Asif Equbal

Efficient heteronuclear decoupling in MAS solid-state NMR using non-rotor-synchronized rCW irradiation.

We present new non-rotor-synchronized variants of the recently introduced refocused continuous wave (rCW) heteronuclear decoupling method significantly improving the performance relative to the original rotor-synchronized variants. Under non-rotor-synchronized conditions the rCW decoupling sequences provide more efficient decoupling, are easier to setup, and prove more robust towards experimental parameters such as radio frequency (rf) field amplitude and spinning frequency. This is demonstrated through numerical simulations substantiated with experimental results under different sample spinning and rf field amplitude conditions for powder samples of U-(13)C-glycine and U-(13)C-L-histidine·HCl·H2O.

A unified heteronuclear decoupling strategy for magic-angle-spinning solid-state NMR spectroscopy

A unified strategy of two-pulse based heteronuclear decoupling for solid-state magic-angle spinning nuclear magnetic resonance is presented. The analysis presented here shows that different decoupling sequences like two-pulse phase-modulation (TPPM), X-inverse-X (XiX), and finite pulse refocused continuous wave (rCW(A)) are basically specific solutions of a more generalized decoupling scheme which incorporates the concept of time-modulation along with phase-modulation. A plethora of other good decoupling conditions apart from the standard, TPPM, XiX, and rCW(A) decoupling conditions are available from the unified decoupling approach. The importance of combined time- and phase-modulation in order to achieve the best decoupling conditions is delineated. The consequences of different indirect dipolar interactions arising from cross terms comprising of heteronuclear and homonuclear dipolar coupling terms and also those between heteronuclear dipolar coupling and chemical-shift anisotropy terms are presented in order to unfold the effects of anisotropic interactions under different decoupling conditions. Extensive numerical simulation results are corroborated with experiments on standard amino acids.

r TPPM: towards improving solid-state NMR two-pulse phase-modulation heteronuclear dipolar decoupling sequence by refocusing.

We present here a simple refocused modification, r TPPM, of the Two-Pulse Phase-Modulation (TPPM) heteronuclear decoupling method, which improves decoupling and makes the sequence much more robust with respect to essential experimental parameters. The modified sequence is compared with the established TPPM sequence and a variety of other decoupling sequences at low to moderate magic-angle spinning frequencies. Simulations are shown to compare TPPM and r TPPM with respect to various experimental parameters. The observations from simulations are corroborated with experimental findings at two spinning frequencies on U-(13)C-glycine and U-(13)C-L-histidine.HCl.H2O.

Relative merits of rCW(A) and XiX heteronuclear spin decoupling in solid-state magic-angle-spinning NMR spectroscopy: A bimodal Floquet analysis.

We present a bimodal Floquet analysis of the recently introduced refocused continuous wave (rCW) solid-state NMR heteronuclear dipolar decoupling method and compare it with the similar looking X-inverse X (XiX) scheme. The description is formulated in the rf interaction frame and is valid for both finite and ideal π pulse rCW irradiation that forms the refocusing element in the rCW scheme. The effective heteronuclear dipolar coupling Hamiltonian up to first order is described. The analysis delineates the difference between the two sequences to different orders of their Hamiltonians for both diagonal and off-diagonal parts. All the resonance conditions observed in experiments and simulations have been characterised and their influence on residual line broadening is highlighted. The theoretical comparison substantiates the numerical simulations and experimental results to a large extent.

Significance of symmetry in the nuclear spin Hamiltonian for efficient heteronuclear dipolar decoupling in solid-state NMR: A Floquet description of supercycled rCW schemes

Symmetry plays an important role in the retention or annihilation of a desired interaction Hamiltonian in NMR experiments. Here, we explore the role of symmetry in the radio-frequency interaction frame Hamiltonian of the refocused-continuous-wave (rCW) pulse scheme that leads to efficient 1H heteronuclear decoupling in solid-state NMR. It is demonstrated that anti-periodic symmetry of single-spin operators (Ix, Iy, Iz) in the interaction frame can lead to complete annihilation of the 1H-1H homonuclear dipolar coupling effects that induce line broadening in solid-state NMR experiments. This symmetry also plays a critical role in cancelling or minimizing the effect of 1H chemical-shift anisotropy in the effective Hamiltonian. An analytical description based on Floquet theory is presented here along with experimental evidences to understand the decoupling efficiency of supercycled (concatenated) rCW scheme.

Sine-squared shifted pulses for recoupling interactions in solid-state NMR

Rotational-Echo DOuble-Resonance (REDOR) is a versatile experiment for measuring internuclear distance between two heteronuclear spins in solid-state NMR. At slow to intermediate magic-angle spinning (MAS) frequencies, the measurement of distances between strongly coupled spins is challenging due to rapid dephasing of magnetisation. This problem can be remedied by employing the pulse-shifted version of REDOR known as Shifted-REDOR (S-REDOR) that scales down the recoupled dipolar coupling. In this study, we propose a new variant of the REDOR sequence where the positions of the π pulses are determined by a sine-squared function. This new variant has scaling properties similar to S-REDOR. We use theory, numerical simulations, and experiments to compare the dipolar recoupling efficiencies and the experimental robustness of the three REDOR schemes. The proposed variant has advantages in terms of radiofrequency field requirements at fast MAS frequencies.

Refocusing pulses: A strategy to improve efficiency of phase-modulated heteronuclear decoupling schemes in MAS solid-state NMR

The strategy of using π pulses in conjunction with continuous-wave radio-frequency fields to refocus spin interactions has lead to robust and efficient family of heteronuclear decoupling schemes in magic-angle spinning solid-state NMR, denoted as, rCW schemes. Here, we investigate the generality of the application of such refocussing pulses in other phase-modulated decoupling schemes, notably the super-cycled XiX decoupling. XiX is a commonly used heteronuclear decoupling scheme under conditions of fast MAS and low-amplitude radio-frequency irradiation. The refocussing of interactions is achieved by inserting π pulses with a phase of 135° in the supercycled XiX scheme. The refocussed XiX, rXiX, scheme has improved decoupling efficiency, better offset tolerance, and easier experimental setup compared to the XiX scheme.

Highly efficient 19F heteronuclear decoupling in solid-state NMR spectroscopy using supercycled refocused-CW irradiation

We present heteronuclear 19F refocused CW (rCW) decoupling pulse sequences for solid-state magic-angle-spinning NMR applications. The decoupling sequences have been designed specifically to ensure suppression of the pertinent 13C-19F dipolar coupling interactions while simultaneously suppressing strong anisotropic chemical shift as well as homonuclear 19F-19F dipolar coupling effects as typically present in perfluorated compounds. In an extensive numerical and experimental analysis using a rigid, organic solid as a model compound, it becomes evident that the supercycled rCW schemes markedly improve the decoupling efficiency, leading to substantial enhancements in resolution and sensitivity when compared to previous state-of-the-art methods. Furthermore, considerable gains in robustness toward rf mismatch as well as offset in the radio-frequency carrier frequency are observed, all of which clearly render the new rCW schemes the methods of choice for 19F decoupling in rigid, fluorinated compounds - which is further supported by a Floquet-based theoretical analysis.