The utility of phase alternated pulses for the measurement of dipolar couplings in 2D-SLF experiments
Bibhuti B. Das, N. Sinha and K.V. Ramanathan
The measurement of hetero-nuclear dipolar coupling using two-dimensional separated local field (SLF-2D) NMR experiments is a powerful technique for the determination of the structure and dynamics of molecules in the solid state and in liquid crystals. However, the experiment is sensitive to a number of factors such as the Hartmann–Hahn match condition, proton frequency off-set and rf heating. It is shown here that by the use of phase alternated pulses during spin-exchange the effect of rf mismatch on the dipolar coupling measurement can be compensated over a wide range of off-sets. Phase alternation together with time and amplitude modulation has also been considered and incorporated into a pulse scheme that combines spin exchange with homonuclear spin decoupling based on magic sandwich sequence and named as SAMPI4. Such time and amplitude averaged nutation experiments use relatively low rf power and generate less sample heating. One of these schemes has been applied on liquid crystal samples and is observed to perform well and yield spectra with high resolution.
Practical aspects of Lee–Goldburg based CRAMPS techniques for high-resolution 1H NMR spectroscopy in solids: Implementation and applications
Cristina Coelho, João Rocha, P.K. Madhu and Luís Mafra
Elucidating the local environment of the hydrogen atoms is an important problem in materials science. Because 1H spectra in solid-state nuclear magnetic resonance (NMR) suffer from low resolution due to homogeneous broadening, even under magic-angle spinning (MAS), information of chemical interest may only be obtained using certain high-resolution 1H MAS techniques. 1H Lee–Goldburg (LG) CRAMPS (Combined Rotation And Multiple-Pulse Spectroscopy) methods are particularly well suited for studying inorganic–organic hybrid materials, rich in 1H nuclei. However, setting up CRAMPS experiments is time-consuming and not entirely trivial, facts that have discouraged their widespread use by materials scientists. To change this status quo, here we describe and discuss some important aspects of the experimental implementation of CRAMPS techniques based on LG decoupling schemes, such as FSLG (Frequency Switched), and windowed and windowless PMLG (Phase Modulated). In particular, we discuss the influence on the quality of the 1H NMR spectra of the different parameters at play, for example LG (Lee–Goldburg) pulses, radio-frequency (rf) phase, frequency switching, and pulse imperfections, using glycine and adamantane as model compounds. The efficiency and robustness of the different LG-decoupling schemes is then illustrated on the following materials: organo-phosphorus ligand, N-(phosphonomethyl)iminodiacetic acid [H4pmida] [I], and inorganic–organic hybrid materials (C4H12N2)[Ge2(pmida)2OH2]·4H2O [II] and (C2H5NH3)[Ti(H1.5PO4)(PO4)]2·H2O [III].
Double-acquisition: Utilization of discarded coherences in a 2D separation experiment using the States method
Masashi Fukuchia, Munehiro Inukai, Kazuyuki Takeda and K. Takegoshi
We propose a new data-acquisition scheme for 2D separation experiments to save the spectrometer time by 1/2. This scheme, referred to as a double-acquisition scheme, is applicable to most of separation experiments with the hypercomplex time-domain data-acquisition scheme (the States method) for data collection.