P.K. Madhu

High-resolution proton solid-state NMR spectroscopy by phase-modulated Lee–Goldburg experiment

Abstract In this Letter, we present a pulse scheme for coherent averaging of spin–spin interactions called phase-modulated Lee–Goldburg (PMLG) which is aimed at achieving high-resolution proton NMR spectra in the solid state. Our objective is mainly to discuss the design and execution of this experiment and to show some preliminary experimental results as evidence of the far-reaching potentials of this technique.

Sensitivity enhancement of the MQMAS NMR experiment by fast amplitude modulation of the pulses

Abstract We report here an improved way of doing the multiple-quantum magic-angle spinning (MQMAS) NMR experiment that relies on the use of amplitude modulated pulses. These pulses were found to yield MQMAS NMR signals that are considerably stronger (≈200–300%) than the ones arising from the usual continuous wave pulse schemes by virtue of a superior efficiency of the triple- to single-quantum conversion process. Numerical simulations and experimental results taking 23 N a and 87 R b nuclei as examples are presented that corroborate the usefulness of this approach.

Proton spectroscopy in solid state nuclear magnetic resonance with windowed phase modulated Lee-Goldburg decoupling sequences

We demonstrate here a new windowed multiple pulse sequence for the detection of protons in solid state NMR. Acquisition windows are inserted in the phase modulated Lee–Goldburg scheme (PMLG). This enables one-dimensional acquisition as against two-dimensional in windowless schemes. The pulse scheme, theoretical justifications and experimental spectra of a few samples are presented. In addition two-dimensional proton–proton and carbon–proton correlation experiments are performed with windowed PMLG proton detection.

Cross-correlations in NMR

The phenomenon of nuclear magnetic resonance (NMR) involves placing magnetically active nuclear spins embedded in a gas, liquid or solid phase in a constant, large and uniform magnetic field, causing a splitting of magnetic energy levels.Energy can be absorbed by these spins from a resonant radiofrequency (RF) field causing transitions between these levels. Immediately following this absorption, the spins start to exchange this energy among themselves and also pass it on to other degrees of freedom, that is, the spins start to relax. Relaxation is central to the NMR phenomenon as a necessary prerequisite for its detection. It is also used as a probe for obtaining information on the local environment of the spins and about the dynamics of the molecules in which the spins are embedded.

Phase modulated Lee-Goldburg magic angle spinning proton nuclear magnetic resonance experiments in the solid state: A bimodal Floquet theoretical treatment

Interference phenomena between sample spinning and radio frequency (RF) irradiation in solid state high resolution proton nuclear magnetic resonance (NMR) spectroscopy are examined. A bimodal Floquet treatment is exploited in order to overcome the limitations of the average Hamiltonian theory (AHT) approach. Frequency switched Lee Goldburg (FSLG) and its variant, phase modulated Lee Goldburg (PMLG-n), homonuclear dipolar decoupling experiments on protons that are rotating at the magic angle are examined. Average Hamiltonian theory (AHT) is used for the synchronous application of FSLG and PMLG-n RF sequences with the sample spinning. A bimodal Floquet approach is introduced to treat both synchronous and nonsynchronous cases. The Floquet approach, providing a general theoretical framework for describing rotating spin systems exposed to periodically applied RF field, reveals several features of the interference between the sample spinning and the RF irradiation. These features can be characterized by mapping...

High-resolution 1H NMR in the solid state using symmetry-based pulse sequences

We demonstrate new rotor synchronised pulse sequences for obtaining high-resolution H-1 NMR spectra in the presence of fast magic angle spinning (MAS). The new sequences exploit selection rules generated by appropriate synchronisation of the radio-frequency (rf) field modulations and the sample rotation. We show preliminary results demonstrating the feasibility of high-resolution proton NMR spectroscopy in the presence of fast MAS, and also demonstrate the resolution of H-1-C-13 J-couplings in the solid state.

Fast radio-frequency amplitude modulation in multiple-quantum magic-angle-spinning nuclear magnetic resonance: Theory and experiments

Multiple-quantum magic-angle-spinning (MQMAS NMR) spectroscopy has become a routine method to obtain high-resolution spectra of quadrupolar nuclei. One of the main problems in the performance of this experiment has been the poor efficiency of the radio-frequency pulses used in converting multiple-quantum coherences to the observable single-quantum signals. As the MQMAS experiment is basically an echo experiment this problem can be related to the efficiency with which continuous wave pulses can normally achieve the multiple- to single-quantum conversion for different crystallites in a spinning powdered sample. In this paper we investigate various aspects involved in this multiple-to-single quantum conversion, in the hope to facilitate the devise of new experimental schemes that can lead to significant MQMAS signal enhancements. We examine in particular a recently suggested experiment for MQMAS spectroscopy which employs amplitude-modulated radio-frequency pulses, and which can yield substantial signal and ...

A bimodal Floquet analysis of phase modulated Lee-Goldburg high resolution proton magic angle spinning NMR experiments

A description of phase modulated Lee–Goldburg experiments PMLG-n, with 2n pulses per rf cycle, on coupled proton systems under magic angle spinning is presented using bimodal Floquet theory. Level crossing conditions between Floquet states are derived and perturbation theory is used to account for spectral changes as a function of the spinning frequency and the number of pulses per PMLG-n sequence. High resolution solid-state proton spectra are shown that are obtained for n equal to 9, 5 and 3. Line broadening and the rf rotor frequency line positions are correlated to level anticrossings.

Strategies for High-Resolution Proton Spectroscopy in Solid-State NMR

We describe radiofrequency (RF) pulse schemes in solid-state NMR applied to samples rotating at the magic angle (MAS) to obtain high-resolution (1)H spectra. Without combined RF schemes and MAS, (1)H spectra are normally severely broadened by the strong homonuclear proton-proton dipolar couplings. This review gives an outline of a representative class of multiple-pulse sequences, designed to work with and without MAS, and commonly used for high-resolution proton spectroscopy in solid-state NMR. We give a theoretical treatment of these sequences based on Floquet theory, both single-mode and bimodal. Using this approach, we define first-order homonuclear decoupling efficiency parameters which provide the line-narrowing characteristics of the various pulse sequences when applied to fast rotating samples. These parameters are used to compare the line-narrowing efficiencies of the multiple-pulse schemes.

Proton line narrowing in solid-state nuclear magnetic resonance: new insights from windowed phase-modulated Lee-Goldburg sequence.

We present here a bimodal Floquet analysis of the windowed phase-modulated Lee-Goldburg (wPMLG) sequence for homonuclear dipolar decoupling. One of the main criteria for an efficient homonuclear dipolar decoupling scheme is an effective z-rotation condition. This is brought about by the presence of radio-frequency imperfections in the pulse sequence together with a systematic manipulation of the wPMLG pulses. Additional improvement in the (1)H spectral resolution was obtained by a proper understanding of the off-resonance dependence of the wPMLG irradiation scheme based on bimodal Floquet theory. Numerical investigations further corroborate both theoretical and experimental findings. Theoretical analysis points to accidental degeneracies between the cycle time of the wPMLG sequence and the rotor period leading to the experimentally observed off-resonance dependence of the resolution. Two-dimensional (1)H-(1)H homonuclear single-quantum correlation spectra of model amino acids are also presented, highlighting the improved spectral resolution of wPMLG sequences.