lines we show that the decoupling performance of EXPORT which is a member of this new family is comparable to the TPPM. To improve the decoupling e ciency further a third modulating eld was added (TOFU) which leads to further reduction in resonance line width. Theoretical analysis of EXPORT-CN sequence for rst and second order terms is shown and some simulations are presented to discuss the role of homonuclear recoupling in order to facilitate the heteronuclear decoupling. Decoupling experiments are performed to explore the behavior of the EXPORTCN and TOFU sequences for decoupling and to compare them with TPPM. These experiments show that the oscillating eld techniques are capable of decoupling heteronuclear interactions e ectively. So using these oscillating eld with carefully chosen amplitudes can lead to a better decoupling by systematical suppression of the higher order terms.