Multiple-pulse sequence structure

The major breakthroughs, however, have come from the use of high magnetic fields and further from the use of different multiple pulse sequences to manipulate the nuclear spins in order to generate more and more information time domain NMR spectroscopy, that is used to probe molecular dynamics in solutions. The latter made it also possible to "edit" sub-spectra and to develop different two-dimensional (2D) techniques, where correlation between different NMR parameters can be made in the experiment (e.g. SH versus 813c, see later). Solid state NMR spectroscopy is used to determine the molecular structure of solids. 

Combined Rotation and Multiple Pulse Spectroscopy (CRAMPS) is a technique in which the dipolar interaction is averaged through a multiple-pulse sequence [54, 55]. The simultaneous spinning around the magic angle, as in MAS NMR, averages the chemical shift anisotropy. Under appropriate conditions, CRAMP spectra can be of greater resolution than MAS NMR spectra. While CRAMPS is not exclusively a surface-sensitive technique, the majority of catalytic applications have focused on the study of adsorbed species, and the information on surface structure that can be extracted from their spectra. 

There are two approaches to pulse sequence classification depending on the user s occupation. For the chemist who has to solve a structural question or characterize a new compound it is the spectra obtained from the pulse sequence that is of primary importance. The NMR spectroscopist is usually more concerned with the pulse sequence structure and choice of experimental parameters and whether a particular pulse sequence can be improved or modified to solve a specific problem. These two different approaches lead to confusion in pulse sequence nomenclature such that names are often a combination of the purpose of the experiment and the sequence layout. For example the commonly used acronyms HMQC, HSQC and HMBC imply a consistent abbreviation system yet HMQC and HSQC describe the coherence state during the evolution time whilst HMBC denotes an experiment to correlate nuclei using multiple bond heteronuclear scalar coupling. 

High-resolution N.M.R. of Solids.— The period under review has seen substantial progress in the application of solid-state high resolution n.m.r. techniques to polymers. The techniques comprise " multiple-pulse sequences (MP), dipolar-decoupling (DD), and magic-angle sample rotation (MAR). In addition, cross-polarization (CP) may be used to enhance weak C resonances. These experiments provide both dynamic and structural information as do conventional broadline measurements, but the resolution of individual resonances provides a detailed description of molecular behaviour at the monomeric level. The papers are conveniently divided into dynamic and structural studies, with some small overlap. 

The inclusion of average Hamiltonian theory derived pulse sequence building blocks specifically designed for the measurement of RDCs is highly desirable. The homonuclear dipolar decoupling sequences used in ref. 190 lead to the less complex multiplet structures observed in isotropic samples with the chemical shift resolution reduced by a factor of 2-3, depending on the multiple pulse... 

A 2002 review by Reynolds and Enriquez describes the most effective pulse sequences for natural product structure elucidation.86 For natural product chemists, the review recommends HSQC over HMQC, T-ROESY (transverse rotating-frame Overhauser enhancement) in place of NOESY (nuclear Over-hauser enhancement spectroscopy) and CIGAR (constant time inverse-detected gradient accordion rescaled) or constant time HMBC over HMBC. HSQC spectra provide better line shapes than HMQC spectra, but are more demanding on spectrometer hardware. The T-ROESY or transverse ROESY provides better signal to noise for most small molecules compared with a NOESY and limits scalar coupling artefacts. In small-molecule NMR at natural abundance, the 2D HMBC or variants experiment stands out as one of the key NMR experiments for structure elucidation. HMBC spectra provide correlations over multiple bonds and, while this is desirable, it poses the problem of distinguishing between two- and three-bond correlations. 

In order to assess magnetization transfer in a multiple-spin system, it is necessary to define a measure that reflects the efficiency of the transfer between two spins i and j. This parameter should reflect the amplitude of the ideal polarization transfer as well as the duration of the mixing process, because, in practice, Hartmann-Hahn transfer competes with relaxation. Relaxation effects result in a damping of the ideal polarization-transfer functions 7j . The damping due to relaxation depends not only on the structure and dynamics of the molecule that hosts the spin system of interest, but also on the actual trajectories of polarizations and coherences under a specific multiple-pulse Hartmann-Hahn mixing sequence (see Section IV.D). For specific sample conditions and a specific experiment, the coherence-transfer efficiency can be defined as the maximum of the damped magnetization-transfer function. 

A simple one-dimensional spectrum is always recorded at the start of any structural analysis. Small modifications to the ID pulse sequence enables the selection of groups of signals or singlet signals, the separation of multiplets for determining coupling constants or multiplicity and the determination of molecular parameters such as relaxation times or diffusion constant. In addition ID pulse sequence can also be converted into 2D sequences (and vice versa) enabling homonuclear and heteronuclear correlation. Table... 

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