Two Dimensional Experiments

2 Two-dimensional Experiments. - Various modifications of the two-dimensional solid state NMR experiments have been proposed. Carravetta et al. have shown that in many cases, the commutation properties of the mixing Hamiltonian leads to selectivity in the allowed coherence transfer processes leading to enhanced resolution in the cross-peaks of 2D solid state NMR spectra.  

Total through-bond correlation technique using adiabatic pulses and fast MAS has been suggested by Hardy et al It has been shown that above a certain threshold, the polarisation transfer achieved with these sequences is largely insensitive to the amplitude and homogeneity of the radiofrequency field employed. An experimental transfer efficiency of up to 76 /o was achieved in a two-spin system. Applications to resonance assignments in the dipeptide L-Val- 

L-Phe and in the cyclic decapeptide antamanide have been demonstrated. 

A new through-bond correlation method for disordered solids has been proposed. The new approach is based on the uniform-sign cross-peak double quantum filtered correlation spectroscopy experiment, which is a refocused version of the popular double quantum filtered correlation spectroscopy experiment in liquids. Its key feature is that it provides in-phase and doubly absorptive line shapes, which renders it robust for chemical shift correlation in solids. It has been shown that both theory and experiment point to distinct advantages of this protocol, which are illustrated by several experiments under challenging conditions, including fast MAS up to 30kHz, anisotropic molecular motion, and correlation spectroscopy at the natural abundance isotope level. 

Ernst et al. have demonstrated that 2D solid state NMR chemical-shift correlation spectra can be recorded under low-power conditions. Except for the CP period, no RF field amplitudes above 40 kHz were used. However, such experiments require the use of fast MAS ( 50 kHz). Low-power spectra were compared to those obtained under high-power conditions. No line broadenings were found, though some changes in the polarisation-transfer dynamics were observed. 

In a 2D experiment one or more scans are acquired with a delay tl that is incremented in subsequent acquisitions to generate a time domain tl. The time domain tl in conjunction with the acquisition time domain t2 generates a 2D data set that upon double Fourier transform gives a 2D spectrum. In a very simplified view all 2D experiments can be described as series of ID experiments but in practise the situation is rather more complicated because to achieve quadrature detection in both dimensions phase cycling or pulse field gradients must be used. Consequently the processing of 2D data sets depends upon the detection mode and the experimental setup. 

For the successful simulation and processing of 2D experiments knowledge of some of the more fundamental 2D parameters is essential and the first part of this section will briefly examine these parameters. For a comprehensive review of 2D parameters the reader is referred to sections 2.3.1 and 2.3.2. 

The second part of this section examines the processing of 2D NMR data using 2D WIN-NMR. By necessity the description of 2D data processing is very brief and the raw 2D data is processed in a single step rather than the stepwise approach used for ID data. Table 3.5 at the end of this section summarizes the recommended processing parameters for a number of the more common 2D experiments. 

To produce an NMR spectrum, a nucleus must possess a nuclear spin. Nuclei with odd mass numbers (e.g. Si, Al) have half-integer spins and are of most interest for solid state NMR. Nuclei with even mass numbers and odd charge (e.g. H, N) have integer spins, and although subject to difficulties they can still be useful NMR nuclei. Of the 120 nuclei suitable for NMR, 9 have spin I = 1, 31 are spin I = V2, 32 are spin I = /2, 22 are spin I = /2, 18 are spin I = I2 and 8 are spin I = 

One factor influencing the usefulness of an NMR nucleus is its natural abundance, which can range from 100 percent down to the vanishingly small. In the latter case, it may only be possible to acquire an NMR spectrum if the sample has been artificially isotopically enriched, as is normally done for N and 0 NMR. The natural abundances of the most useful spin I = V2 nuclei are listed in Table 1.1 and those of the quadmpolar nuclei in Table 1.2. The standard substances against which the chemical shifts of the various nuclides are quoted are listed for the spin I = V2 nuclei in Table 1.1 and for the quadmpolar nuclei in Table 1.3. 

Nucleus Natural abundance (%) V() at 7.05 T (MHz) Relative receptivity Standard substance 

Nucleus I Natural Vo (MHz) Relative Quadrupole Quadrupole Stemheimer  

One of the most powerful 2D NMR strategies is based on inverse detection [6,14], i.e. indirect observation of an insensitive nucleus through coupling with a sensitive one. This approach enables an impressive reduction of the acquisition time for nuclei of low receptivity such as Fe, Rh or Os 

Such an experiment can also give the relative signs of the coupling constants (see Chapters 1 and 4). 

Two-dimensional exchange spectroscopy might also be a valuable tool in the study of the metal skeleton. No example has been found in the literature. Preferably for nuclei with 100% natural abundance, homonuclear COSY or double-quantmn filtered (DQF)-COSY spectra can give the connectivity pattern of the different metal atoms in a cluster. This holds even, in some cases, for quadnipolar nuclei such as Co [21]. 

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Muns ENDOR mvolves observation of the stimulated echo intensity as a fimction of the frequency of an RE Ti-pulse applied between tlie second and third MW pulse. In contrast to the Davies ENDOR experiment, the Mims-ENDOR sequence does not require selective MW pulses. For a detailed description of the polarization transfer in a Mims-type experiment the reader is referred to the literature [43]. Just as with three-pulse ESEEM, blind spots can occur in ENDOR spectra measured using Muns method. To avoid the possibility of missing lines it is therefore essential to repeat the experiment with different values of the pulse spacing Detection of the echo intensity as a fimction of the RE frequency and x yields a real two-dimensional experiment. An FT of the x-domain will yield cross-peaks in the 2D-FT-ENDOR spectrum which correlate different ENDOR transitions belonging to the same nucleus. One advantage of Mims ENDOR over Davies ENDOR is its larger echo intensity because more spins due to the nonselective excitation are involved in the fomiation of the echo. 

The nOe experiment is one of the most powerful and widely exploited methods for structure determination. nOe difference (NOED) or the two-dimensional experiment, NOESY, is used extensively for stereochemical assignments. It provides an indirect way to extract information about internuclear distances. The other use of nOe is in signal intensification in certain NMR experiments, such as the broad-band decoupled C-NMR experiment. 

Recent Developments in NMR Spectroscopy Soft Two-Dimensional Experiment... 

NOESY NMR spectroscopy is a homonuclear two-dimensional experiment that identifies proton nuclei that are close to each other in space. If one has already identified proton resonances in one-dimensional NMR spectroscopy or by other methods, it is then possible to determine three dimensional structure through NOESY. For instance, it is possible to determine how large molecules such as proteins fold themselves in three-dimensional space using the NOESY technique. The solution structures thus determined can be compared with solid-state information on the same protein obtained from X-ray crystallographic studies. The pulse sequence for a simple NOESY experiment is shown in Figure 3.23 as adapted from Figure 8.12 of reference 19. 

J speetra are able to yield ehemieal shifts experimentally but this is a projeetion of a two-dimensional experiment. It is inevitable that onedimensional pulse and eolleet will be the most rapid experiment available. An effieient method of post-extraeting ehemieal shifts from the experimental speetrum is therefore required. 

In Chapter 2 it was seen that a response surface for a one-factor system can be represented by a line, either straight or curved, existing in the plane of two-dimensional experiment space (one factor dimension and one response dimension). In two-factor systems, a response surface can be represented by a true surface, either flat or curved, existing in the volume of three-dimensional experiment space (two factor dimensions and one response dimension). By extension, a response surface associated with three- or higher-dimensional factor space can be thought of as a hypersurface existing in the hypervolume of four- or higher-dimensional experiment space. 

Figure 6.43 H spectrum of diclofenac glucuronide obtained after single trapping from a 100 pi injection of female urine 4 h after dosage of 50 mg of diclofenac. The spectrum was recorded using a cryogenic flow probe at 600 MHz. The spectrum indicates that the sensitivity is sufficient to run aU two-dimensional experiments needed for structure elucidation. Reproduced from [82] with permission from Elsevier.

The level and structural complexity of the impurities. Complex impurities require higher analyte concentrations to acquire two-dimensional experiments in reasonable time frames. Low-level impurities may not give the desired spectral quality. 

An aspect of general interest in organometallic chemistry is the equilibrium between contact and solvent-separated ion pairs, because metal cations which are sun ounded by an individual solvent cage are expected to show different reactivity towards basic centres than those closely attached to carbanions or amines. At the same time, the anionic centre is less shielded in an SSIP than in a CIP and thus expected to be more reactive. In solution, the differentiation by NMR methods between both structural motifs relies in most cases on chemical shift interpretations and, if possible, on heteronuclear Overhauser (NOE) measurements. The latter method is especially powerful in the case of lithium organic compounds, where H, Li or even H, Li NOE can be detected by one- and two-dimensional experiments. ... 

Selecting the C-bound protons before performing a homonuclear two-dimensional experiment enables to measure small heteronuclear coupling constants [16]. Such an experiment with a sample of natural isotopic abundance was first published by Otting and Wuthrich in 1990, where the half-filter element with spin-lock purge pulse was used to select the C-bound protons in a small protein in aqueous solution [6]. Later applications illustrated the usefulness of the same half-filter element with smaller molecules [17, 18]. 

An experiment intended to measure a relaxation rate consists in general of three elements the preparation period, the relaxation period and the detection period. The scheme differs a little from the famous four-period division of two-dimensional experiments [7]. In the case of two-dimensional... 

The rehydrated samples were obtained by exposing dehydrated samples to water vapor at least three days over saturated NH4GI solution at room temperature. A duraction of 0.5 s between scans were allowed for nuclear spin to recover to their equilibrium magnetization. The one—dimensional Na NMR spectra were recorded by using the spin—echo technique. The strength of the radio-frequency field for the two dimensional nutation experiments was 80 kHz and 128 ti values were used (0 250 /is). Each two- dimensional experiment took about 12 hours of spectrometer time. 

In contrast to applications in structural biology where X/Y correlations are nowadays normally executed as H detected, three-dimensional experiments because of sensitivity reasons,14 many studies on inorganic or organometallic compounds are still performed as two-dimensional experiments with direct detection of one heteronucleus and under -decoupling. As compared to these two categories, one-dimensional polarisation transfer methods such as (semi) selective X/Y-INEPT or INDOR-type techniques, which had in the past been shown to be particularly useful for the characterisation of substrates with only one or two heteronuclei,11 have recently received less attention.15 NOE-based correlations, which are frequently employed for the structure elucidation of bio-molecules, remain rare, and apart from an earlier report of a 13C/6Li HOESY experiment,16 have not been further investigated. 

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